First-principles study of electronic structure of Ce3 + centers in alkaline- earth fluorides including spin-orbit and scalar relativistic effects. N.V. Popov, A.S.Mysovsky, E.A.Radzhabov National Research Irkutsk State Technical University (NR ISTU), Irkutsk, 83 Lermontov street A.P. Vinogradov Institute of Geochemistry SB RAS, Irkutsk, 1a Favorsky St.
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First-principles study of electronic structure of Ce3+ centers in alkaline-
earth fluorides including spin-orbit and scalar relativistic effects.
N.V. Popov, A.S.Mysovsky, E.A.Radzhabov
National Research Irkutsk State Technical
University (NR ISTU), Irkutsk, 83 Lermontov street
A.P. Vinogradov Institute of Geochemistry SB RAS,
Irkutsk, 1a Favorsky St.
Introduction
Due to the fast 5d → 4f emission of Ce3+ in the blue and UV spectralregions cerium-doped compounds(Ce3+ in CaF2,SrF2) have considerableinterest in application of scintillators and solid state lasers.
Picture of 4f-5d transitions state depend strongly on the local crystallineenvironment of the dopant Ce3+, due to the large crystal-field interactionexperienced by the 5d electron.
ProblemOur purpose is to study electronic structure and optical transitions incrystal-defect systems (CaF2, SrF2) using embedding quantum clusterformalism, including scalar and spin-orbit effects.
Calculation details
● We developed a set of utilities for embedded quantum cluster calculation, including:
– embedding.exe — calculating classical gradient and energies– optimus_lbfgs.exe — “glue” for our classical code and third-
party quantum chemistry package, it performs geometry optimization in a combined QM/MM fashion
– Pre- and postrpocessing tools, scripts
● As a “calculator” we use Molcas 7.8
[1] http://www.chokkan.org/software/liblbfgs/
● We used 2-th order Douglas-Kroll-Hess method to includescalar relativistic effects
● Basis set - ANO Relativistic core correction full electron basis(B.O. Roos, V. Veryazov and P.-O. Widmark)
● Non-quantum cluster atoms were described by Ab initio modelpotential (AIMPs)
● For large-scale nonlinear optimization problem we use LimitedMemory Broyden-Flebsche-Gordano (aka L-BFGS)[1]
● Restricted Active Space State Interaction (RASSI) for accountingfor spin-orbit effects
Quantum cluster
Point charges, pair potentials & AIMPs; geometry opt. available
Fixed point charges
Ce3+ cubic centre
“Cubic” Ce3+ centre
Due to asymmetric charge localizationgeometry optimization brings structure tothe lower symmetry.Symmetry breaks from Oh to D4h!
F-
Ce3+
CeF8 clusterCaF2
X, Å Y, Å Z, Å
1.38 1.38 1.38
ΔX, Å ΔY, Å ΔZ, Å
-0.043 -0.043 0.024
SrF2
X, Å Y, Å Z, Å
1.47 1.47 1.47
ΔX, Å ΔY, Å ΔZ, Å
-0.095 -0.095 -0.026
Ce3+ 4f orbitals (“cubic” centre)
Eu
ε=-0.5351 a.u.Eu
ε=-0.5351 a.u.A2u
ε=-0.5351 a.u.Eu
ε=-0.5334 a.u
Eu
ε=-0.5334 a.uB1u
ε=-0.5334 a.uB1u
ε=-0.5259 a.u.
Eg
ε=-0.5064 a.u.Eg
ε=-0.5064 a.u.B2g
ε=-0.5064 a.u.
B1g
ε=-0.3901 a.uA1g
ε=-0.3901 a.u
Ce3+ 5d orbitals (“cubic” centre)
CAS states of CaF2:Ce3+
Without spin-orbit coupling With spin-orbit coupling
SF State
Symm. State energy(eV)
State energy (cm-1)
1 Eu 0.00 0.02 Eu 0.00 2.33 ? 0.02 147.54 ? 0.03 258.95 Eu 0.04 284.16 Eu 0.04 285.97 B1u 0.30 2411.38 ? 3.91 31526.39 ? 4.09 33027.9
[1] L. van Pieterson, FM Reid, RT Wegh, S Soverna, A Meijerink, PRB 65, 045113 (2002)[2]A. Myasnikova, A. Mysovsky, E. Radzhabov, Opt. i Specktr. 114, 445 (2013)
SrF2:CeF8 «Oh» absorption spectrum
Spectral lines
Transition energy, cm^-1[calculated]
Oscillator Strength
Line frequencycm^ -1[1]
34302.703 1,92E-02 33955.943
34302.703 3,54E-03
35647.767 3,36E-05
35647.767 4,31E-04
53603.017 1,67E-03 46538.192
53603.017 2,10E-02
55634.871 2,24E-03
55634.871 1,74E-04
56877.555 5,99E-04
56877.555 6,06E-04
References:
[1]First Principle Calculation of 4fn-4f(n-1) 5d Absorption Spectra of Ce3+ and Pr3+ Ions in Alkaline Earth FluoridesAlexandra Myasnikova, Andrey Mysovsky, and Evgeny Radzhabov
Insufficient correlation accounting come from small active space: only one 4f electron for Ce3+
Absorption spectrum
Ce3+ with interstitial fluorine ion
Atom X, Å Y, Å Z, Å ΔX, Å ΔY, Å ΔZ, ÅCE 1.38 0 0 -0.16 0 0
CA0-CA3 -1.38 0 2.76 -0.03 0 -0.15
CA4 -4.14 0 0 0.16 0 0FI -1.38 0 0 0.04 0 0
Ce
CA4
Interstitial Fluorine
CA0-CA3
Ce3+Fi- in CaF2
Atom X, Å Y, Å Z, Å ΔX, Å ΔY, Å ΔZ, ÅCE0 1.47 0 0 -0.09 0 0
[1] Calculations by Alexandra Myasnikova, Andrey Mysovsky, andEvgeny Radzhabov
Absorption spectrum
SrF2:Ce3+Fi- absorption spectrum
Spectral lines
Transition energy, cm^-1
[calculated]Oscillator strength
Line frequencycm^ -1[1]
35269,89 0,013039 34117
35269,89 0,009742
39762,28 0,000746 40247
39762,28 0,000695
51888 0,002082 46861
51888 0,002113
52522,33 0,005922 47829
52522,33 0,006764
54419,72 0,003892
54419,72 0,004181
References:
[1]First Principle Calculation of 4fn-4f(n-1) 5d Absorption Spectra of Ce3+ and Pr3+ Ions in Alkaline Earth FluoridesAlexandra Myasnikova, Andrey Mysovsky, and Evgeny Radzhabov
Absorption spectrum
SrF2:Ce3+ absorption spectrum
Experimental Reference:Cubic and tetragonal Ce3+ ions in strontium fluoride (E. Radzhabov, T. Kurobori)
Experimental: Calculated:
Conclusion
1. We have developed an approach for embedded cluster QM/MM calculations with MOLCAS quantum chemistry package used for electronic structure calculations. The approach itself is similar to GUESS method (AL Shluger, PV Sushko).
2. This allows to use the strong side of MOLCAS - sophisticated post-SCF and multiconfigurational techniques – for calculations of defects in solids.
3. 4f-5d Ce3+ electronic transitions in CaF2 and SrF2(Oh and C4v centers) were studied using CASPT2 and scalar-relativistic Douglas-Kroll-Hess approach. Spin-orbit coupling was treated with the restrictive active space state interaction (RASSI).
4. It is shown that cubic Ce3+ centers in CaF2 and SrF2 undergoes asymmetric relaxation due to Jahn-Teller effect. Optical absorption spectrum calculated with this asymmetric relaxation demonstrates good agreement with experiment, moreover, allows to explain and identify the absorption lines.
5. Calculated optical absorption for Ce3+ with interstitial fluorine ion shows good agreement with experiment as well.