BLEACHING OF F* CENTRES IN SrCl2 DOPED WITH AN ALKALINE CATION

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BLEACHING OF F* CENTRES IN SrCl2 DOPED WITH AN ALKALINE CATION

S. Lefrant, J. Chapelle, E. Rzepka

To cite this version:

S. Lefrant, J. Chapelle, E. Rzepka. BLEACHING OF F* CENTRES IN SrCl2 DOPED WITH AN ALKALINE CATION. Journal de Physique Colloques, 1973, 34 (C9), pp.C9-511-C9-513.

�10.1051/jphyscol:1973986�. �jpa-00215462�

J O U R N A L D E P H Y S I Q U E Colloque C9, supplklnent au

1 1 - 12, Torue 34, Novembre-De'cembre 1973, page C9-511

BLEACHING OF F* CENTRES IN SrCI, DOPED WITH AN ALKALINE CATION

S. L E F R A N T , J. P. CHAPELLE and E. R Z E P K A Laboratoire de Physique Cristalline (*)

Universite d e Paris-Sud, Centre d70rsay, Biitiment 490, 91405 Orsay, France

RCsumC. - Les proprietes generales (absorption optique, RPE) des centres F qui apparaissent dans SrC12 : M b ( M i = Na+, K 1 , R b l ) sont indiquees.

L'etude du blanchiment a T

78 K des centres F* formes dans SrClz : Na+ met en Cvidence la symttrie ternaire de ces centres. Dans le cas de SrC12 : Rb-, les centres F* donnent naissance a des associations constituees par des centres FZ qui ne possedent pas la symetrie axiale.

Abstract.

The general properties (optical absorption, ESR) of F centres which appear in SrC12 : M + ( M '

Nai, K ' , R b l ) are reported.

F* centre bleaching in SrC12 : N a r at LNT shows that the centres have a trigonal symmetry.

In the case of SrC12 : R b b , F* centre bleaching leads to the formation of associations composed of FZ centres which do not have axial symmetry.

1. General properties. - The identification of electron centres which appear in SrC1, after jX-irra- diation at L N T has given rise to several different interpretations [I], [2], [3]. So, a few experimental results which seem to be well established, are first described.

1) With a n SrCI, crystal grown in chlorine atmo- sphere from a material purified by zone melting in the same atmosphere, it is impossible to form color centres by X-irradiation, at RT, LNT o r LHeT.

2) When this material is doped with a n alkali metal cation and when a single crystal is grown by the Bridgman method in chlorine atmosphere, it is very easy t o form color centres by X-rays a t LNT o r LHeT.

F o r these two temperatures, we obtain a n absorption spectrum (Fig. 1) which consists of the following : first, one broad band due t o positive hole centres ;

FIG. 1. - Optical absorption spectra after X-irradiation at T

77 K. 1) SrC12 : Na+ ; 2) SrCI2 : K-I- ; 3) SrCIZ : Rb-+.

(*) Equipe de recherche associee at1 CNRS, n o 13.

then, a double band more or less resolved according to the nature of the impurity. We have attributed it t o F centres perturbed by the alkali metal cation [I].

The ESR spectrum of the perturbed F* centres consists of a broad line superimposed on the V, spectrum [4].

3) By thermal annealing, the perturbed F * centres transform into F centre associations, and then into unperturbed F centres (Fig. 2).

FIG. 2. -Optical absorption spectra of SrClz : Na+ at T - ^{77 K.}

- - - After irradiation at ^T

77 K and thermal annealing at T = 163 K.

- After irradiation at T

77 K and thermal annealing at T

228 K.

These conclusions are based on the following deductions. There exists a n absorption band B, with a maximum at A = 580 nm which is well-defined whatever the nlonovalent cation. We again observe this band when we transform U centres into U, centres by X-ray treatment at L N T followed by thermal annealing at 240 K [4].

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973986

C9-5 12 S. LEFRANT. J. P. CHAPELLE A N D E. RZEPKA

This absorption band is associated with an ESR spectrum, seen so far in crystals doped with hydrogen.

In the other cases, the centre concentration was too small to observe a signal.

In fact, when we choose a suitable irradiation tem- perature, we can observe an ESR spectrum. Recently, we have carried out such experiments in SrCl, : Rb+

treated with X-rays at T = 200 K. The ESR spectrum of the F centres is represented in figure 3 for H // [I 111.

X-rayed ol T = 2 0 0 ° ~

FIG. 3 .

ESR spectrum of F centres in SrC12 : Rbf.

This orientation of the magnetic field give the best resolution of the hyperfine structure due to the inter- action with six neighbouring Cl- ions, in agreement with the fact that the F centre is located in a chlorine ion site.

4) These properties seem to characterize doped alkaline-earth halides and can account for the following results :

a) they allow us to explain the results of Scouler and Smakula on CaF, : Na' [ 5 ] .

b) BaCl, - an orthorhombic crystal - cannot be colored through X-irradiation when it is pure.

By additive or electrolytic coloration, we get a spec- trum due to F centres [6].

In contrast, it is possible to color BaCI, : K f through X-irradiation at LNT. We observe an optical spectrum slightly different from the preceding one due to perturbed F* centres [Table I]. By thermal

[- [

I

I

annealing at 180 K, the absorption spectrum due to the perturbed F* centres is converted into the absorp- tion spectrum due to the pure F centres. For example, we indicate the evolution of the absorption spectrum at T ⁼ 180 K of BaCI, : K' X-irradiated at T = 78 K (Fig. 4) [7].

2. Bleaching of perturbed F* centres. - Our aim is to specify the symmetry of perturbed F" centres

FIG. 4.

Transformation of F* centres into F centres in BaClz : K-I-.

using optical bleaching experiments. In particular, we have studied SrCl, : Naf because, in this case, the double band (B, + B,) is the best resolved [8].

The most interesting case is the one where the B, and B, bands are selectively bleached by polarized light parallel to the [01 I] direction. The absorption

k / / refers t o E / / [011]

k I refers ~ O E ~ [ O I I ] . The results obtained are shown in figure 5. It appears that both a bleaching process and a reorien- tation occur together.

FIG. 5. -

Variations of the rclativc absorption coefficients of the BI and Bz bands. Bleaching in the

band. 1) krill ; 2) k B l l l ; 3) k B L l l ; 4) k u z l (case h). 6) Variations of the relative absorption coefficients of the B I and B l bands. Bleaching in the Bz band. I ) k13,11 ; 2)

3)

krill/ (case h).

We can interpret all the results assuming that the F* centre has a trigonal symmetry and using the following phenomenological equations.

Let drljl' be the variation, due to the recombination process, of the number of the perturbed F* centres populating the ith configuration in unit volume during the bleaching time dt. We can write

d n f " = An,(t) cos' ( O i ) D dt

where A is a constant factor containing the transition

oscillator strength. D is a recombination probabili(y

coefficient.

BLEACHING O F F* CENTRES I N SrClz DOPED WITH AN ALKALINE CATION C9-5 13

Similarly, for the reorientation process, we can write

dni2' = - Ani(t) cos2 (Oi) R dt

where R is the probability coefficient of finding an F * centre of the ith configuration in another confi- guration.

For example, we obtain the following results for the B1 band bleaching.

dkB,,

- - A

- -

[9 D l - 4 R,]

dt t = o 18

We have 3 D l = 4 R 1 .

All the other relations are well verified.

Before interpreting these last results, it is necessary to have a better understanting of the locations of the V, centres in the lattice. Otherwise, the simplest hypothesis is t o assume that the monovalent cation takes the place of a bivalent cation and is associated by a vacancy. During irradiation, the last step is the trapping of an electron by such a vacancy. Therefore, the perturbed F * centre is surrounded by three cations

Sr'+ and one of the alkali metal. It has the C,, symmetry (Fig. 6).

Fccntre 0 5r2* O Alkaline callm

.CI-

B+J

FIG. 6.

Model of the F* centre in SrC12: Na-.

3. Bleaching of M centres.

The crystals doped with K f or Rb+ have the same properties as those doped with Na'. If we bleach perturbed F* centres, this causes destruction and reorientation. But new bands appear

These bands are observed, whatever the nature of the alkali cation, by annealing at 140 K the centres obtained at LNT.

Bleaching experiments allow us to establish that these bands are due to the same centres which do not have axial symmetry and we can postulate that they are composed of perturbed F2 centres.

References

[ l ] BALTOG, I., LEFRANT, S., HOULIER, B., YUSTE, M., CHAPELLE, [5] SCOULER, W. J. and SMAKULA, A., PIIJJS. Rev. 120 (1960) J. P. et TAUREL, L., Phys. Stat. Sol. (6) 48 (1971) 345. 1 154.

[2] W., LAMBOURN, R. F., G.and

[6] HOULIER, B., YUSTE, M., CHAPELLE, J. P. and TAUREL, L., I. M., J. Phys. C 6 (1973). P ~ I J I S . Stat. Sol. (b) 51 (1972) 881.

[3] DEN HARTOG, H. W., MOLLEMA, P. and SCHAAFSMA, A,,

Phys. Stat. Sol. (b) 55 (1973) 721. [7] YUSTE, M., HOULIER, B. (to be published).

[4] LEFRANT, S., JUMEAU, D. and TAUREL, L., PIIJ~s. Stat. Sol. [8] RZEPKA, E., BALTOG, I., LEFRANT, S., YUSTE, M. and (b) 50 (1972) K 101. TAUREL, L., P11ys. Stat. Sol. (6) 57 (1973) 383.

DISCUSSION

A. M. STONEHAM. -In your model of the per- and this suggests a configuration with more widely turbed F-centre, the electron is bound by a dipole, separated impurity and vacancy.

with no net charge. For such a short dipole, the elec-

tronic levels and wavefunctions are very different from J. P. CHAPELLE.

The symmetry of the F* centre

those of the isolated F-centre. I should be surprised is trigonal but it is possible that the Na' ion does

if the changes in spectra were as small as you find, not be in nn position.