1. The refinement of self-trapped excitons
structure in CaF2 and SrF2 crystals: an
ab initio study
Andrey S. Mysovsky, Evgeny A. Radzhabov
Vinogradov Institute of
Geochemistry SB RAS
1a Favorsky Street, 664033
Irkutsk, Russia
5. Short-wave excitonic luminescence bands
Crystal Main bands New bands
BaF2 4.25 eV 5.05 eV
SrF2 4.4 eV 5.33 eV
CaF2 4.5 eV 5.66 eV
•New bands are not influenced by impurities in wide concentration
range (in contrast to the quenching of main bands)
•New bands disappear in temperature region 90-150K
•The exciton configuration responsible for these bands must have
the formation mechanism different from that of normal STE
7. Calculation method
Quantum cluster:
•BH&HLYP density functional
•6-31G basis set, +2d functions
on cations
Classical region:
•Buckingham pair potentials
Interface
•LANL1 ECPs on cations to avoid the distortions of electronic
density by positive point charges
Code
•Modified PC GAMESS 7.12
11. Off-centre STE
M Adair, C H Leung, K S Song,
J. Phys. C: SolidStatePhys. 18
(1985)L909-L913
I
III
II
IV
12. Off-centre STE
Conf. I II IV
Electron
MO
Hole
MO
E (eV)
CaF2
SrF2
0.00
0.00
0.23
0.28
0.40
0.36
13. Off-centre STE
Calculated Expt.
R(F2
¯), Å Aiso, G Biso , G Aiso, G Biso , G
I 1.96 319 272
CaF2 II 1.95 404 291 386.7 314.3
IV 1.95 376 282
I 2.02 330 271
SrF2 II 1.96 385 298 409.7 318.3
IV 1.96 347 261
Aiso – hyperfine coupling for interstitial fluorine
Biso – for lattice fluorine bonded to interstitial
Ground state parameters
14. Off-centre STE
Luminescence energies
Triplet
lumin. (eV)
TDDFT
energy (eV)
fosc Expt.
lumin. (eV)
I 2.34 2.14 0.028
CaF2 II 3.62 3.73 0.004 4.5
IV 2.88 3.20 0.057
I 1.88 1.75 0.029
SrF2 II 3.24 3.44 0.002 4.4
IV 2.42 2.72 0.041
20. Electron self-trapping
•First discussed in: C.R.A. Catlow, J. Phys. C: Solid State Phys.,
12, 969 (1979)
•Stable according to our calculations
Trigonal symmetry (C3V)
FI – vacancy distance = 2.32 Å
FI – central interstitial
position distance = 0.06 Å
9.1%
6.6%
5.1%
5.3%
6.6%
5.3%
9.1%
21. Electron self-trapping
•Self-trapping of electron is (according to calculations)
energetically favorable. The energy gain ≈ 0.3 eV
•This value may be within the calculation error and is not to be
taken as absolute truth
•The barrier for self-trapping EB ≈ 0.3 eV
•The existence of barrier and two minima (corresp. to conductive
band electron and self-trapped electron) has been checked in
several clusters.
24. Conclusion
There is a new excitonic band in CaF2, SrF2 and BaF2 crystals about 1
eV higher the main band of STE luminescence.
Its intensity does not depend on impurities in wide range of impurity
concentration, in contrast to the quenching of main bands.
The defect specie responsible for this band must differ from usual off-
centre STE by the formation mechanism.
On-centre STE (VK+e) should have the luminescence sufficiently higher
then the energy of new band. Besides, on-centre STE is unstable and relaxes
into off-centre configuration after overcoming very small energy barrier.
It follows from calculation results that self-trapping of electron is possible
in CaF2 and SrF2 crystals. Self-trapped electron is a centre with trigonal
symmetry and can be considered as perturbed F-centre. There is a barrier
for self-trapping about 0.3 eV.
It can be tentatively suggested that self-trapped electrons are the
precursors of yet unknown excitonic configuration responsible for the new
luminescence bands.
25. Thank you!
Acknowledgements:
• Alex A. Granovsky
(http://classic.chem.msu.su/gran/gamess/index.html)
for the source code of PC GAMESS 7.12
• Institute of System Dynamics SB RAS for granting
access to BLACKFORD supercomputer facility
• Alexandra Myasnikova for preparing nice schematic
picture
• RBFR for the possibility to be here now (travel grant
09-02-09332-моб_з)
