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S. Lefrant, A. Harker, M. Yuste, B. Houlier

To cite this version:


c7-232 JOURNAL DE PHYSIQUE Colloque C7, supplkment au no 12, Tome 37, Dkcembre 1976



S. LEFRANT ("), A. H. HARKER (*), M. YUSTE (") and B. HOULIER (") (") Laboratoire de Physique Cristalline (*), UniversitC Paris-Sud,

BBtiment 490, 91405 Orsay Cedex, France

( b ) Theoretical Physics Division, AERE Harwell,

Oxfordshire, OX1 1 ORA, U. K.

RBsum6. - Nous avons utilisk la mkthode du reseau rigide ponctuel pour calculer la structure Clectronique des centres F dans BaClF, SrClF et BaC12. Les positions relatives des bandes d'absorp- tion des centres F en site F- ou Cl- ont 6tk dkterminkes dans BaClF et SrClF. Dans BaC12, les calculs de niveaux d'energie suggerent I'existence d'un seul type de centres F, ceux qui sont entoures par quatre ions Ba++.

Abstract. - We present point-ion calculations of the electronic structure of F centres in BaClF, SrClF and BaC12. The relative positions of the singlet and doublet absorption bands of the F centres at F- and C1- sites have been determined in BaClF and SrCIF. In BaC12, calculations of energy levels suggest the existence of only the F centres which are surrounded with four Ba++ ions.

1. Introduction. - Theoretical models of F centres have been very successful in the past in the case of alkali-halides and alkaline earth-halides [I, 21. In particular they have given results in good agreement with experimental F-band energies. We have extended this theory to F centres in crystals of lower symmetry such as the quadratic BaClF and SrClF crystals and BaCI, which is orthorombic. These examples are interesting because the calculations of energy levels lead to the interpretation of experimental results. 2. BaCIF and SrClF crystals.


In the mixed

crystals BaClF and SrClF, the anion vacancies are of two types and so, it is possible to create two types of F centres : an F centre in fluoride site with D,,

symmetry and an F centre in chloride site with C,, symmetry (Fig. 1). In these conditions, the F centre excited state will be split into a doublet and a singlet. The optical absorptions observed fit this pattern, with four F bands, two polarised parallel to and two per- pendicular to the four-fold axis of the crystal.

The first optical work on BaClF was done by Nicklaus and Fischer [3] and more recent studies by Yuste et al. [4] has covered both BaCIF and SrClF crystals by means of EPR and optical techniques. F centres may be produced in these crystals by various means such as X-ray irradiation at RT, addi- tive coloration, X-irradiation at low temperatures of substitutional H - ions [4].

(*) Equipe de Recherche Associke au C. N. R. S. no 13.


o F -orCL vacancy

FIG. 1. - F centre at F- or C1- site in BaClF or SrCIF.

Since we are interested by the F centres in these crystals and as their identification is not obvious, we have labelled them I and J and therefore the associated absorption bands I, and J,, I, and J, (Fig. 2). The J, band is a doublet split by about 0.10 eV. Magnetic circular dichroism experiments [5] have shown that is was due to a Jahn-Teller coupling to the non-cubic modes of vibration of the ions surrounding the centre.

In BaClF, we observe for the I centre two bands at 2.83 eV (E


c) and 2.25 eV (E -L c) and for the J

centre 2.33 eV (E


c) and 2.82 eV (E


c). Similar

results are observed for SrClF.




400 500 a 8a31OOO

11 (2.83eV)

FIG. 2.


Optical absorption spectra of F centres in BaClF at L N T : l ) E / c ; 2 ) E l c .

In order to determine which centre is the F centre in fluoride or chloride site, we have applied the point- ion model developped by Gourary and Adrian [I] to calculate the energy levels, using a program written by one of us (A. H. H.).

For the F(F-) centre, its D,, symmetry allows transitions A, + B, (E


c) and A, + E (E


c) and so, we have taken for the angular part of the wave-function s for A,, p, for B, and p,, py for E. The

results are indicated on the table I.

for the A, and A; and (p,, py) for the E levels (see Table I). Another set of calculations, which does not take account of the small axial field mixing the s and p, components gives results quite similar to those indicated here.

These calculations show that the F(F-) centres have the doublet transition energies greater than the singlet and the reverse order holds for the F(C1-) centres. We must therefore make the identification :

I centre


F(C1-) and J centre r F(F-). This interpretation differs from that previously sug- gested by Nicklaus and Fischer [3].

3. BaCI,.


BaC1, is an orthorombic crystal. Its crystallographic structure is such that two different sites of C1- ions exist. So, two types of F centres can also be expected to be created : the F1 centre with four Ba++ ions in nearest neighbours and the F,, centre with five Ba++ ions (Fig. 3). Both have C,

FIG. 3.


Surrounding of the FI and FII centres in BaC12. symmetry. In this case, the first excited state will be split into three levels and for each centre F, or F,,, one may expect to observe three absorption bands, one polarised parallel to and two polarised perpen- dicular to the b axis of the crystal. In fact, the absorption spectra are complicated (Fig. 4). A main band at 2.5 eV for E

// b

and two bands for both

a and c directions are observed. Calculated a ~ d experimental transition energies (in

QD, eV) of F centres at F - and C1- sites in BaClF and SrClF crystals

F(F-) J centre F(C1-) I centre Calcul. Exp. Calcul. Exp.

- -




E / / c E l c


A1 + E A1 + E FIG. 4. -Optical absorption spectra of F centres in BaClz E l c 3.63 3.52 2.11 2.67 at LNT for E parallel to a, b, c respectively.

The F(C1-) centre with C,, symmetry allows A calculation of energy levels has been performed in A, -+ A; (E


c) and A, + E (E


c) transitions. the point-ion approximation using a program adapted



TABLE 11 i) the single absorption band when E



ii) from the EPR spectrum of the F centres, one

Transition energies and oscillator strengths calculated can deduce only one [g] tensor,

for the F centres in BaCl,, with (( s )), (( p, )>, ( ( p , >> iii) the EPR spectrum (Fig. 5) has previously been

and (( pp, )> type trial functions. (The oscillator strength interpreted [6]. The central line is due to the hyperfine

value is $xed arbitrarily at 1, when E


b.) interaction with Baf + nuclei with a nu1 magnetic Rela- oscil-

Transi- Energies tive lator strengths BaC12 tions

- - (eV> - E//a EE//b


E//c - FI centre A' + A ' 2.15 0.71 0


0 Calculated va- A' + A" 2.7 0 1 0

Iues A 1 + A ' 1.8 - 0 0 0.65 FII centre A' + A ' 2.2 1.36 0 - 0 Calculated va- A' + A " 1.7 0 1 0 lues A ' + A ' 3.0 - 0 0 1.56 AI 1.94


0.6 0 0 B 2.17 - 0 . 4 0 0 Experimental values D 2.5 0 1 0 (5 K) A2 1.93 0 0


0.6 C 2.25 0 0



(B. H.) for the two types of F centres. The results are summarized on the table II. The results, in best agree- ment with the main absorption energies, are obtained for the F, centre and the hypothesis of only one type of F centres seems to be reasonable. Other arguments are in favour of the existence of only the F, centre :

moment. Similar spectra are observed for the F centres in BaClF, whatever the considered site. A detailed

analysis of the EPR spectra confirms the interpreta-

tion of one type of F centre.

FIG. 5. - EPR spectrum of F centres in BaClz at LNT, H



4. Conclusion. - Theoretical calculations of energy levels of F centres lead to their identification in F- ou C1- site for the quadratic crystals BaClF and SrCIF. For the orthorombic BaCI,, we can assume that only the F, centre (with four Ba++ neighbours) can exist.


[I] GOURARY, B. S. and ADRIAN, F, J., Sol. Stat. Phys. 10 [4] YUSTE, M., ~ H M A N I , M., LEMOYNE, D. and TAUREL, L., (1960) 127. J. Phys. & Chem. Solids, 37 (1976) 961.

[2] BARTRAM, R. H., STONEHAM, A. M. and GASH, P., Phys. [5] LEMOYNE, D., DURAN, J., YUSTE, M. and BILLARDON, M.,

Rev. 176 (1968) 1014-24. J. Phys. C : Solid. StatePhys. 8 (1975) 1455.




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