Ba(MoO F) (XO (X = Se, Te): First Cases of ...

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Ba(MoO2F)2(XO3)2 (X = Se, Te): First Cases of Noncentrosymmetric

Fluorinated Molybdenum-Oxide Selenite/Tellurite Through Unary

Substitution for Synchronously Enlarging Band Gap and Second

Harmonic Generation

Lin Lin,†,§ Xingxing Jiang,‡,§ Chao Wu,†,§ Longhua Li,† Zheshuai Lin,‡ Zhipeng Huang,† Mark G.

Humphreyδ and Chi Zhang*,†

† China-Australia Joint Research Center for Functional Molecular Materials, School of Chemical

Science and Engineering, Tongji University, Shanghai 200092, China

‡ Key Lab of Functional Crystals and Laser Technology, Technical Institute of Physics and

Chemistry, Chinese Academy of Sciences, Beijing 100190, China

δ Research School of Chemistry, Australian National University, Canberra, ACT 2601, Australia

KEYWORDS : nonlinear optical materials, second harmonic generation, unary substitution,

selenite, tellurite

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ABSTRACT: Nonlinear optical (NLO) materials have critically important applications in

advanced laser technologies. However, achieving a good balance between the mutual competing

optical performance within one molecular structure remains a great challenge. In this study, two

alkaline-earth-metal fluorinated molybdenum-oxide selenite/tellurite, Ba(MoO2F)2(XO3)2 (X =

Se BMFS, Te BMFT), were synthesized through a facile unary substitution: BMFS was obtained

by partial substitution of oxygen atoms with highly electronegative fluorine in parent compound

BaMo2O5(SeO3)2 (BMS), while BMFT was achieved by further replacing lone-pair Se4+ cations

in BMFS with heavier Te4+ cations in the same main-group. By partial replacing oxygen with the

fluorine, BMFS shows broaden band gap and enhanced second harmonic generation (SHG)

response compared to BMS owing to the high electronegativity of fluorine anions and the

favorable orientation and alignment of NLO-active [MoO5F]5- and [SeO3]2− groups, which is

relatively rare for unary anion substitution. BMFS and BMFT are isostructural and both

crystallize in noncentrosymmetric (NCS) space groups Aba2, featuring a three-dimensional (3D)

double-layered framework comprised of 2D [MoO4F(XO3)]∞ anionic layers interconnected by

divalent barium cations. Both BMFS and BMFT exhibit good optical performance, including

large SHG responses (3 × and 4 × KH2PO4), wide band gaps (3.30 and 3.27 eV) and optical

transparency window (0.38−10.3 and 0.38−10.5 μm), and large laser damage thresholds (60 ×

and 53 × AgGaS2), suggesting their potential applications as promising NLO materials. The DFT

calculations based on real-space atom-cutting technique and SHG-weighted electronic density

analysis have elucidated the crucial role of the [MoO5F]5- groups to the enlarged band gaps and

enhanced SHG responses in BMFS and BMFT. This work proposes a feasible unary substitution

strategy for synthesizing first NCS fluorinated molybdenum-oxide selenite/tellurite with

synchronously enlarged band gap and SHG efficiency.

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INTRODUCTION

Nonlinear optical (NLO) material is an important research frontier of inorganic, physical and

materials chemistry owing to their promising applications in laser frequency conversion, optical

parametric oscillation, and optical parametric amplification, which play a significant role in

advanced laser technology.1-3 Noncentrosymmetric (NCS) structure is indispensable for a

second-order NLO material, in which microscopic polarizations of asymmetric units would be

superimposed along one specific direction, thus leading to a macroscopic net polarization and

second harmonic generation (SHG).4-6 To achieve crystalline materials with a NCS structure, one

strategy is to apply cations that manifest a preference for distorted coordination environment

induced by second-order Jahn-Teller (SOJT) effect.7-9 Such a SOJT effect can be evoked by two

different types of cations, including transition-metal (TM) cations with d0 electronic

configuration (Ti4+, V5+, Nb5+, Mo6+, W6+, etc.) and cations with sterically active lone pairs (Se4+,

Te4+, I5+, Bi3+, etc.).10-14 According to the SOJT effect theory, a multiatomic system of highly

symmetric configuration with near degeneracy is unstable and spontaneous distortion would

occur to minimize the internal energy of the system.8 As a result, SOJT-active groups are under

asymmetric coordination environments and favor to increase the incidence of a crystallographic

NCS material. Among these d0 TM cations, Mo6+ cation has been classified as a strong distorter

with the highest magnitude of the out-of-center distortion, which suggests that strong distortion

of molybdenum-oxide octahedra may lead to strong SHG effects.8,11

In principle, strong optical absorption of NLO crystals under intense laser beams may lead to

large laser damage,15 small band gaps of the crystals always accompany by a low laser damage

threshold (LDT),16 and a material with large band gap shows weak SHG response in many

cases.17-22 Recently, numbers of new NLO materials have been obtained by substituting relative

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low NLO-active components of known compounds to optimize the optical performance.23-26 For

example, α-/β-Ba2[GaF4(IO3)2](IO3) have been synthesized via the aliovalent substitutions of V5+

cations and oxygen anions by Ga3+ cations and fluorine anions from the known α-/β-

Ba2[VO2F2(IO3)2](IO3), which results in the improvement of band gaps of the parent

compounds.23 Partially replacing I− anions with Cl− in Cs2Hg3I8 leads to the construction of

Cs2HgCl2I2 with an increased band gap and LDT.24 Fluoroiodate CsIO2F2 with a [IO2F2]− unit is

achieved by fluorine substitution of CsIO3 and exhibits a widened optical band gap.25 However,

these cases mentioned above all suffer from the drawback of the reduction in SHG efficiency in

comparison with their parent compounds. On the other hand, study on Pb2TiOF(SeO3)2Cl and

Pb2NbO2(SeO3)2Cl reveals that fluorinated d0 TM octahedron in selenides could enhance the

SHG effects of the compounds by changing the band structures but lead to an obvious reduction

in band gaps.26 It is therefore necessary to undertake careful molecular modification to design

new compounds which can achieve an optimized balance between band gaps and SHG

coefficients in one molecular system. We consequently suppose that replacing oxygen anions in

SOJT-active d0 TM-oxide octahedron with highly electronegative fluorine anions might enhance

the distortion scale of [MOxF6-x] octahedron owing to the symmetry reduction. Further

combining the distorted [MOxF6-x] octahedra with asymmetric [SeO3]2−/[TeO3]2− pyramids could

not only regulate the alignment of both NLO-active components but also facilitate a large SHG

response. In addition, heavy elements with more electronic shells are usually favorable to an

extended IR transparent window and a large refractive index as well as a large dipole

moment,19,27 a new NLO material with enhanced SHG response and a wider transparent window

can be accordingly achieved through rational heavy element substitution.

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Following the above considerations, an alkaline-earth molybdenum-oxide selenite

BaMo2O5(SeO3)228 (BMS) was chosen as a parent compound. Partial substitution of oxygen

anion in the parent compound by highly electronegative F− anions and further substitution of

Se4+ cation with heavier Te4+ cation, two NCS Ba(MoO2F)2(XO3)2 (X = Se BMFS, Te BMFT)

were successfully synthesized via the facile hydrothermal reaction. BMFS and BMFT feature a

three-dimensional (3D) double-layered framework comprised of 2D [MoO4F(XO3)]∞ anionic

layers interconnected by divalent Ba2+ cations. In contrast to their parent compound BMS,

BMFS and BMFT possess more distorted basic building octahedra [MoO5F] instead of [MoO6]

in BMS, showing enhanced dipole moments owing to the substitution of fluorine anions. Optical

property studies reveal that both BMFS and BMFT exhibit excellent NLO properties, including

strong SHG responses (3 ×, 4 × KH2PO4 (KDP)), large optical band gaps (3.30, 3.27 eV), wide

optical transparency windows (0.38−10.3, 0.38−10.5 μm), and large LDTs (60 ×, 53 × AgGaS2),

respectively. To the best of our knowledge, BMFS and BMFT are the first examples of NCS

fluorinated molybdenum-oxide selenite/tellurite. In sharp contrast to examples of substitution

mentioned above, the BMFS shows enhanced SHG response and larger band gap simultaneously

compared to BMS through a unary anion substitution. Theoretical calculations have been

performed on BMFS and BMFT, demonstrating that the strong SHG responses and large band

gaps mainly originate from the combination and particularly alignment arrangements of the

[MoO5F]5− octahedra and [SeO3]2−/[TeO3]2− pyramids. This work propose a feasible unary

substitution strategy by the introduction of highly electronegative F− anions into d0 TM-oxide

octahedra for the designed syntheses of NLO crystal materials with optimized optical

performance.

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EXPERIMENTAL SECTION

Reagents and Instruments. BaF2 (99%, AR), MoO3 (99%, AR), SeO2 (99%, AR), TeO2

(99%, AR) and hydrofluoric acid (40%, AR) were purchased from commercial sources and used

without further purification. Elemental analyses were measured using a field emission scanning

electron microscope (FESEM, Hitachi S-4800) with an energy dispersive X-ray spectroscope

(EDS). Bruker D8 advance diffractometer equipped with Cu Kα radiation (λ = 1.540598 Å) was

used to collect the powder X-ray diffraction (PXRD) patterns for crystalline samples.

Thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) were performed

on a Netzsch STA 409PC instrument. Infrared (IR) spectra were collected using a Thermo

Scientific Nicolet iS10 FT-IR spectrometer. Agilent Cary 5000 UV-Vis-NIR spectrophotometer

was employed to collect the UV-Vis-NIR diffuse reflectance spectra.

NLO Property Measurements. Powder second-harmonic generation measurements were

performed on polycrystalline compounds BMFS and BMFT using a modified Kurtz-Perry

method.29 Radiation (λ = 1064 nm) generated by a Q-switched Nd:YAG solid-state laser was

employed as the fundamental frequency light. The polycrystalline samples of BMFS and BMFT

were ground and sieved into several different particle size ranges (<26, 26–50, 50–74, 74–105,

105–150, 150–200 and 200–280 μm), and pressed into disks with diameters of 6 mm that were

placed between glass microscope slides and secured with tape in a 1 mm thick aluminum holder.

Crystalline KDP (serving as the reference) was also sieved into the same particle size ranges and

tested under the same conditions. The SHG efficiency was evaluated by comparing oscilloscope

traces of the SHG signal of BMFS, BMFT and KDP at the particle size range of 105–150 μm.

The LDTs of polycrystalline samples of BMFS, BMFT and AgGaS2 were measured using a

single-pulse method with a pulse laser beam (1064 nm, 10 ns) generated from Q-switched

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Nd:YAG solid-state laser.15 BMFS, BMFT and AgGaS2 were ground into the size range of 105–

150 μm and then pressed between two glass slides into the sample loading area. During the

measurements, the pulse energy was gradually raised up from 1 mJ until obvious superficial

damage on the samples were observed under a magnifier.

Synthesis. Ba(MoO2F)2(SeO3)2 and Ba(MoO2F)2(TeO3)2 were synthesized via facile

hydrothermal reactions. An initial reactant mixture of BaF2 (0.263 mg, 1.5 mmol), MoO3 (0.288

mg, 2.0 mmol), SeO2/TeO2 (0.222/0.319 mg, 2.0 mmol), hydrofluoric acid (0.10 mL) and

deionized water (2.0 mL) was tightly sealed in a 23 mL autoclave equipped with a Teflon liner.

The autoclave was heated at 220 ℃ for 72 h, and then cooled slowly to 30 ℃ at a rate of 4 ℃/h.

The product was collected by vacuum filtration, washed with deionized water, and then dried in

the air. Colorless crystals of BMFS (0.376 g, 55% based on Se) and BMFT (0.188 g, 24% based

on Te) were isolated using a microscope.

Single-crystal Structure Determination. Single-crystal structural data collections were

carried out on a Bruker D8 Venture CMOS X-ray diffractometer with a Mo Kα radiation (λ =

0.71073 Å) at 293(2) K using the APEX II software. The data were obtained with scan widths of

1.00° and an exposure time of 3 second per frame. Multi-scan absorption corrections were

applied and the crystal structures were solved by direct methods and refined on F2 by full-matrix

least-squares methods using the SHELXTL-97 software package.30,31 All atoms were refined

with anisotropic displacement parameters. The space group for both structures were determined

to be NCS Aba2, which were checked and confirmed with the PLATON software.32 In BMFS,

there is an obvious Q peak (9.59 e∙Å-3) appearing near the Se1 atom, which can be identified as

Se1A atom and refined the occupancy as 0.8:0.2, leading to a split of selenium atoms. Based on

the results of bond valence calculations, the (0.36159, 0.33594, -0.20177) site in BMFS showed

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bond valence of −1.41 or −1.12 when it occupied by oxygen or fluorine atom, respectively.33

This site should be full occupancy by fluorine atoms in consideration of the charge balance

simultaneously. Similar situation occurred in dealing with BMFT, but the occupancy was refined

as 0.7:0.3. The detailed crystallographic data and structural refinement parameters of BMFS and

BMFT are listed in Table 1. Other crystallographic information (Atomic coordinates, selected

bond lengths and angles, and equivalent isotropic displacement parameter,) is provided in Tables

S1 and S2.

Table 1. Crystallographic data and refinement parameters for title compounds.

Formula Ba(MoO2F)2(SeO3)2 Ba(MoO2F)2(TeO3)2

Formula weight 685.13 782.41 Temperature 293(2)K 293(2)K Crystal system Orthorhombic Orthorhombic

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Computational Descriptions. Theoretical calculations for optical and electronic properties were

conducted based on the single-crystal structure data of BMFS and BMFT by using first-

principles simulations. The calculations were performed based on a plane-wave basis set and

pseudo-potential density functional theory (DFT) using the CASTEP package.34,35 The

generalized gradient approximation (GGA) scheme of Perdew-Burke-Ernzerhof (PBE) was

utilized to calculate the exchange correlation interaction.36,37 A kinetic energy cutoff of 400 eV

and dense Monkhorst–Pack38 k-point mesh spanning less than 0.2 Å-3 in the Brillouin zone were

using to guarantee the veracity of the calculation. Due to the discontinuity of the exchange–

correlation of standard DFT, the calculated band is usually smaller than the experimental result.

Therefore, a scissor operator was adopted to shift the conduction band (CB) to match the

Space group Aba2 Aba2 a(Å) 12.3643(15) 12.5591(14) b(Å) 11.0325(16) 11.0137(9) c(Å) 7.2945(10) 7.2753(8) α(°) 90 90 β(°) 90 90 γ(°) 90 90 V(Å3) 995.1(2) 1006.34(18) Z 4 4 ρcalc / g·cm-3 4.573 5.164 µ / mm-1 13.787 12.083 F(000) 1223 1368 θ (deg) 3.30-26.39 3.24-27.12

Limiting indices -15<=h<=14 -14<=h<=16 -13<=k<=13 -14<=k<=12 -8<=l<=9 -8<=l<=9

Rint 0.0493 0.0305 Reflections collected / unique 2416/905 2500/1015 GOF on F2 1.098 1.196 R1/ wR2 [I > 2σ (I)]a 0.0438/0.0886 0.0297/0.0643 R1/ wR2 (all data) 0.0540/0.0916 0.0358/0.0856 Largest diff. peak and hole(eÅ-3) 1.544 and -1.205 1.378 and -1.693 aR1 = ∑||Fo| – |Fc||/∑|Fo|; wR2 = [∑w(Fo

2–Fc2)2]/∑w(Fo

2)2]1/2

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measured value.39 Based on the scissor-corrected electronic structure, the imaginary part of the

dielectric function can be calculated by the electronic transition from the forbidden band and the

real part, such as the refractive index, can be obtained using the Kronic–Kramer

transformation.40 The SHG coefficients were determined using the formulae modeled by virtual

electron transition in the length-gauge framework.41

RESULTS AND DISCUSSION

Synthesis. Crystalline compounds BMFS and BMFT were synthesized through facile

hydrothermal reactions according to the equations (1)−(2):

BaF2 + 2MoO3 + 2SeO2 → Ba(MoO2F)2(SeO3)2 (1)

BaF2 + 2MoO3 + 2TeO2 → Ba(MoO2F)2(TeO3)2 (2)

The reaction temperature adopted for the preparation of BMFS and BMFT is higher than that of

reaction for BMS. In general, under mild hydrothermal conditions, the high temperature is

favorable for introducing fluorine element into the molecular system.26 During the reactions (1)

and (2), a small amount of hydrofluoric acid is necessary for the generation of target compounds,

lacking of hydrofluoric acid fails to obtain BMFS and BMFT. It is thus assumed that small

amount of hydrofluoric acid provides an acidic condition and serves as mineralizer

simultaneously. Similar reactions for synthesizing BMFS and BMFT were also performed with

BaCO3 served as barium source instead of BaF2 but none of target crystals was obtained, which

may imply that the fluorine source is all from BaF2 rather than hydrofluoric acid. The initial and

final pH values of two reaction systems are 1.4 and 3.1 for BMFS and 1.9 and 4.0 for BMFT,

respectively. The two reactions have an obvious difference in the yields of BMFS and BMFT

that may be due to the difference in solubility of SeO2 and TeO2. SeO2 can be easily dissolved in

water and almost all participates in the hydrothermal reactions. In contrast, TeO2 can be hardly

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dissolved in water or a moderate acid condition so that a considerable amount of TeO2 is left

unreacted and the preparation of BMFT is impeded by incomplete reactions. We also used the

same method to prepare other divalent cation (e.g. Sr2+, Zn2+, Cd2+, Pb2+) homologues but no

target crystal was achieved. It may be owing to the obvious difference between ionic radii of

these cations. The purities of the two crystalline products were confirmed by PXRD studies, the

powder patterns were shown in Figure S1.

Structure Description. Crystals BMFS and BMFT are isostructural possessing one same basic

building units (BBUs) ([MoO5F]5− octahedra) and one homogeneous BBU ([SeO3]2− in BMFS

and [TeO3]2− in BMFT) in the molecular structures, both crystallize in the orthorhombic system

with NCS space group Aba2 (No. 41). As a representative, BMFS will be discussed in detail. The

structure of BMFS features a three-dimensional (3D) double-layered framework comprised of

2D [MoO4F(SeO3)]∞ anionic layers interconnected by divalent Ba2+ cations (Figure 1). The

asymmetric unit cell of BMFS contains crystallographically independent one Ba, two Mo, two

Se, ten O and two F atoms. Within the structure of BMFS, each Mo6+ cation adopting an

octahedral coordinating mode is connected with five oxygen and one fluorine atoms to form a

distorted [MoO5F]5− octahedron, while every selenium atom is coordinated with three

oxygen/fluorine atoms to form [SeO3]/[SeO2F] showing the tripod-like configuration. The

[MoO5F]5− octahedron shows a C2 distortion (toward an edge) with two elongated Mo−O/F

bonds (2.069(10) and 2.224(14) Å), two intermediate (1.956(7) and 1.991(8) Å) and two short

(1.695(13) and 1.737(12) Å) Mo−O bonds. The out-of-center distortion (Δd) of [MoO5F]5−

octahedron is calculated to be 0.91, indicating that Mo6+ cation manifests a strong distortion (Δd

> 0.8). It is noticeable that there is a crystallographic position disorder on the Se atoms in

BMFS, where the Se atom occupies one of two specific sites with possibilities of 80% and 20%

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corresponding to Se1 and Se1A, respectively. However, the Se atom could actually only locate in

one of such positions in the crystal structure of BMFS. As the symmetry elements decrease, the

crystal structure has a lower symmetry. As each Se atom has an opportunity of 80% to appear at

Se1 site, it is coordinated to three oxygen atoms from three adjacent [MoO5F]5− octahedra with

bond lengths range from 1.711(13) to 1.730(8) Å. If the Se locates at the Se1A site, the Se-

centered pyramid is constructed with two normal Se−O bonds (1.717(16) and 1.825(14)Å) as

well as an elongated Se−F bond of 2.091(19) Å. Each Ba2+ cation is tenfold coordinated to

nearby eight O and two F atoms which is located on the two-fold axis. The Mo-centered

[MoO5F]5− octahedra and [SeO3]2− pyramids are alternately connected with each other via

corner-sharing apex O atoms to construct a six-numbered ring (6-MR), which serves as the

anionic building block. By further connecting these building blocks, corrugated 2D

[MoO4F(SeO3)]5− anionic layers parallel with the ac plane have been constructed. Each of the 2D

layer is isolated with others by the Ba2+ counter cations and further interconnected via Ba−O/F

bonds to form a 3D framework.

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Although BMFS can be simply considered as aliovalent substitution of one oxygen sites with

two fluorine atoms from BMS, a distinct change in their geometrical structure occurs from 3D to

a 2D anionic framework owing to the fluorine substitution. Both BMFS and BMS have 6-MRs in

their structure that is composed of [MoO6]6−/[MoO5F]5− and [SeO3]2− BBUs, but their

configurations and connection modes are definitely different. The 6-MR in BMFS are composed

of three [MoO5F]5− octahedra and three [SeO3]2− pyramids linking alternately via bridged O

atoms, while four [MoO6]6− and two [SeO3]2− in BMS construct a non-planar 6-MR containing

Mo−O−Mo bonds (Figures 1e and 1f). The Mo−O−Mo bonds connecting adjacent pseudo-layers

in BMS are broken by the substitution of the bridged O atoms with F atoms, resulting in the

break of 3D anionic framework and the transformation into 2D anionic layer structure in the ac

plane (Figures 2a and 2b). After the structural modulation, the structure of BMFS shows a short

Figure 1. (a) Single layer structure of BMFS. (b, c) Basic building units of BMFS with atom

identity marked. The Se1 atom randomly distributes over the two sites. (d) The 3D structure of

BMFS with the Ba–O/F bonds omitted for clarity. The 6-MR in BMFS (e) and BMS (f).

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distances between layers of approximately 1.70 Å. Comparing the two cases of BMFS and BMS,

we find that the F atoms in BMFS tend to act as terminal groups by playing the role of “chemical

scissor” and help to reduce the connection between the building units. A combination of

asymmetrically coordinated [SeO3]2− pyramids and F− anions is supposed to be an efficient way

for constructing low-dimensional structures. Although [SeO3]2− groups all connect to three

[MoO6]6− or [MoO5F]5− octahedra via sharing three oxygen apexes, the distortion scale of

[SeO3]2− groups varies along with the structure modification by F− anion substitution from BMS

to BMFS. The [SeO3]2− groups in BMFS are restricted in a compact 2D layer structure with

layered thickness of 3.81 Å, while those in BMS are situated in a more flexible 3D pseudo-layers

framework (layered thickness 6.44 Å). Consequently, the [SeO3]2− groups in BMFS experience

greater bond strain than those in BMS in order to retain extending in a 2D direction, resulting in

longer Se−O bonds (ave. 1.7188 Å for BMFS and ave. 1.6991 Å for BMS) and thus larger

distortion scale of [SeO3]2− groups in BMFS that is favorable for a larger SHG response. The

structural evolution in this system has been further explored by the substitution of Se atoms in

BMFS with the heavier Te atoms, leading to isostructural compound BMFT with subtle

distinctions in layer thickness and interlayer distance (Figure 2c). More obviously, the [TeO3]2−

groups show smaller bond angles (88.5−105.1°) as well as longer Te−O bond distances

(1.897(8)−1.930(12) Å) than those of [SeO3] in BMFS (92.3−107.3°, 1.711(13)−1.730(8) Å),

which are agreed well with the reported geometries of [SeO3]2− and [TeO3]2−.42 This reduction in

the O−X−O angles and increase of X−O bond length from [SeO3]2− to [TeO3]2− groups indicate

an increase of repulsive force between the lone-pairs and oxygen atoms and the geometrical

structures of elongated [TeO3]2− pyramids are more favorable to obtain larger dipole moments.

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Thermal Stability Studies. Thermogravimetric analysis reveals that crystals BMFS and BMFT

exhibit similar thermal behavior. The TGA plots (Figure S3) shows that BMFS is thermally

stable up to around 360 °C under a flow of nitrogen gas while BMFT is stable up to 380 °C.

BMFS quickly loses weight in the temperature region 360−900 ℃, showing a sharp weight-loss

of 43.2% that corresponds to the removal of 1 F2, 2 SeO2 and 1 O2 per formula unit (calculated

value 42.6%). The TGA curve of BMFT also exhibits a one-step weight loss; the weight loss of

52.2% in the range 380−800 °C is attributed to the release of 1 F2, 2 TeO2 and 1.5 O2 per formula

unit (calculated value 51.8%).

IR Measurements. Attributed to the lack of large-sized crystals of compounds BMFS and

BMFT, their accurate IR transmission cutoff edges cannot be obtained. However, the IR

transmission spectra of small-sized single crystals can at least preliminarily provide the

transparent region of two compounds. As shown in Figure S4, no obvious optical absorption

peaks is observed in the range of 4000−400 cm−1 in their IR spectra of BMFS and BMFT. The

peaks around 956, 912 and 854 cm−1 in the IR spectrum of BMFS and those around 950 and 892

Figure 2. Structure comparison of BMS (a, d), BMSF (b, e) and BMST (c, f).

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cm−1 in the IR spectrum of BMFT can be ascribed to Mo−O/F stretching vibrations.43 Peaks

around 663 and 650 cm−1 in the IR spectra of BMFS and BMFT can be assigned to the

combination of the Mo−O/F, Se/Te−O, and Mo−O−Se/Te vibrations.44,45 Peaks around 555 and

454 cm−1 in spectrum of BMFS and 535 and 452 cm−1 in spectrum of BMFT are attributed to

Mo−O/F bending modes.44 The IR spectra confirmed the existence of Mo6+ and

[SeO3]2−/[TeO3]2− groups, albeit with slight position shift which may be associated with the

position disorders or attributed to chemical bonds in significantly different environments. Both

compounds have wide transparent regions, from 2.5 to 10.3 μm for BMFS and 2.5 to 10.5 μm for

BMFT, covering almost of the two important atmospheric transparent windows (3−5 and 8−12

μm).46 As a consequence, BMFS and BMFT may have important potential applications in the IR

range.

UV-Vis-NIR Diffuse Reflectance Spectra. The UV-Vis-NIR diffuse reflectance spectra

indicate strong reflectance in the region of λ = 400–2500 nm for two compounds. As UV-Vis-

NIR spectra shown in Figure S5, BMFS and BMFT reveal the UV cutoff edges of 376 and 379

nm, respectively. There is almost no optical absorption during 380−780 nm, which is consistent

Figure 3. Optical properties of BMFS (a) and BMFT (b). The insert figures show the

corresponding band gaps.

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with the colorlessness of these two compounds BMFS and BMFT. As a result, crystals BMFS

and BMFT have wide transparent regions from UV to IR in the range 0.38−10.3 and 0.38−10.5

μm (Figure 3), respectively. The optical band gaps of BMFS and BMFT can be deduced from the

plots of F(R) versus energy based on the Kubelka−Munk equation:47 F(R) = (1 − R)2/2R = K/S.

Accordingly, the band gap values of BMFS and BMFT are approximately 3.30 and 3.27 eV,

respectively. These relatively large band gap values indicate that both compounds are wide band

gap semiconductors and may possess higher LDTs than that of commercialized IR NLO material

AgGaS2 (2.75 eV).48

LDT Measurements. As BMFS and BMFT possess large optical band gaps, LDT measurements

were performed on polycrystalline samples of BMFS and BMFT using AgGaS2 as a reference.

The results reveal large LDT values of 126.3 and 113.1 MW/cm2 for BMFS and BMFT, which

are approximately 60 and 53 times higher than that of AgGaS2 (2.12 MW/cm2), respectively. The

BMFS and BMFT therefore exhibit both larger band gaps and higher LDTs than AgGaS2, further

confirming the potential in high-energy laser application.

SHG Measurements. Since both BMFS and BMFT crystallize into NCS space group Aba2,

powder SHG measurements were performed on the ground crystals BMFS, BMFT and

polycrystalline KDP (as a reference) using a modified Kurtz NLO system with a 1064 nm light

source, while the signals of green light of 532 nm were detected and displayed via an

oscilloscope. As showed in Figure 4, powder SHG measurements revealed that BMFS and

BMFT are both phase-matched compounds and exhibit large SHG signals, which are

approximately 3 and 4 times that of KDP sample in the particle size range of 105–150 μm,

respectively. It is noteworthy that, such a SHG efficiency of BMFS is obviously larger than that

of non-fluorinated parent compound BMS (80 × α-SiO2, approximately 2 × KDP),49 which

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confirms our hypothesis that the introduction of highly electronegative F− anions into metal

molybdenum-oxide selenite structure may lead to more distorted configuration and consequently

a stronger SHG effect.

Theoretical Studies. To clarify the origins of the optical properties of crystals BMFS and

BMFT, theoretical calculations have been performed employing DFT methods. The calculated

results on band structures (Figures S6) indicate that due to the structural similarity, the band

structures of BMFS and BMFT are very similar to each other: the bands in both valence band

(VB) and conduction band (CB) are very flat, but the bands in CB are looser than those in VB.

The results demonstrate that BMFS and BMFT are direct band gap compounds with the valence

band maximum (VBM) and the conduction band minimum (CBM) both located at G point. The

band structure calculations also give band gaps of 2.82 and 2.75 eV for BMFS and BMFT,

respectively, which are smaller than the experimental values (3.30 eV for BMFS and 3.27 eV for

BMFT). The band gap underestimation by the theoretical calculations should be attributed to the

limitation of the exchange and correlation function of GGA-PBE. Hence, to accurately describe

the optical properties, the scissors of 0.48, and 0.52 eV have been adopted to match the

Figure 4. (a) SHG intensities of BMFS and BMFT with KDP as the reference in the particle size

range of 105–150 μm. (b) Phase-matching curves of BMFS, BMFT and the reference KDP.

19

experimental gaps in the following analyses for BMFS and BMFT, respectively. The electronic

structure calculation has also been performed on parent compound BMS in the same condition

and a theoretical band gap of 2.45 eV is obtained, which is obviously smaller than that of its

fluorinated analogue BMFS. These results further confirm the assumption that F− anion

substitution may unambiguously change the electronic structures of materials and lead to an

increased optical band gap.

The partial density of states (PDOS) is a powerful technique for assigning the band structure

and understanding the bonding interactions in the crystal. The PDOS diagrams of the two

compounds behave similarly, so only BMFS is taken as an example to describe the PDOS

diagram in detail. As shown in Figures 5a and 5b, the electronic states of Se and Mo atoms are

fully overlapped with those of O atoms in the whole energy region, exhibiting the strong

interactions of Se−O and Mo−O bonds in the compound BMFS. The peaks below −8.0 eV in the

VB region mainly originate from the Ba 6p, O 2s, F 2s and Se 4s4p states; the VB region near

the Fermi level (−8.0 to 0 eV) is contributed by O 2p, F 2p, Mo 4d, Se 4p, mixed by a small

amount of Se 4s state. In CB, the electronic states are mainly comprised of the unoccupied Mo

4d, Se 4s4p, O 2p orbitals. For BMFS, the highest VB is dominated by the 2p nonbonding states

of O atoms, while the lowest CB consists of the empty Mo 4d with small amount of Se 4p and O

2p orbitals; therefore, the band gap of BMFS is mainly determined by Mo and O atoms. As

shown in the electronic density maps (Figure 6), VBM of two compounds are mainly contributed

by non-bonding O 2p and Se 4p/Te 5p stereochemically active lone pairs, while CBM are mainly

originated from the strong covalent bonding between Mo and ligand atoms. Therefore, the SHG

effects of two compounds are mainly attributed to the [SeO3]2− (BMFS)/[TeO3]2− (BMFT)

groups and [MoO5F]5− octahedra.

20

Owing to the Kleinman symmetry, the space group Aba2 of BMFS and BMFT have three

independent SHG components. The calculated SHG coefficients are showed in Table S3. The

largest calculated component for BMFS and BMFT is d13 with the value of 4.36 and 4.88 pm/V,

respectively, which are corresponding to 11 × and 12 × KDP. These calculated SHG coefficients

are somewhat larger than the corresponding experimental values, because the SHG calculations

are based on ideal periodic structural models which are deem to generate larger SHG effects. A

real-space atom-cutting technique has also been employed to identify the contribution of the

SHG response from the respective constituent groups quantitatively (Table S3). The largest SHG

coefficient d13 for BMFS is mostly attributed to [SeO3]2− (38%) and [MoO5F]5− (62%) and the

contribution from Ba2+ cations (0.7%) is negligible. However, in contrast to BMFS, the SHG

response of BMFT originates largely from [TeO3]2− groups (52%) and [MoO5F]5− octahedra

(47%). The increased proportion of the contribution from [SeO3]2− to [TeO3]2− implies that

Figure 5. The total density of states (DOS) and partial density of states (PDOS) of BMFS (a)

and BMFT (b).

21

polarization of the three-coordinated pyramids is greatly increased by the substitution of Se with

Te atoms that [TeO3]2− groups dominate the contribution of SHG properties in BMFT. The SHG-

weighted electronic density analysis was carried out on BMFS and BMFT to intuitively display

the electronic clouds of groups which dominate the SHG responses (Figures 6). It is clear that the

SHG-weighted electronic clouds in virtual-electron (VE) process are mainly located on

[MoO5F]5− octahedra and [SeO3]2−/[TeO3]2− groups. In BMFS, the SHG-related electronic clouds

are concentrated on both the [MoO5F]5− octahedra and [SeO3]2− groups (Figure 6a) for the

occupied states; but for the unoccupied states, the electronic clouds mainly arise from the

[MoO5F]5− octahedra (Figure 6b), which means that the SHG response is more likely attributed

to [MoO5F]5− octahedra rather than that of [SeO3]2− groups under the perturbation of

optoelectronic fields. For the occupied states in BMFT, the SHG-weighted electronic clouds are

almost concentrated on O 2p orbitals, and are predominately located on Mo 4d orbitals and lone-

pairs on Te4+ cations for the unoccupied states, which is consistent with the calculated SHG

coefficients contributing from both [MoO5F]5− and [TeO3]2− groups. The calculated refractive

index curves are shown in Figure S7 and the birefringence for BMFS and BMFT at 1064 nm

were calculated to be 0.18 and 0.27, respectively, which are large enough to achieve phase-

matchable conditions.

22

Unary Anion Substitution for Properties Modulation. BMFS is achieved by partial replacing

oxygen anions in BMS with fluorine anions, while BMFT is obtained by further substituting Se4+

with Te4+ from BMFS, but they exhibit different optical performance. The observed macroscopic

SHG responses are resulting from the local polarizations of the asymmetric polyhedra added

constructively. To better understand the origin and magnitude of SHG effect, direction and

magnitude of the polarizations of asymmetric polyhedra in the structures of BMFS and BMFT

must be determined and analyzed. The local dipole moments for the [SeO3]2−, [TeO3]2−, and

[MoO5F]5− groups and the net dipole moments within a unit cell for BMFS and BMFT were

therefore calculated by using the Debye equation.50,51 The calculated dipole moments for the

[MoO5F]5− and [SeO3]2− groups for BMFS are 5.17 and 9.06 D, respectively. These values are

larger than those of [MoO6]6− (4.56 D) and [SeO3]2− (8.73 D) in BMS,49 implying higher

Figure 6. SHG-weighted electronic clouds of the occupied (a, BMFS; c, BMFT) and unoccupied

(b, BMFS; d, BMFT) states in the VE process.

23

asymmetric coordination environments of Mo6+ and Se4+ after the introduction of F− anions into

BMS. The Mo6+ cation in [MoO5F]5− octahedron shows preference for distorting toward a

terminal oxide ligand but away from a fluorine ligand because of the structural compensation for

bond valence. In the structure of BMFS, substituting an O2− anion by a F− anion opposite to a

terminal oxide ligand may lead to a [MoO5F]5− octahedron with stronger out-of-center distortion.

As for parent compound BMS, the net dipole moments for [MoO6]6− octahedra and [SeO3]2−

groups point at the opposite directions and the net dipole moment for [SeO3]2− groups is larger

than that of [MoO6]6− octahedra, which means that the NLO property is mainly arising from

[SeO3]2− groups but cancel to a large extent by the components of [MoO6]6− octahedra.

Therefore, the superior SHG property of BMFS compared to BMS might result from the

favorable arrangement and the larger dipole moments of BBUs ([MoO5F]5− and [SeO3]2−). The

net dipole moments of BMFS in a unit cell are calculated to be 25.64 D along the c-axis.

Similarly, the calculated dipole moments for the [MoO5F] octahedra and [TeO3] groups are 5.32

and 9.42 D, respectively, and add to a net value of 45.27 D along the c-axis in a unit cell for

BMFT. These calculation results demonstrate that for both BMFS and BMFT, the calculated net

dipole moments are consistent with their moderate strong SHG signals and the trend of increase

SHG intensity from BMFS to BMFT.

In addition to its enhanced SHG response, BMFS also shows an enlarged band gap compared

to BMS owing to the high electronegative F− anions increasing the ionicity in Mo−F bonds,

which has been confirmed by the theoretical calculation results. It is noteworthy that BMFS

synchronously exhibits enlarged band gap and SHG response in comparison with the parent

compound BMS by unary anion substitution. This is in sharp contrast to all previously reported

examples (Table 2), where unary anion substitution can only improve one of two key optical

24

properties (band gap and SHG response) of NLO crystals but decrease the other one. For

example, partial fluorine substitution of RbIO3 leaded to the enhanced band gap but sharply

decreased SHG response of RbIO2F2,52 while replacing Cl− in Pb2BO3Cl with Br− obtained

Pb2BO3Br with increased SHG response but narrow band gap.56,57 As the theoretical calculation

results showed above, the band gap of BMFS is mainly determined by [MoO5F]5− groups;

therefore, the further substitution of Se4+ cations in BMFS with Te4+ has little impact on band

gap of BMFT. As a consequence, BMFT exhibits an enhanced SHG response with negligible

reduction in band gap, which means BMFT achieves an optimized balance between the

competing SHG and band gap performance through a two-step unary substitution from parent

compound BMS.

Table 2. Examples for Unary Anion Substitution

compound SHG (× KDP) Band Gap (eV) RbIO3

52 20 4b RbIO2F2

52 4 4.2b

CsIO325 15 4.2b

CsIO2F225 3 4.5b

RbCdI3·H2O53 3.6 4.1b Rb2CdBr2I2

54 4 3.35b

Cs2Hg3I855 10 2.56b

Cs2HgCl2I224 1 3.15b

Pb2BO3Cl56 9 3.99b Pb2BO3Br57 9.5 3.33b

BMS49 2 2.45b BMFSa 3 3.30b / 2.82c BMFTa 4 3.27b / 2.75c a This work; b experimental value; c Theoretical value.

25

CONCLUSION

In summary, we have synthesized the first NCS fluorinated molybdenum-oxide selenite/tellurite

(BMFS/BMFT) by a facile unary substitution strategy. The NCS crystalline compounds BMFS

and BMFT exhibit large SHG responses (3 × or 4 × KDP) and LDT values (60 × or 53 ×

AgGaS2), wide band gaps (3.30 or 3.27 eV) and optical transparency window (0.38−10.3 or

0.38−10.5 μm), and high thermal stabilities, which make them new promising NLO crystals in

visible and IR spectral regions. In particular, BMFS possesses synchronously enhanced SHG

efficiency and larger band gap in comparison with the parent compound BMS, which is achieved

for the first time by a unary anion substitution approach. The introduction of highly

electronegative F− anions into molybdenum-oxide octahedra to obtain a [MoO5F]5− octahedron

with larger out-of-distortion scale may lead to the further distortion of adjacent asymmetric

groups and, as a result an improving SHG response. A further substitution from BMFS to BMFT

achieve optimized balance between the competing properties of band gap and SHG effect, which

suggests that BMFT will be a potential candidate for second-order NLO material. The theoretical

calculations based on real-space atom-cutting technique and SHG-weighted electronic density

analysis further confirm that the enhanced SHG responses in BMFS and BMFT originate from

the synergistic effect of [MoO5F]5− and [SeO3]2−/[TeO3]2− anions, and particularly the fluorinated

[MoO5F]5− octahedra play a crucial role in the enlarged band gaps and enhanced SHG responses

in both NLO crystals. This work provides a facile strategy for the synthesis of NCS fluorinated

molybdenum-oxide selenite/tellurite crystalline materials with an optimized combination of

SHG, LDT and optical transparency performances.

ASSOCIATED CONTENT

26

Supporting Information

Atomic coordinates and anisotropic displacement parameters, selected bond lengths and angles,

list of the dipole moment calculations, PXRD patterns, figures of EDS, TGA, IR spectra, UV-

Vis-NIR spectra, and calculated birefringence.

AUTHOR INFORMATION

Corresponding Author

Chi Zhang

China-Australia Joint Research Center for Functional Molecular Materials, School of Chemical

Science and Engineering, Tongji University, Shanghai 200092, China

Phone: (+86)2165988860

Email: [email protected]

Author Contributions

§These authors contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

This research was financially supported by the National Natural Science Foundation of China

(no. 51432006), the Ministry of Education of China for the Changjiang Innovation Research

Team (no. IRT14R23), the Ministry of Education and the State Administration of Foreign

Experts Affairs for the 111 Project (no. B13025), and the Innovation Program of Shanghai

27

Municipal Education Commission. M. G. H. and C. Z. thank the Australian Research Council for

support (DP170100411).

REFERENCES

1. Ok, K. M.; Chi, E. O.; Halasyamani, P. S. Bulk Characterization Methods for Non-

centrosymmetric Materials: Second-harmonic Generation, Piezoelectricity, Pyroelectricity,

and Ferroelectricity. Chem. Soc. Rev. 2006, 35, 710-717.

2. Ok, K. M. Toward the Rational Design of Novel Noncentrosymmetric Materials: Factors

Influencing the Framework Structures. Acc. Chem. Res. 2016, 49, 2774-2785.

3. Lin, T. C.; Cole, J. M.; Higginbotham, A. P.; Edwards, A. J.; Piltz, R. O.; Perez-Moreno, J.;

Seo, J. Y.; Lee, S. C.; Clays, K.; Kwon, O. P. Molecular Origins of the High-performance

Nonlinear Optical Susceptibility in a Phenolic Polyene Chromophore: Electron Density

Distributions, Hydrogen Bonding, and ab Initio Calculations. J. Phys. Chem. C 2013, 117,

9416-9430.

4. Li, Z.; Yang, Y.; Guo, Y. W.; Xing, W. H.; Luo, X. Y.; Lin, Z. S.; Yao, J. Y.; Wu, Y. C.

Broadening Frontiers of Infrared Nonlinear Optical Materials with π‑conjugated Trigonal-

planar Groups. Chem. Mater. 2019, 31, 1110-1117.

5. Zhang, J. J.; Zhang, Z. H.; Zhang, W. G.; Zheng, Q. X.; Sun, Y. X.; Zhang, C. Q.; Tao, X. T.

Polymorphism of BaTeMo2O9: A New Polar Polymorph and the Phase Transformation.

Chem. Mater. 2011, 23, 3752-3761.

6. Hu, C.; Zhang, B. B.; Lei, B. H.; Pan, S. L.; Yang, Z. H. Advantageous Units in Antimony

Sulfides: Exploration and Design of Infrared Nonlinear Optical Materials. ACS Appl. Mater.

Interfaces 2018, 10, 26413-26421.

28

7. Lin, L.; Li, L. H.; Humphrey, M. G.; Wu, C.; Huang, Z. P.; Zhang, C. Incorporating Rare-

earth Cations with Moderate Electropositivity into Iodates for the Optimized Second-order

Nonlinear Optical Performance. Inorg. Chem. Front. 2020, 7, 2736-2746.

8. Halasyamani, P. S. Asymmetric Cation Coordination in Oxide Materials: Influence of Lone-

pair Cations on the Intra-octahedral Distortion in d0 Transition Metals. Chem. Mater. 2004,

16, 3586-3592.

9. Zhang, J. J.; Zhang, Z. H.; Sun, Y. X.; Zhang, C. Q.; Zhang, S. J.; Liu, Y.; Tao, X. T.

MgTeMoO6: A Neutral Layered Material Showing Strong Second-harmonic Generation. J.

Mater. Chem. 2012, 22, 9921-9927.

10. Wu, C.; Lin, L.; Jiang, X. X.; Lin, Z. S.; Huang, Z. P.; Humphrey, M. G.; Halasyamani, P.

S.; Zhang, C. K5(W3O9F4)(IO3): An Efficient Mid-Infrared Nonlinear Optical Compound

with High Laser Damage Threshold. Chem. Mater. 2019, 31, 10100-10108.

11. Ok, K. M.; Halasyamani, P. S.; Casanova, D.; Llunell, M.; Alemany, P.; Alvarez, S.

Distortions in Octahedrally Coordinated d0 Transition Metal Oxides: A Continuous

Symmetry Measures Approach. Chem. Mater. 2006, 18, 3176-3183.

12. Fan, H. X.; Lin, C. S.; Chen, K. C.; Peng, G.; Li, B. X.; Zhang, G.; Long, X. F.; Ye, N.

(NH4)Bi2(IO3)2F5: An Unusual Ammonium-containing Metal Iodate Fluoride Showing

Strong Second Harmonic Generation Response and Thermochromic Behavior. Angew.

Chem. Int. Ed. 2020, 59, 5268-5272.

13. Wu, C.; Jiang, X. X.; Lin, L.; Lin, Z. S.; Huang, Z. P.; Humphrey, M. G.; Zhang, C.

AGa3F6(SeO3)2 (A = Rb, Cs): A New Type of Phase-matchable Hexagonal Tungsten Oxide

Material with Strong Second-harmonic Generation Responses. Chem. Mater. 2020, 32,

6906-6915.

29

14. Wu, C.; Li, L. H.; Lin, L.; Huang, Z. P.; Humphrey, M. G.; Zhang, C. Enhancement of

Second-order Optical Nonlinearity in a Lutetium Selenite by Monodentate Anion Partial

Substitution. Chem. Mater. 2020, 32, 3043-3053.

15. Zhang, M. J.; Jiang, X. M.; Zhou, L. J. Guo, G. C. Two Phases of Ga2S3: Promising Infrared

Second-order Nonlinear Optical Materials with Very High Laser Induced Damage

Thresholds. J. Mater. Chem. C 2013, 1, 4754-4760.

16. Abudouwufu, T.; Zhang, M. Cheng, S. C.; Zeng, H.; Yang, Z. H.; Pan, S. L.

K2Na(IO3)2(I3O8) with Strong Second Harmonic Generation Response Activated by Two

Types of Isolated Iodate Anions. Chem. Mater. 2020, 32, 3608-3614.

17. Zhao, B. Q.; Yang, Y.; Zhao, S. G.; Shen, Y. G.; Li, X. F.; Li, L. N.; Ji, C. M.; Lin, Z. S.;

Luo, J. H. A New Phase-matchable Nonlinear Optical Silicate: Rb2ZnSi3O8. J. Mater. Chem.

C 2017, 5, 11025-11029.

18. Lu, J.; Yue, J. N.; Xiong, L.; Zhang, W. K.; Chen, L.; Wu, L. M. Uniform Alignment of

Non-π-conjugated Species Enhances Deep Ultraviolet Optical Nonlinearity. J. Am. Chem.

Soc. 2019, 141, 8093-8097.

19. Lu, W. Q.; Gao, Z. L.; Liu, X. T.; Tian, X. X.; Wu, Q.; Li, C. G.; Sun, Y. X.; Liu, Y.; Tao,

X. T. Rational Design of a LiNbO3‑like Nonlinear Optical Crystal, Li2ZrTeO6, with High

Laser-damage Threshold and Wide Mid-IR Transparency Window. J. Am. Chem. Soc. 2018,

140, 13089-13096.

20. Li, Y. Q.; Liang, F.; Zhao, S. G.; Li, L. N.; Wu, Z. Y.; Ding, Q. R.; Liu, S.; Lin, Z. S.; Hong,

M. C.; Luo, J. H. Two Non-π-conjugated Deep-UV Nonlinear Optical Sulfates. J. Am.

Chem. Soc. 2019, 141, 3833-3837.

30

21. Chen, J.; Hu, C. L.; Mao, F. F.; Feng, J. H.; Mao, J. G. A Facile Route to Nonlinear Optical

Materials: Three-site Aliovalent Substitution Involving One Cation and Two Anions.

Angew. Chem. Int. Ed. 2019, 58, 2098-2102.

22. Wang, Z. J.; He, J. G.; Hu, B.; Shan, P.; Xiong, Z. Y.; Su, R. B.; He, C.; Yang, X. M.; Long,

X. F. Ca2B5O9Cl and Sr2B5O9Cl: Nonlinear Optical Crystals with Deep-ultraviolet

Transparency Windows. ACS Appl. Mater. Interfaces 2020, 12, 4632-4637.

23. Huang, J. H.; Jin, C. C.; Xu, P. L.; Gong, P. F.; Lin, Z. S.; Cheng, J. W.; Yang, G. Y.

Li2CsB7O10(OH)4: A Deep-ultraviolet Nonlinear-optical Mixed Alkaline Borate Constructed

by Unusual Heptaborate Anions. Inorg. Chem. 2019, 58, 1755-1758.

24. Zhang, G.; Li, Y. J.; Jiang, K.; Zeng, H. Y.; Liu, T.; Chen, X. G.; Qin, J. G.; Lin, Z. S.; Fu,

P. Z.; Wu, Y. C.; Chen, C. T. A New Mixed Halide, Cs2HgI2Cl2: Molecular Engineering for

a New Nonlinear Optical Material in the Infrared Region. J. Am. Chem. Soc. 2012, 134,

14818-14822.

25. Zhang, M.; Hu, C.; Abudouwufu, T.; Yang, Z. H.; Pan, S. L. Functional Materials Design

via Structural Regulation Originated from Ions Introduction: A Study Case in Cesium Iodate

System. Chem. Mater. 2018, 30, 1136-1145.

26. Cao, X. L.; Hu, C. L.; Xu, X.; Kong, F.; Mao, J. G. Pb2TiOF(SeO3)2Cl and

Pb2NbO2(SeO3)2Cl: Small Changes in Structure Induced a Very Large SHG Enhancement.

Chem. Commun. 2013, 49, 9965-9967.

27. Cao, L. L.; Song, Y. X.; Peng, G.; Luo, M.; Yang, Y.; Lin, C. S.; Zhao, D.; Xu, F.; Lin, Z.

S.; Ye, N. Refractive Index Modulates Second-harmonic Responses in RE8O(CO3)3(OH)15X

(RE = Y, Lu; X = Cl, Br): Rare-earth Halide Carbonates as Ultraviolet Nonlinear Optical

Materials. Chem. Mater. 2019, 31, 2130-2137.

31

28. Harrison, W. T. A.; Dussack, L. L.; Jacobson, A. J. Hydrothermal Investigation of the

Barium/Molybdenum(VI)/Selenium(IV) Phase Space: Single-crystal Structures of

BaMoO3SeO3 and BaMo2O5(SeO3)2. J. Solid State Chem. 1996, 125, 234-242.

29. Kurtz, S. K.; Perry, T. T. A Powder Technique for the Evaluation of Nonlinear Optical

Materials. J. Appl. Phys. 1968, 39, 3798-3813.

30. G. M. Sheldrick, SHELXL-97: Program for the Refinement of Crystal Structures; University

of Göttingen: Göttingen, Germany, 1997.

31. G. M. Sheldrick, SHELXS-97: Program for the Solution of Crystal Structures; University of

Göttingen: Göttingen, Germany, 1997.

32. Spek, A. L. Single-crystal Structure Validation with the Program PLATON. J. Appl.

Crystallogr. 2003, 36, 7-13.

33. Brese, N. E.; Okeeffe, M. Bond-valence Parameters for Solids. Acta Crystallogr. B 1991, 47,

192-197.

34. Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M.

C. First Principles Methods Using CASTEP. Z. Kristallogr. 2005, 220, 567-570.

35. Kohn, W.; Sham, L. J. Self-consistent Equations Including Exchange and Correlation

Effects. Phys. Rev. 1965, 140, A1133-A1138.

36. Perdew, J. P.; Wang, Y. Pair-distribution Function and Its Coupling-constant Average for

the Spin-polarized Electron Gas. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46,

12947-12954.

37. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple.

Phys. Rev. Lett. 1996, 77, 3865-3868.

32

38. Monkhorst, H. J.; Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B:

Solid State 1976, 13, 5188-5192.

39. Godby, R. W.; Schlüter, M.; Sham, L. J. Self-energy Operators and Exchange-correlation

Potentials in Semiconductors. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 10159-

10175.

40. E. D. Palik, Handbook of Optical Constants of Solids; Academic: New York, 1985.

41. Lin, Z. S.; Jiang, X. X.; Kang, L.; Gong, P. F.; Luo, S. Y.; Lee, M. H. First-principles

Materials Applications and Design of Nonlinear Optical Crystals. J. Phys. D: Appl. Phys.

2014, 47, 253001.

42. Nguyen, S. D.; Kim, S. H.; Halasyamani, P. S. Synthesis, Characterization, and Structure

Property Relationships in Two New Polar Oxides: Zn2(MoO4)(SeO3) and Zn2(MoO4)(TeO3).

Inorg. Chem. 2011, 50, 5215-5222.

43. Balraj, V.; Vidyasagar, K. Syntheses and Characterization of Novel Three-dimensional

Tellurites, Na2MTe4O12 (M = W, Mo), with Intersecting Tunnels. Inorg. Chem. 1999, 38,

5809-5813.

44. Chi, E. O.; Ok, K. M.; Porter, Y.; Halasyamani, P. S. Na2Te3Mo3O16: A New Molybdenum

Tellurite with Second-harmonic Generating and Pyroelectric Properties. Chem. Mater. 2006,

18, 2070-2074.

45. Nguyen, S. D.; Halasyamani, P. S. Synthesis, Structure, and Characterization of Two New

Polar Sodium Tungsten Selenites: Na2(WO3)3(SeO3)·2H2O and Na6(W6O19)(SeO3)2. Inorg.

Chem. 2013, 52, 2637-2647.

33

46. Lin, H.; Chen, H.; Zheng, Y. J.; Yu, J. S.; Wu, X. T.; Wu, L. M. Coexistence of Strong

Second Harmonic Generation Response and Wide Band Gap in AZn4Ga5S12 (A = K, Rb, Cs)

with 3D Diamond-like Frameworks. Chem. Eur. J. 2017, 23, 10407-10412.

47. Kortüm, G. Reflectance Spectroscopy; New York, 1969.

48. Chemla, D. S.; Kupecek, P. J.; Robertson, D. S.; Smith, R. C. Silver Thiogallate, a New

Material with Potential for Infrared Devices. Opt. Commun. 1971, 3, 29-31.

49. Oh, S. J.; Lee, D. W.; Ok, K. M. Influence of the Cation Size on the Framework Structures

and Space Group Centricities in AMo2O5(SeO3)2 (A = Sr, Pb, and Ba). Inorg. Chem. 2012,

51, 5393-5399.

50. Sivakurnar, T.; Chang, H. Y.; Baek, J.; Halasyamani, P. S. Two New Noncentrosymmetric

Polar Oxides: Synthesis, Characterization, Second-harmonic Generating, and Pyroelectric

Measurements on TlSeVO5 and TlTeVO5. Chem. Mater. 2007, 19, 4710-4715.

51. Izumi, H. K.; Kirsch, J. E.; Stern, C. L.; Poeppelmeier, K. R. Examining the Out-of-center

Distortion in the [NbOF5]2− Anion. Inorg. Chem. 2005, 44, 884-895.

52. Wu, Q.; Liu, H. M.; Jiang, F. C.; Kang, L.; Yang, L.; Lin, Z. S.; Hu, Z. G.; Chen, X. G.;

Meng, X. G.; Qin, J. G. RbIO3 and RbIO2F2: Two Promising Nonlinear Optical Materials in

Mid-IR Region and Influence of Partially Replacing Oxygen with Fluorine for Improving

Laser Damage Threshold. Chem. Mater. 2016, 28, 1413-1418.

53. Ren, P.; Qin, J. G.; Liu, T.; Wu, Y. C.; Chen, C. T. Characterization and Properties of a

Nonlinear Optical Crystal in IR Region: RbCdI3·H2O. Opt. Mater. 2003, 23, 331-334.

54. Wu, Q.; Meng, X. G.; Zhong, C.; Chen, X. G.; Qin, J. G. Rb2CdBr2I2: A New IR Nonlinear

Optical Material with a Large Laser Damage Threshold. J. Am. Chem. Soc. 2014, 136, 5683-

5686.

34

55. Zhang, G.; Qin, J. G.; Liu, T.; Zhu, T. X.; Fu, P. Z.; Wu, Y. C.; Chen, C. T. Synthesis,

Characterization, and Crystal Growth of Cs2Hg3I8:A New Second-order Nonlinear Optical

Material. Cryst. Growth Des. 2008, 8, 2946-2949.

56. Zou, G. H.; Lin, C. S.; Jo, H.; Nam, G.; You, T. S.; Ok, K. M. Pb2BO3Cl: A Tailor-made

Polar Lead Borate Chloride with Very Strong Second Harmonic Generation. Angew. Chem.

Int. Ed. 2016, 55, 12078-12082.

57. Luo, M.; Song, Y. X.; Liang, F.; Ye, N.; Lin, Z. S. Pb2BO3Br: A Novel Nonlinear Optical

Lead Borate Bromine with a KBBF-type Structure Exhibiting Strong Nonlinear Optical

Response. Inorg. Chem. Front. 2018, 5, 916-921.

35

Synopsis

The two-step substitution in barium molybdenum-oxide selenite system is beneficial to the

comprehensive enhancement of both second harmonic generation efficiencies and optical band

gaps. Guided by this substitution design strategy, Ba(MoO2F)2(XO3)2 (X = Se, Te), the first

cases of noncentrosymmetric fluorinated molybdenum-oxide selenite and tellurite, are

synthesized by facile hydrothermal methods. This work may have significant implications for the

design of novel nonlinear optical materials with optimized performance for practical

applications.

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