Studies on Optical, Dielectric and Magnetic Properties of Mn2+, Fe3+ \u0026 Co2+ Ions Doped LFBCd Glasses

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Studies on Optical, Dielectric andMagnetic Properties of Mn2+, Fe3+ & Co2+

Ions Doped LFBCd GlassesV. Naresh a & S. Buddhudu aa Department of Physics, Sri Venkateswara University,Tirupati-517502, IndiaVersion of record first published: 18 Dec 2012.

To cite this article: V. Naresh & S. Buddhudu (2012): Studies on Optical, Dielectric and MagneticProperties of Mn2+, Fe3+ & Co2+ Ions Doped LFBCd Glasses, Ferroelectrics, 437:1, 110-125

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Ferroelectrics, 437:110–125, 2012Copyright © Taylor & Francis Group, LLCISSN: 0015-0193 print / 1563-5112 onlineDOI: 10.1080/00150193.2012.741987

Studies on Optical, Dielectric and MagneticProperties of Mn2+, Fe3+ & Co2+ Ions Doped

LFBCd Glasses

V. NARESH AND S. BUDDHUDU∗

Department of Physics, Sri Venkateswara University, Tirupati-517502, India

This paper discussed important results pertaining to thermal, optical, dielectric, ac-conductivity, and magnetic properties of certain transition metal (Mn2+, Fe3+ & Co2+)ions doped optical glasses in the chemical composition of Li2O-LiF-B2O3-CdO whichare labelled as LFBCd glasses. For these glasses, absorption spectra have been mea-sured in the (UV-Vis-NIR) region and are analysed. From XRD profiles, the amorphousnature of the doped glasses has been confirmed. Thermal analysis has been carried outfor the prepared glasses and their glass transition temperature (Tg) and crystallizationtemperature (Tc) are evaluated using their DSC profiles. The dielectric properties (di-electric constant (ε′) and tanδ) ac conductivity (σ ac) and electrical modulus (M′, M′ ′)have been investigated at the room temperature in the frequency range of 1 Hz to 1 MHz. It is found that the dielectric properties are decreasing as the ac conductivity in-creases with an increase in frequency variation because of hoping of electrons betweendifferent valence states. The high value of dielectric constant at low frequencies couldbe due to space charge polarization. Besides these, magnetic properties have also beenstudied for the transition metal oxides doped glasses. It is noticed that the transitionmetal (Mn2+, Fe3+ & Co2+) ions separately exhibited paramagnetic behaviours in theLFBCd glass matrix.

Keywords Glasses; characterization

1. Introduction

Glassy materials have become more promising materials as that are exhibiting quite en-couraging results in their optical, electrical and magnetic properties because of the presenceof various dopant ions in the glassy host matrices. Alkali fluoro-borate glasses have ex-tensively been studied over the years due to their unique properties like hardness, goodstrength, transparency, UV-transmission ability and corrosion resistance [1, 2]. Borateglasses are given importance due to the ability of boron atom to change its coordinationnumber between three and four with oxygen providing an anionic environment coordinatedwith modifying metal cations. The presence of divalent oxide like CdO in Lithium fluoro-borate glass exhibit dual nature as network former in four fold co-ordinate/ modifier withsix to eight- fold co-ordinations [3]. Adding of such divalent oxide with the host glassesimparts good chemical durability, low glass transition temperature, wide glass forming

Received July 25, 2012; in final form October 6, 2012.∗Corresponding author. E-mail: profsb [email protected]

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Properties of Mn2+, Fe3+ & Co2+ Ions 111

compositional range and high transmission in the visible region with interesting applica-tions have widely been reported [4, 5]. Glasses containing transition metal oxides are used toprobe the glass structure and other related properties [6–14]. Recently, we have reported [15]the ac conductivity of LFBCd glass which is found to be 1.24 × 10−6 S/cm. In the presentwork, we have undertaken Mn, Fe and Co ions each separately doped glasses, in order tostudy their optical absorption, thermal and magnetic properties alongside measurements ofionic conductivity for their use as glassy electrodes and battery related cathode materials.

2. Experimental Studies

Three transition metal ions such as Mn, Fe, & Co each of them doped separately into theLithium Fluoro-Boro Cadmium (LFBCd) glass matrix and found them as brightly colouredglasses as shown in Figure 1 were prepared in the following chemical compositions by amelt quenching method.

30Li2O-20LiF-(46-x)B2O3-4CdO-xMnO2

30Li2O-20LiF-(46-x)B2O3-4CdO-xFe2O3

30Li2O-20LiF-(46-x)B2O3-4CdO-xCoCl2 (where x = 0.5mol%).

Besides, a reference glass of the composition 30Li2O+20LiF+46 B2O3+4CdO (LFBCd)was also prepared. The starting chemicals used were in analytical grade such as H3BO3,Li2CO3, LiF, CdO, MnO2, Fe2O3 and CoCl2.6H2O. All the chemicals were weighed in10 g batch each separately, thoroughly mixed using an agate mortar and a pestle andthen each of those was collected into porcelain crucible and heated in an electric furnacefor melting them for an hour at 950◦C. Melt mixture was repeatedly stirred to ensure atotal homogenization. This melt was then quenched in between two smooth surfaced brassplates; bubble free glasses in circular designs having 2–3 cm in diameters and a thicknessof 0.3 cms were obtained. Due to the homogeneous distribution of transition metal ionsin the glass matrices, these bubble free transparent glasses have displayed brighter coloursas shown in Fig. 1. Mn2+: LFBCd glass has exhibited brown colour, Fe3+: LFBCd glasshas shown an yellow colour and Co2+: LFBCd glass has demonstrated blue colour and acolourless reference glass.

XRD profiles were recorded for the glass samples on a Seifert X-ray Diffractometer(model 3003TT) with Cu Kα radiation (λ = 1.5406 Å) at 40 KV and 20 mA with a Sidetector and 2θ = 10◦ and 60◦ at the rate of two degrees per minute. DSC measurementfor the four powdered samples of host glass and transition metal ions doped host glasseswere carried out on a NetZsch STA 409 in a dry nitrogen atmosphere. The four samplesof each composition weighing 10 mg were heated at a uniform rate of 20◦C/min usingplatinum crucibles in the temperature range of 30◦C–700◦C. The DSC Instrument UniversalAnalysis Program was used to determine the average value of glass transition temperature

Figure 1. Photographs of reference LFBCd, Mn2+: LFBCd, Fe3+: LFBCd, & Co2+: LFBCd glasses.

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112 V. Naresh and S. Buddhudu

(Tg) and crystallization temperature (Tc) were evaluated from the DSC profiles. The opticalabsorption spectra of LFBCd glasses doped with certain transition metal ions (Mn2+, Fe3+,& Co2+) were recorded at room temperature in the spectral range of 250 nm–2500 nm ona Varian-Cary-Win Spectrometer (JASCO V-570). Electrical conductivity measurementswere carried using a two electrode cell configuration by sandwiching the glass samplesbetween brass electrodes at room temperature over a frequency range of 1 Hz–1 MHz at anac voltage strength of 0.5 Vrms on a Phase Sensitive Multimeter (PSM 1700) in LCR modewhich is employed with the software for acquiring the data of real and imaginary partsof complex impedance. The calculation of conductivity (σ ac), real (ε′) and imaginary (ε′′)parts of dielectric constant (ε∗), dielectric loss (tan δ) as well as real (M’) and imaginary(M′′) parts of the complex electric modulus (M∗) were performed using raw impedance databased on the capacitance, sample dimensions and electrode area. The magnetic moment asa function of applied field was measured for the Mn2+, Fe3+ and Co2+ ions doped glasseson a vibrating Sample Magnetometer (VSM, Lakshore- 7410) within the range of ± 11 KOe at room temperature.

3. Theoretical Analysis

Manganese

Manganese ion exists in different valance states from +2 to +7 [Mn2+ (3d5), Mn3+ (3d4),Mn4+ (3d3) and Mn5+ (3d2) are prominent] with different co-ordinations in glass matrices.The free ion levels of Mn2+ are arranged as follows, based on their energy: 6S, 4G, 4P, 4Dand 4F etc., In a cubic crystalline field of low to moderate strength, the 5d electrons ofMn2+ ion are distributed in the t2g and eg orbitals, with three in the former and two in thelatter orbitals. Thus, its ground state configuration is (t2g)3 (eg)2, gives rise to energy levelssuch as 6A1g, 4A1g, 4Eg, 4T1g, 4T2g and to a number of doublet states of which 6A1g lieslowest according to Hund’s rule. The energy levels in octahedral environment (CN = 6)are 6A1g (6S), 4T1g (4G), 4T2g (4G), 4Eg-4A1g (4G), 4T2g (4D) and 4Eg (4D). The 4Eg-4A1g

(4G) and 4Eg (4D) levels have relatively less influence compared to the other levels by thecrystal field [16–19]. The ground state of Mn3+ is 5E state belonging to t23 e electronicconfiguration, where as luminescent states are 5T2 from t22e2 electronic configuration and1T2 from the t24 electronic configuration. In case of Mn4+ in octahedral coordination theground state is the 4A2 state of electronic configuration t23 Manganese ion shows Mn3+

in borate glasses with octahedral coordination having large magnetic anisotropy due to itsstrong spin-orbit interaction of the 3d orbital whereas in silicate and germinate glasses asMn2+ with both tetrahedral and octahedral environment possess small magnetic anisotropydue to zero angular momentum [20]. Mn2+ and Mn3+ are well known paramagnetic ionsand Mn2+ and Mn4+ are luminescent activators [21]. The ground state of Mn3+ is 5E statebelonging to t23 e electronic configuration, where as luminescent states are 5T2 from t22e2

electronic configuration and 1T2 from the t24 electronic configuration. In case of Mn4+ inoctahedral coordination the ground state is the 4A2 state of electronic configuration t23.

Iron

Iron mainly exists in Fe3+ state and occupies both octahedral and tetrahedral sites in theglass matrix [17,18]. The electronic configuration of Fe3+ is (Ar) 3d5. The Fe3+ free iongives rise to 2S, 2P, 2D, 2F, 2H, 4P, 4D, 4F, 4G and 6S terms with 6S as ground state [22]. Ina strong cubic crystal field d5 electrons are distributed into degenerate t2g and e2g orbitals

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Properties of Mn2+, Fe3+ & Co2+ Ions 113

with a ground state t32g e2g. The sextet and quartet terms transform as follows: 6S→6A1g,

4G→4T1g + 4T2g + 4Eg + 4A1g, 4D→4T2g + 4Eg, 4P→4T1g. Of the above terms, accordingto Hund’s rule 6A1g lies lowest and it is the ground state. For d5 configuration the transitionsare spin forbidden and appear with less intensity. The transitions 6A1g(S)→4T1g(G) and6A1g(S)→4T2g(G) depend on the crystal field strength Dq and give rise to broad bands. Fortransitions like 6A1g(S) →4Eg (G), which are independent of Dq, the bands would be lessbroadened [23, 24].

Cobalt

Cobalt (d7) is divalent ion having free ion states 4F, 4P, 2P, 2D, 2G, 2H, and 2F in octahedralor tetrahedral coordination. In octahedral co-ordination Co2+ free ion has 4F ground statewhich splits into two triplets 4T1g, 4T2g and a singlet 4A2g state while the next lowest freeion state 4P remains un-split with the 4T1g state as the lowest. In regards of Co2+ in thisco-ordination has three bands which correspond to the spin allowed transitions 4T1g (F) →4T2g (F), 4T1g (F) → 4A2g (F) and 4T1g (F) → 4T1g (P). The 4T1g (F) → 4A2g (F), transactioncould be seen in a low intensity due to a forbidden two-electron jump in the visible region[16–19]. In a tetrahedral symmetry, the energy levels of Co2+ ion are 4T2g (4F), 4T1g (4F),2E2g (2G) and 4T1g (4P) etc., with the ground state of 4A2g (4F). In a tetrahedral symmetry,Co2+ ion doped materials mainly show two-spin forbidden transitions 4T1g (F) →4A1g (4F)and 4T1g (4F) →4A2g (4F) respectively. The high intensity of the tetrahedrally co-ordinatedband could be attributed to the mixing of the 3d-orbitals with 4p-orbitals and ligand orbitals[25–27].

4. Results and Discussion

In Fig. 2, XRD profiles show broad hollow pattern (diffused peak) at 2θ (10◦–60◦) for allglass samples, which could be the characteristic of the short range order and this confirmsamorphous (vitreous state) nature.

DSC profiles for each composition in the temperature range 200◦C–700◦C are shownin the Fig. 3. In DSC profiles, for the doped and undoped glass samples two peaks werenoticed, caused by the exothermic events above the glass transition temperature. Thepresence of two peaks could be due to distinct phase transformations or due to differentcrystallization mechanisms. The first peak above Tg, is a sign of crystallization (Tc). It isobserved that there has been some shift in the Tg and Tc values for the transition metals(Mn2+, Fe3+ & Co2+) doped glasses when compared with undoped reference LFBCd glasstowards higher temperature suggesting that transition metal ions enters in to glass matrixas modifiers by modifying the glass structure. The glass transition temperatures (Tg),exotherms corresponding to crystallization temperature (Tc) of all compositions are shownin the figure 3, and their values are presented in the Table 1.

The absorption spectra of transition metal ions are influenced by the nature of the hostmatrices into which those ions are accommodated owing to the excitation spectra of 3delectrons. The absorption spectra of transition metals are fairly broader and are sensitiveto the changes in coordination and symmetry. Due to the presence of various oxidationstates, each of the states can give rise to different absorption spectra which can be explainedby the application of ligand field theory. Figure 4 (a) represents the absorption spectrumfor reference LFBCd glass. The absorption edge for this pure glass is observed at 350 nmwhich is found to shift towards higher wavelengths because of the adding of transitionmetal ions. In Figure 4(b), the absorption spectrum of the Mn2+: LFBCd glass is shown.

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114 V. Naresh and S. Buddhudu

Table 1Absorption bands with assignments, CF parameters, Tg and Tc values of (0.5 mol%) Mn2+,

Fe3+ & Co2+: LFBCd glasses.

CrystalAbsorption field

Glass Bands Electronic parametersamples (nm) Assignment (Dq) (cm−1) B (cm−1) Tg TC

LFBCd 467◦C 568◦CMn2+: LFBCd 460 5Eg (D) →5T2g (D) 2173 488◦C 581◦CFe3+: LFBCd 456 6A1g (S) →4T2g(G) 2192 490◦C 587◦CCo2+: LFBCd 520 4A2g (F) →2T2g (G) 865 759 467◦C 579◦C

590 4T1g (F) →4T1g (P)1416 4A2g (F) →4T1g (F)1207 4A2g (F) →4T1g (F)1671 4A2g (F) →4T1g (F)

∗Calculations for Dq and B values are adapted from references [31 and 33].

Figure 2. XRD profiles of reference LFBCd, Mn2+: LFBCd, Fe3+: LFBCd, & Co2+: LFBCd glasses(Figure available in color online).

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Properties of Mn2+, Fe3+ & Co2+ Ions 115

Figure 3. DSC profiles of reference LFBCd, Mn2+: LFBCd, Fe3+: LFBCd, & Co2+: LFBCd glasses(Figure available in color online).

The manganese ion is incorporated into octahedral sites of the LFBCd glass network asMn2+ and Mn3+ ions. The spectrum clearly reveals an intense broad absorption band withthe maximum centred at 460 nm assigned to the 6A1g(S)→ 4T1g(G) transition of Mn2+, witha single spin allowed transition having 3d5-elelctronic configuration in octahedral sites ofthe LFBCd glass network [28–30]. In the octahedral symmetry the ground state of Mn2+ ionsplits into lower 6A1g(S) and excited 4T1g(G) states positioned by higher 10Dq. Therefore,the crystal field parameter (Dq = ν/10) is evaluated to be 2173 cm−1 [31].

Fig. 4(c) represents the optical absorption spectrum of Fe3+: LFBCd glass. A broadband is observed due to the change of configuration from (t2g)3 (eg)2 to (t2g)4 (eg)1 inthe visible region at 456 nm and it is ascribed to the d-d transition of 6A1g(S) →4T2g(G)[32, 33]. The crystal field (Dq) parameter is calculated to be 2192 cm−1 [31]. The bandposition observed in the present work indicates the iron ions present in trivalent state (Fe3+)with distorted octahedral symmetry. It is known that Fe3+ have no spin –allowed transitionand the present band is due to spin-forbidden transition only. The observed band positionis compared with the reported positions for many glasses containing iron ion [34, 35].

Figure 4 (d)shows the absorption spectrum of Co2+: LFBCd glass with five absorptionbands, two in the visible region (520 nm, 595 nm) and three weak bands in the infrared

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116 V. Naresh and S. Buddhudu

Figure 4. Optical absorption spectra of (a) Reference LFBCd, (b) Mn2+: LFBCd, (c) Fe3+: LFBCd& (d) Co2+: LFBCd glasses (Figure available in color online).

region (1207 nm, 1416 nm, and 1671 nm). The intense band in the visible region at 595 nmis assigned to the spin allowed transition of 4T1g (F) →4T1g (P) having octahedral symmetry,a small shoulder band at 520 nm is due to spin forbidden transition tetrahedrally assigned to4A2g (F) → 2T2g (G). A broad band in the infrared region (1000 nm–2000 nm) is assignedto the 4A2g (F) → 4T1g (F), splits into three sub maxima (1207 nm, 1416 nm, and 1671 nm),which could be caused by low symmetry distortion of the tetrahedrally coordinated Co2+

ion [29, 36]. The high intensity of the tetrahedrally co-ordinated band could be attributedto the mixing of the 3d-orbitals with 4p-orbitals and ligand orbitals [37, 38]. The relativeintensities of the bands in the visible and in IR region depends on the energy separation of thedoublet states from the 4T1g (P) and 4T1g (F) states. The values of Dq = (ν2-ν1)/10 (crystalfield parameter) and Racah coefficient B (electrostatic parameter which is a measure ofthe interelectronic repulsion) are calculated from the energies corresponding to the spectraltransition using Tanabe – Sugano equation [32, 33].

4T1g(4F) → 4T1g(4P) = 15B4A2g(F) → 2T2g(G) = 4B + 3C

The Dq value is evaluated to be 865 cm−1 and B is found to be 759 cm−1 where as C valueis estimated to be 3893 cm−1 for C/B = 4.5 [33].The absorbance, wavelengths and theircorresponding transitions are presented in Table 1.

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Properties of Mn2+, Fe3+ & Co2+ Ions 117

In ionic conducting glasses, the dielectric properties arise due to ionic motions. Itis well known that charge carriers cannot move freely in the glass matrix but they canbe displaced and gets polarized as a response to applied electric field. Two mechanismscontribute to the dielectric properties (i) rotation of the ions around their negative sites and(ii) short distance transport (hoping). Glasses containing larger alkali content exhibits lowerdielectric constant [39, 40]. The dielectric response is explained by complex permittivity,ε∗ = ε′ – jε′′ where real ε′′ and imaginary ε′′ are the dielectric components for energystorage and energy loss of applied electric field. Dielectric properties of ionic conductingglasses are due to the contribution of electronic, ionic, dipole orientations and space chargepolarizations. The complex permittivity of the glass is obtained from the impedance data:

ε∗ = 1

(jωC◦Z∗)= ε′ − jε′′ (1)

where Z∗ is the complex impedance, Co is the capacitance of free medium. The real part ofpermittivity (dielectric constant) ε′ represents the polarizability, while the imaginary part(dielectric loss) ε′′ represents the energy loss due to polarization and ionic conduction. Thedielectric constant (ε′) is calculated from:

ε′ = Cd

ε◦A(2)

where C is the capacitance of the sample, εo is the permittivity of the free space (8.85× 10−12 F/m) and A is the cross-sectional area of electrode. Dielectric loss tangent isdetermined from:

tan δ = ε′′

ε′ (3)

where tanδ is the loss tangent.The frequency dependence of ε‘ for the Mn2+, Fe3+, & Co2+: LFBCd glasses are

shown in the Figs. 5 (a) & (b)In Fig. 5 (a),at lower frequencies (ε′ α ωs-1), the values of ε′ is found to be high

due to the space charge accumulation at the electrode-electrolyte interface and due to thepolarization effect, this overall effect causes immobilization of charge carriers within thesample, whereas at higher frequencies it comes to a saturation because of the periodicreversal of electric field occurs so fast that no excess ion diffusion in the direction of thefield. The polarization due to the charge accumulation decreases leading to the decrease inthe values of ε′ is observed [41].

Loss tangent or loss factor (tanδ) represents the energy dissipation in the dielectricsystem. Figure 5 (b)shows the variation of tan δ with frequency at room temperature for thetransition metal ion (Mn2+, Fe3+ & Co2+) doped LFBCd glasses. Dielectric loss tangenthas high dispersion at low frequency is attributed to interfacial polarization mechanism,which decrease gradually with a tailing edge as frequency increases due to low reactanceoffered by the glasses causing less contribution of ions in the direction of applied field,could also be due to domination of the polarization by migrating charges in the lowerfrequency and low dipolar relaxations in the higher frequency. The absence of relaxationpeaks in the dielectric loss spectra also indicates the short range ionic conduction due tohoping of charges in dielectric response at lower frequency [42–44].

The ionic conduction in glasses is believed to takes place in a random potential, whosefluctuations are partly determined from the structure of amorphous matrix and partly from

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118 V. Naresh and S. Buddhudu

Figure 5. (a) Dielectric constant (ε′) profiles of Mn2+: LFBCd, Fe3+: LFBCd, & Co2+: LFBCdglasses. (b) Variation of dielectric loss factor (tan δ) of Mn2+: LFBCd, Fe3+: LFBCd, & Co2+: LFBCdglasses.

the interactions between ions. The ac conductivity of the sample (σ ac) is determined fromthe dielectric data using the formula:

σac = ωε◦ε′′ (4)

where ω ( = 2π f) is the angular frequency, σ ac is the ac conductivity. The profiles of the acconductivity (log σ ac) as a function of frequency (log ω) at room temperature for transitionmetal oxide (Mn2+, Fe3+, & Co2+) doped LFBCd glasses is shown in Fig. 6. The glasses

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Properties of Mn2+, Fe3+ & Co2+ Ions 119

Figure 6. Conductivities (log σ ac) of Mn2+: LFBCd, Fe3+: LFBCd, and Co2+: LFBCd glasses.

under study show mixed conductivity, electronic conductivity due to polaron hoping andionic conductivity due to migration of cations (Li+) between two energetically favourablesites over a potential barrier. At room temperature with increasing frequency, conductiontakes place via charge transfer over a potential barrier between lower and higher valancestates.

The ac conductivity is analysed on the basis of Jonscher universal power law [45]:

σ (ω) = σdc + AωS 0 < s < 1 (5)

Where σ dc is the dc conductivity of the samples, A(= σdc/ωSP) is temperature dependent

constant, ω = 2πf is the angular frequency of the applied field and s is the power lawexponent in the range 0 < s < 1, represents the degree of interaction between the mobileions. From the Fig. 6, it is observed that all the three plots have shown same trend withnearly flat portion at lower frequencies and tends to merge at higher frequencies, suggestingthat at lower frequencies, conductivity is independent of the frequency and increase linearlyas a function of higher frequencies. This attributes to the number of charge carriers havinghigh relaxation time responds less due to high energy barrier in the low frequency region, soconductivity is small at lower frequencies [39]. However, due to high frequency the energybarrier height decreases and more number of charge carriers respond easily and possesshort-range Li+ ion migration due to short time periods available, so higher conductivityis dominant at higher frequencies. The other possible way of explanation attributed toenhancement in ac conductivity: at lower frequencies, random distribution of ionic chargesvia activated hopping gives rise to a frequency independent conductivity while at higherfrequencies, conductivity exhibits dispersion which increases linearly following power lawrelation in the higher frequency region (σ dc = 0):

σ (ω) = AωS 0 < s < 1 (6)

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120 V. Naresh and S. Buddhudu

Table 2The ac-conductivities of Mn2+, Fe3+ & Co2+: LFBCd glasses.

Glass samples σ ac (S cm−1) Exponent (s)

Mn2+: LFBCd 2.69 × 10−5 0.86Fe3+: LFBCd 2.58 × 10−5 0.86Co2+: LFBCd 2.62 × 10−5 0.85

The values of the exponent s is evaluated from the slopes of log σac (ω) versus log ω by usingthe equation S = d(Lnσac(ω))

d(Ln(ω)) at room temperature for all compositions [46]. The parameter sis associated with the modification of network structure and interaction with the ions. Thevalue of the exponent s is found to be in the range of 0.8 to 0.9 as shown in the Table 2.

The analysis of frequency dependent electrical properties show that the value of dielec-tric constant (ε′) decreases with increasing frequency and accordingly conductivity (σ ac)increases due to small polaron hoping. Hence, the above equation (Eq (6)) corresponds tothe short range hopping conduction of Li+ charge carriers. The ac conductivity values ofthe Transition metal ions doped glasses have been represented in the Table 2.

In ionically conducting materials, dielectric relaxation behaviour is more successfullyanalysed in terms of electric moduli, which suppress the effect of electrode polarization.The use of electric modulus approach helps us in understanding the bulk response ofmoderately conducting samples. This would help to circumvent the problems caused byelectrical conduction which might mask the dielectric relaxation process. Figures 7 (a) &(b) depicts the log frequency dependence of M′ and M′′ respectively at room temperaturefor the glass sample, which can be evaluated from the following relations using dielectricdata [47]:

M∗ = 1

ε∗ (7)

M∗ = M ′ + jM ′′ = ε′((ε′)2 + (ε′′)2

) + jε′′

((ε′)2 + (ε′′)2

) (8)

M ′ = ε′((ε′)2 + (ε′′)2

) and M ′′ = ε′′((ε′)2 + (ε′′)2

)

It is observed that the real (M′) and imaginary (M′′) parts of the modulus increases with anincrease in frequency. At lower frequencies, the value of M′ and M′′ approaches to zero,indicating the contribution of electrode polarization is negligible (suppression of electrodepolarization) and long tail of the M′′ peak represents the long range diffusion of the ions.At higher frequencies, M′ reaches to M’∞ (= 1/ε∞) due to relaxation process where as M′′

spectra is border than ideal Debye curve and asymmetric in nature with a relaxation peak.The frequency region below the M′′ peak indicates the range in which Li+ ions drift to longdistances. In the frequency region above M′′ which represents that the Li+ ions motionis spatially confined and localized within the potential wells, i.e., ions diffuses to shorterdistances. The frequency range where the maximum peak occurs suggests the transitionfrom long-range to short-range mobility of ions. The M′′ asymmetry in glasses is usuallyrepresents the distribution of relaxation times in the conduction process. The relaxationbehaviour occurring in the present conducting glasses is observed to be non–Debye type.The profiles of electric modulus formalism reveals that the relaxation peaks which are

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Properties of Mn2+, Fe3+ & Co2+ Ions 121

Figure 7. (a). Dependence of real electrical modulus (M’) on log (ω) for Mn2+: LFBCd, Fe3+:LFBCd, and Co2+: LFBCd glasses (b) Dependence of imaginary electrical modulus (M”) on log (ω)for Mn2+: LFBCd, Fe3+: LFBCd, and Co2+: LFBCd glasses (Figure available in color online).

suppressed in the case of dielectric loss tangent due to accumulation of charges is prominentin the plots and also the ions diffuse through longer distance in the M′′ spectra.

According to RKKY (Ruderman-Kittel-Kasuya-Yosida) theory, magnetism is due tothe exchange interaction between local spin-polarized electrons and conduction electrons.This interaction leads to the spin polarization of conductive electrons. Therefore the long-range exchange interaction makes all the ions to have the same spin directions [48–50].

The M-H hysteresis loops (magnetization curves) are measured on VSM in the range+11K Oe to -11K Oe at room temperature for doped (Mn2+, Fe3+, & Co2+) LFBCd glassesas shown in the Figs. 8 (a), (b) & (c). The curves are symmetric and are supposed to be

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122 V. Naresh and S. Buddhudu

Figure 8. M vs H curves of (a) Mn2+: LFBCd (b) Fe3+: LFBCd & (c) Co2+: LFBCd glasses (Figureavailable in color online).

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Properties of Mn2+, Fe3+ & Co2+ Ions 123

composed of reversible and irreversible contribution displaying no coercivity, remanenceand small magnetic moments.

M = Mrev + Mirr

Only the irreversible term contributes to the hysteresis and the reversible term is assumedto be linear with the magnetic field, i.e.

Mrev = χH

where χ is magnetic susceptibility.In Fig. 8 (a), the magnetization curve for Mn2+: LFBCD glass is almost linear with

a zero remnant magnetization (Mr), magnetic saturation (Ms), coercive field (Hc) and nomagnetic moment remains even on reversing the field direction. The curve in the profilereveals that the sample exhibits paramagnetic behaviour at room temperature which couldbe attributed to the presence of Mn ions. The species (molecules or ions) having weakmagnetic interaction yield a straight line exhibiting paramagnetic behaviour [51]. Figure 8(b) represents the magnetization curve of Fe3+: LFBCd glass. The paramagnetic behaviourof the M-H curve suggests the presence for the iron in the glass matrix in trivalent stateas Fe3+, having zero remnant magnetization (Mr), magnetic saturation (Ms), coercive field(Hc). The paramagnetic behaviour could be due to the presence of permanent magneticdipole moments in the iron doped in the glass matrix. The paramagnetic trend shown by irondoped glass sample is in coincidence with the earlier reported literature [52]. Figure 8(c)shows the M vs H curve for Co2+: LFBCd glass. The curve is thin exhibiting paramagneticnature and no saturation magnetization, loop can be found around the origin can be observedin the measured region at room temperature.

Conclusion

It is concluded that, we have successfully developed stable and UV transmitting certaintransition metal ions doped LFBCd glasses using melt quenching method. Manganesedoped LFBCd glass has exhibited a brown colour in the visible region with a broad peakat 460 nm attributed to 6A1g (S)→ 4T2g (G) suggesting manganese exists in Mn2+ state.Iron doped LFBCd glass has displayed an yellow colour with a broad peak at 456 nmassigned to 6A1g(S) →4T2g (G) transition concluding iron is present in trivalent state (Fe3+)having octahedral symmetry, where as Cobalt doped LFBCd glass has shown a blue colourwith five absorption bands, two in the visible region (520 nm, 595 nm) attributed to 4A2g

(4F) →4T1g (G) and to 4T1g (F) → 2T1g (P) and three weak bands in the infrared region(1207 nm, 1416 nm, and 1671 nm) have been assigned to 4A2g (F) → 4T1g (F). Based onthe intense peak in the visible absorption region it has been concluded that cobalt in LFBCdglass mainly exists as Co2+ in octahedral symmetry. Their crystal field splitting factors(Dq) are calculated to be 2173 cm−1(Mn2+), 2192 cm−1(Fe3+), and 865 cm−1(Co2+). Theanalysis of frequency dependent electrical properties shows that the value of dielectricconstant (ε′) and loss (tan δ) are found to be decreasing with an increase in frequency andaccordingly conductivity (log σ ac) increases due to small polaron hoping of mobile Li+

ions. It is also observed that there has been enhancement in the ac conductivity by oneorder with the incorporation of transition metal ions (Mn2+, Fe3+, & Co2+) when comparedwith the ac conductivity value of reference glass reported earlier in the literature. TheVSM results show that, Mn2+: LFBCd, Fe3+: LFBCd & Co2+: LFBCd glasses possess

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124 V. Naresh and S. Buddhudu

paramagnetic behaviours. Thus, based on the results we could conclude their suitability asglassy electrodes and cathode materials in battery applications.

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