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IEEE TRANSACTIONS ON NUCLEAR SCIENCE 1
First Principle Calculation of
Absorption Spectra of and Ions in
Alkaline Earth Fluorides
Alexandra Myasnikova, Andrey Mysovsky, and Evgeny Radzhabov
Abstract—In this paper we present the results of an ab initio
calculations of df-absorption spectra for alkaline earth fluoride
matrix doped with and ions. The density-func-
tional theory (DFT) calculations have been performed in em-
bedded-cluster approach. All our results were obtained with
taking into account the lattice relaxation in the presence of triva-
lent impurity ion. The calculation have been made both for cubic
and tetragonal centers. For the tetragonal centers we estimate the
crystal field splitting of level.
Index Terms—Absorption, alkali-earth fluoride, -doping,
embedded cluster approach, -doping.
I. INTRODUCTION
TODAY many excellent scintillators doped with rare earth
ions such as or have been developed [1], [2].
The fast luminescence of and activators is due to
dipole allowed transition from configuration to
ground state (df-emission). ion still remains the most pop-
ular activator for halide and oxide scintillators introducing both
high efficiency of registration and fast decay time [3]. For alka-
line earth fluorides ( , and ) doped with
or ions df-emission in the region of 4–6 eV and 4 eV are
observed respectively [4].
The absorption spectra of lanthanides have
been studied for a long time in different lattice environment
[5]–[7]. It was shown that the 5d level of rare earth ions is very
sensitive to local surrounding, and so the crystal field splitting of
5d level in accordance with the defect symmetry was observed.
The experimental data are summarized in Table I. In the
or doped alkali earth fluorides the impurity ions replaced
the cation lattice positions and therefore the interstitial fluorine
ions are formed for compensating the excess charge. According
to theoretical results [8] the rare-earth sites compensated by in-
terstitial fluorine in nearest-neighbor (NN) positions are
favored by small lattice constants and sites compensated by a
in the next-nearest-neighbor
Manuscript received November 15, 2011; revised January 19, 2012; accepted
February 06, 2012. This work was supported in part by the Russian Foundation
for Basic Research (grant 11-02-00717-a).
The authors are with the A. P. Vinogradov Institute of Geochemistry,
Siberian Branch, Russian Academy of Science, 664033 Irkutsk, Russia (e-mail:
sasham@igc.irk.ru).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TNS.2012.2187931
TABLE I
PEAK POSITIONS OF ABSORPTION BANDS IN
ALKALINE-EARTH FLUORIDE CRYSTALS DOPED WITH OR IONS.
(NNN) positions are favored by larger lattice constants.
Therefore, tetragonal symmetry sites are predicted for
lattice, trigonal sites for lattice, and both
types are predicted for . In addition to these types of
charge-compensating the cubic symmetry of rare earth
ions may be observed when the distance between impurity
and interstitial fluorine is several times greater than the lattice
constant. Moreover for crystal the defect symmetry
may be assumed with single ion in accordance with the
large distance and slight influence on the level
splitting. In accordance with the different symmetry sites the
energy of 5d level of rare earth ions split by the cubic or tetrag-
onal crystal fields and so one may expect a number of bands in
absorption spectra connected with the defect symmetry.
In this work we present the results of an ab initio calculations
of lattice relaxation and the energies of the
absorption bands of and ions embedded in alkaline
earth fluoride matrix. We used clusters with and symme-
tries for both obtaining the crystal-field splitting and comparing
the calculated results with experimental ones.
II. CALCULATION DETAILS
The calculations have been performed in embedded cluster
approach implemented in the GUESS computer code [9]. The
0018-9499/$31.00 © 2012 IEEE

2 IEEE TRANSACTIONS ON NUCLEAR SCIENCE
TABLE II
CALCULATED PARAMETERS OF ION IN DIFFERENT MATRIX
embedded cluster method was described in [10] in detail, and
so we do some remarks about our calculations. In the approach
the system of interest is divided into several groups:
(i) quantum—mechanical cluster (QM), which includes the
defect and its nearest neighbors and is treated with B3LYP
(40% of Hartree–Fock and 60% of DFT exchange) den-
sity functional method. This modified functional gives the
accurate electron state localization and was successfully
used for DFT calculations of different defects in alkali
earth fluorides [11]–[13].
(ii) classical region consists of several hundreds of ions are
treated classically in the shell model. We used pair poten-
tial parameters in buckingham form from [13]. Both QM
and classical region are allowed to move to provide the
accurate lattice distortion around the defect.
(iii) the classical region, in its turn, is surrounded by point
charges necessary to provide correct Madelung potential
inside the classical and QM regions. These atoms are fixed
during the geometry optimization run.
We obtained the time dependent DFT (TD DFT) method for
calculation of transition energies and oscillators strength with
the Gaussian 03 [14] computer code. We used 6–31G basis set
of George Petersson and coworkers [15] on fluorine and cal-
cium ions and SDD basis on strontium, barium, cesium and
praseodymium ions. We replaced all cations in the radius of 6
around QM cluster with the interface ions, which had LANL1
ECP (Hay-Wadt large core pseudopotential) and no basis func-
tions.
The following QM clusters were selected and used to
model the point defects in the present work:
for modeling a cubic symmetry , and for
representation of selectively charge-compensated site
.
III. RESULTS AND DISCUSSION
A. Ion
The absorption spectra of doped alkaline earth fluorides
have been experimentally investigated in detail in [5]. It was
shown that the absorption spectra consist of three main parts, la-
beled as A, B and C (see Table I), with the band C observed only
for and crystals. It was also shown that the intensity
of the band C grows steadily with increasing the concen-
tration. This band was associated with the “cluster-ion” absorp-
tion, when the ion and nearest interstitial fluorine ion tend
to cluster with another identical pair. For the crystal
only bands in regions A and B were observed.
First of all we have calculated the lattice distortion induced by
the presence of ion in crystalline matrix. The calculations
have been performed both for the single (with symmetry)
and for ion locally compensated by the interstitial fluo-
rine ( symmetry). The displacements of nearest 8 fluorine
ions induced by the single ion were 0.015, 0.12 and 0.32
toward the impurity ion for , and crystals
respectively. In the case of the presence of interstitial fluorine
the displacements of nearest fluorine ions were lager. The di-
rections of shifts are shown in Fig. 1. One can see that the in-
terstitial fluorine and ions get closer. The calculated dis-
tances between these ions in different matrices are summarized
in Table II. The displacements of the ion cause the large dis-
placements of four fluorines, which are situated between
and ions. The maximum displacements of fluorine ions were
0.21 and 0.17 for and respectively. These results
are in agreement with the results in [16].
For the barium fluoride crystal the interstitial and cerium ions
shifted in opposite directions. Therefore, we conclude that this

MYASNIKOVA et al.: FIRST PRINCIPLE CALCULATION OF ABSORPTION SPECTRA 3
Fig. 1. Cluster representing the interstitial fluorine position. Ar-
rows indicate the displacements of ions during the lattice relaxation induced by
defect.
Fig. 2. One-electron 5d states of or participating in the optical
absorption transition.
symmetry group is unstable in . This result agrees well
with theoretical work [8] as was mentioned above.
Using the geometry of the ground state we performed the cal-
culation of absorption spectra. The results
of TD DFT calculations are listed in Table II. For the single
ion calculation only two bands were observed. These bands are
connected with the crystal-field splitting of 5d state to
and levels. Our results agree well with experimental data
of investigation of cubic centers in [17] and
[18] crystals. Authors of [17] showed the excitation spectra of
doped with and , where only cubic centers
were formed. Two bands at 4.04 and 6.51 eV were observed
in this spectrum. For the cubic centers in also two
bands at 4.14 and 6.7 eV were observed [17]. So we can con-
clude that the bands A and B for crystal correspond
to cubic centers too. In all cases of single ions the 6–7 eV
band consists of three bands which are due to spin-orbit split-
ting of level.
In the case of the interstitial fluorine ion presence in NN
position the 5d level splits into four levels: , ,
and . Two former states correspond to crystal
field splitting of level, the latter two corresponds level.
Fig. 2 shows molecular orbitals (MO) of 5d state participating
in TD DFT calculations. The MO located largely on the cerium
ion, with the shape of each orbital corresponds to an orbital
type as identified in the figure.
TABLE III
CALCULATED PARAMETERS OF ION IN DIFFERENT MATRIX
According to our calculations (Table II) the level is more
sensitive to the distance between and ions. We ob-
served three main bands in absorption spectra as was obtained
in experiments for and crystals. The
calculated energies agree well with experimental data, but the
high-energy bands were underestimated by TDDFT calculation.
We have been making our calculations without taking into ac-
count relativistic effects. We assume that these effects can cause
the error in DFT results of transition energies as was presented
in [19].
In spite of the assumption of authors [5] that bands C are due
to “cluster-ion” absorption we can attribute the band near 5 eV
to the transitions of ions. This conclusion
agrees with experimental data [18], where the 5 eV band was ob-
served only for tetragonal center. The complicated struc-
ture of the band B is also due to both crystal field and spin-orbit
splitting of the level.
B. Ion
The experimental investigation of the ion absorption in
, and crystals is presented in [6], [20]. The UV
absorption spectra of ion in alkali earth fluorides are sim-
ilar to those of : as listed in Table I the
spectra consist of three bands A, B and C.
The results of calculation are presented in Table III. Ac-
cording to our calculation of lattice relaxation of cubic and
tetragonal centers, the size and directions of nearest
fluorine displacements are the same as in the case of
ion. But the distances between and interstitial fluorine
are somewhat larger, so the crystal-field splitting of level
is smaller than in the doped fluorides. All obtained TD
DFT results for are comparable with those for
ion: (i) for single ion we observed only two bands,
which correspond to cubic crystal field splitting of 5d band as
experimentally obtained for ; (ii) for ion with

4 IEEE TRANSACTIONS ON NUCLEAR SCIENCE
NN charge compensator we observed four bands which agree
with tetragonal splitting of 5d state; (iii) the band C correspond
to transitions of tetragonal center. The latter
result agrees well with the theoretical work presented in paper
[21], [22].
IV. CONCLUSION
We have performed ab initio calculations of lattice distortion
and absorption of and doped al-
kaline earth fluorides. The calculations have been made for both
cubic and tetragonal centers. For the tetragonal centers we esti-
mate the crystal field splitting of the level. We conclude that
the C bands of experimental spectra connect not only with the
defects aggregation but also with transitions of rare
earth ions.
ACKNOWLEDGMENT
The authors gratefully appreciate the use of Blackford and
MVS-1000 computational clusters located in the Institute of
System Dynamics and Control Theory SB RAS.
REFERENCES
[1] P. A. Rodnyi, “Progress in fast scintillators,” Radiat. Meas., vol. 33,
pp. 605–614, 2001.
[2] P. Dorenbos, “Fundamental limitations in the performance of Ce, Pr
and Eu activated scintillators,” IEEE Trans. Nucl. Sci., vol. 57, no. 3,
pp. 1162–1167, Jun. 2010.
[3] K. W. Kramer, P. Dorenbos, H. U. Gudel, and C. W. E. vanEijk, “De-
velopment and characterization of highly efficient new cerium doped
rare earth halide scintillator materials,” J. Mater. Chem., vol. 16, pp.
2773–2780, 2006.
[4] E. Radzhabov and V. Nagirnyi, “Excitation of ions in alkaline-
earth fluorides,” in Proc. IOP Conf. Ser.: Mater. Sci. Eng., 2010, vol.
15, pp. 012029–012029.
[5] E. Loh, “Ultraviolet absorption spectra of in alkaline-earth fluo-
rides,” Phys. Rev., vol. 154, no. 2, pp. 270–276, 1967.
[6] E. Loh, “Ultraviolet absorption spectra of ion in alkaline-earth
fluorides,” Phys. Rev., vol. 158, no. 2, pp. 273–279, 1967.
[7] P. Dorenbos, “The transitions of the trivalent lan-
thanides in halogenides and chalcogenides,” J. Lumin., vol. 91, pp.
91–106, 2000.
[8] J. Corish, C. R. A. Catlow, P. W. M. Jacobs, and S. H. Ong, “Defect
aggregation in anion-excess fluorites. Dopant monomers and dimers,”
Phys. Rev. B, vol. 25, pp. 6425–6438, 1982.
[9] P. V. Sushko, A. L. Shluger, and C. R. A. Catlow, “Relative energies
of surface and defect states: Ab initio calculations for the MgO(001)
surface,” Surface Science, vol. 450, pp. 153–153, 2000.
[10] A. S. Mysovsky, P. V. Sushko, E. A. Radzhabov, M. Reichling, and
A. L. Shluger, “Structure and properties of oxygen centers in
crystals from ab initio embedded cluster calculations,” Phys. Rev. B,
vol. 84, pp. 064133–064133, 2011.
[11] A. Myasnikova, E. Radzhabov, A. Mysovsky, and V. Shagun, “Im-
purity luminescence in and Yb crystals,” IEEE
Trans. Nucl. Sci., vol. 55, no. 3, pp. 1133–1137, Jun. 2008.
[12] A. Myasnikova, E. Radzhabov, and A. Mysovsky, “Ab initio calcula-
tion of cross-luminescence spectrum,” J. Lumin., vol. 129, pp.
1578–1580, 2009.
[13] A. S. Myasnikova, E. A. Radzhabov, and A. S. Mysovsky, “Ab initio
calculation of charge-transfer absorption in and crystals
with and Yb impurities,” IEEE Trans. Nucl. Sci., vol. 57, no.
2, pp. 1193–1195, Apr. 2010.
[14] M. J. Frisch et al., Gaussian 03 (Revision E.1). Pittsburgh, PA:
Gaussian Inc., 2007.
[15] G. A. Petersson, A. Bennett, T. G. Tensfeldt, M. A. Al-Laham, W. A.
Shirley, and J. Mantzaris, “A complete basis set model chemistry. I.
The total energies of closed‐shell atoms and hydrides of the first‐row
elements,” J. Chem. Phys., vol. 89, pp. 2193–2218, 1988.
[16] T. A. Gavasheli, D. M. Daraseliya, D. L. Dzhaparidze, R. I. Miri-
anashvili, O. V. Romelashvili, and T. I. Sanadze, “Method for esti-
mating local lattice distortions near a magnetic ion based on the pa-
rameters of the ligand hyperfine interaction: in the fluorite ho-
mologous series,” Phys. Solid State, vol. 44, pp. 1880–1884, 2002.
[17] L. vanPieterson, M. F. Reid, R. T. Wegh, S. Soverna, and A. Meijerink,
“ transitions of the light lanthanides: Experiment and
theory,” Phys. Rev. B, vol. 65, pp. 045113–045113, 2002.
[18] E. Radzhabov and T. Kurobori, “Cubic and tetragonal Ce ions in stron-
tium fluoride,” J. Phys.: Condens. Matter, vol. 16, pp. 1871–1877,
2004.
[19] J. Andriessen, E. van der Kolk, and P. Dorenbos, “Lattice relaxation
study of the 4f-5d excitation of -doped LaCl3, LaBr3, and
NaLaF4: Stokes shift by pseudo Jahn-Teller effect,” Phys. Rev. B, vol.
76, pp. 075124–075124, 2007.
[20] E. Radzhabov, “Photon cascade emission of ion in alkaline-earth
fluorides,” J. Lumin., vol. 159, pp. 1581–1585, 2009.
[21] T. Chen, C. K. Duan, S. Xia, and M. Yina, “Calculations for 4f-5d exci-
tation spectra of site-selective F charge-compensated
and ,” J. Lumin., vol. 122–123, pp. 51–54, 2007.
[22] K. Ogasawara, S. Watanabe, H. Toyoshima, and M. G. Brik,
“First-principles calculations of transition
spectra,” in Handbook on the Physics and Chemistry of Rare
Earths. North-Holland: Elsevier, 2007, vol. 37, ch. 231.

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