Study of aggregation of F-centers in NaCl by Raman scattering

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Study of aggregation of F-centers in NaCl by Raman

scattering

L. Taurel

To cite this version:

Study of aggregation of F-centers in NaCl by Raman scattering

Abstract. — Raman spectra of F-centers in N a d are very dependent on coloration method. At the very initial

stage of colloidal aggregate formation, the change of the r ,+ Raman spectrum induced by F-centers may be

inter-preted assuming clusters of F-centers having an average separation lower than three nearest-neighbours. Raman spectra of colloids are composed of peaks superimposed to a background due to broadening of the Rayleigh line which vanishes at T ~ 10 K. It is suggested that Raman peaks come from excitation of NaCl lattice modes at the interface colloid-NaCl host crystals, while the background may be due to single particle excitation of the electron-gas of the metallic particles.

We report Raman and optical studies by additively coloured NaCl crystals at several stages of F-center aggregation. It was shown that Raman spectra of F-centers in additively coloured NaCl (T ^ 630 °C) depend strongly on the cooling rate; these spectra are different from those obtained in crystals X-irradiat-ed at 78 K [1].

When additively coloured samples are quenched at 78 K, the centers are not in thermal equilibrium, and so the quenched samples were subsequently annealed at selected temperatures for a few hours and their Raman and optical absorption spectra were studied at 78 K. Three ranges of temperature (Tj) can be distinguished from optical absorption and Raman spectra.

1. 120 °C < TA < 150 °C. — Raman spectra of

the samples subsequently annealed between 120 and 150 °C are similar to those obtained by samples X-irradiated at 78 K. The absorption spectrum contains an F-band with the same features (/•max = 450 nm, half-width = 0.27 eV) attributed to isolated pure F-centers. Raman spectra are inde-pendent of the laser line wavelength /.L in the spectral

range 476.5-620 nm, except the overall increase of intensity when the spectra are taken in resonance with the F-band [1]. Since no breaking of the usual selection rules [2] was shown, standard calculations

(*) Equipe de recherche associee au C.N.R.S. n° 13.

of Raman scattering by independent centers have been used [3]. Very good agreement is obtained between calculated and experimental spectra (Fig. 1)

Fig. 1. — Experimental and calculated first-order Stokes Raman spectra ofNaCl(F), irradiated with X-rays at T= 78 K. (a) Absorp-tion spectrum (T = 78 K). (b), (c), (d) Experimental first-order Stokes Raman spectra at T=78 K. Cut-off frequency for phonons

o), = 264 cm"1, Xh = 514.5 nm. (&'), (c'), (d") Calculated spectra

at T = 78 K. linear electron-phonon coupling with two ionic shells ; the same with five ionic shells.

JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 7, Tome 4 1 , Juillet 1980, page C6-468

L. Taurel, M . G h o m i , E . R z e p k a a n d J. P . Chapelle Laboratoire de Physique Cristalline (*), Université de Paris-Sud, Centre d'Orsay, Bâtiment 490, 91405 Orsay, France

Résumé. — Les spectres Raman des centres F dans NaCl dépendent fortement de la méthode de coloration. Il

existe un stade précurseur dans la formation des colloïdes où la modification du spectre rf induit par les centres F peut être interprétée en supposant l'existence de groupements de centres F dont la séparation moyenne est infé-rieure à trois distances interioniques. Les spectres Raman des colloïdes sont constitués de bandes superposées à un fond continu dû à un élargissement de la raie Rayleigh, qui disparaît à T ^ 10 K. On suggère que les pics de diffusion Raman proviennent de l'excitation de modes du réseau NaCl, à l'interface colloïde-NaCl, tandis que le fond continu pourrait être dû à l'excitation des particules individuelles qui constitue le gaz électronique des colloïdes métalliques.

STUDY OF AGGREGATION O F F-CENTERS IN NaCl BY RAMAN SCATTERING C6-469 introducing changes cik,,(lj, 6 k , , ( l ) for the longitu-

dinal interionic force constant between the F-center and its 1 nn's ( p l =

-

dkO1(1)/Al2 = 70

%,

p2 = - 6 k I 4 ( 1 ) / A 1 2 = 10

%)

provided the interaction between the F-center and the displacements of the five ionic shells surrounding the center is considered. A change 6 k , , ( l ) for the longitudinal force constant between the 1 nn's and 2 nn's of the F-center does not improve the results [I].

2. 150 "C < TA < 200 "C.

-

Annealing in this temperature range leads to a broadening of the F-band mainly on its low-energy side, and to a modification of the Raman spectrum. Both results are connected, they depend on TA and on duration of annealing. Raman spectra are now dependent on 3., [I]. All the spectra can be interpreted if we suppose that the

R g . 2. - First-order Stokes Raman spectra of Fx-centers

(I,, = 514.5 nm, T = 78 K). (a) Absorption spectrum of the sample showing the broadened F-band ; (b) Experimental

[100]-+ [loo] first-order Raman spectrum; (c) Calculated [I001

-

[I001 first-order Raman spectrum.

samples contain isolated F-centers and another type of F-centers - we called (( Fx )) centers - giving

rise to a spectrum strongly resonant at i.

--

514 nm. This spectrum (Fig. 2b) is close to that described in paragraph 1, except an overall increase of intensity. It may be calculated using the parameter p, and electron-phonon coupling coefficients of the F-center provided that additional softening in the interionic longitudinal force constants

are .antroduced in calculations (Fig. 2c). This suggests that three dimensional clusters of F-centers - rather than pairs of F-centers - are responsible for Raman scattering. The enhancement of scattering efficiency can be explained supposing that mean radius of the clusters are smaller than the laser line wavelength. An average separation of the F-centers lower than three nearest neighbours can give a qualitative interpretation for the further softenings of force constant. Formation of loose aggregates [F] has been yet assumed in other alkali-halides [4] in order to explain the narrowing of the E.S.R. line of isolated F-centers and the decrease of their relaxation time. In NaC1, we have always observed such a modification of the E.S.R. line of the F-center when cc Fx >) centers

seen through Raman and optical absorption spectra are formed.

3. 200 "C

<

T A < 400 O C ,

-

Annealing at

T A > 200 0C leads to the formation of colloids detected through their absorption band C whose peak-wavelength ibc depends on TA. Two samples have been specially studied : sample (A) contains very small colloi'ds (so-called X-centers [5]) responsible for an absorption band located at 2c

--

560nm; sample (B) contains larger particles exhibiting a colloidal band (I.,

-

640 nm ; R

-

500 from Savostianova's curves [5]), and situated mainly in grain boundaries as it was seen from ultramicroscopic observation. The most typical results are described below :

i) Raman spectra depend somewhat on i., but both samples give similar results if th'ey are studied by excitation in resonance with the C band. Therefore,

Fig. 3. - Raman spectra of sodium colloids In NaCl at selected temperatures I,,,,, = 514.5 nm

.

FWHM of Rayleigh line

C6-470 L. TAUREL, M. GHOMI, E. RZEPKA AND J. P. CHAPELLE

X-centers are not atomically dispersed centers as it

was sometimes supposed [6].

I

-

ii) Raman spectra are always formed of more or

_-

= less resolved bands superimposed to a background _{Y A} due to the broadening of the Rayleigh line. This .rn

52

broadening vanishes at 10 K [7] (Fig. 3). -Y

-

iii) The frequency shifts of Raman peaks are lower

than the cut-off frequency of the host-lattice phonons.

5

E NaBr +CoUoids(Na) A similar result appears in the case of Raman spectra

of sodium colloYds in NaBr and in LiF ( I ) [7] (Fig. 4).

'1.9.

,

,

L

l

0 100 200 O(crn-')-

Moreover, some critical point phonon frequencies are consistent with the shift frequencies of Raman

peaks induced by sodium colloids in NaCl [7]. This

I

shows that the phonons of the host lattice - to a

slight extend perturbed - modulate the electronic

-

m

X

polarizability of the collo'ids. e

c

Y An important result is the broadening of the

-

Rayleigh line. We tentatively propose that it comes _z 6

from single-particle excitation of the electron gas of I: 6

~r Li F+Colldids(Na) the metallic particle ; indeed it was previously proved

0

that in the case of small particles of definite size

coupling of the plasma oscillation with the individual 0 200 400 600 0 (cm-'1-

electronic states quantized in discrete levels must be _{Fig, 4, -Stokes Raman spectra of NaBr and LiF containing} taken into account to explain the phsma line broaden- sodium cofloids ( T = 78 K). (a) Absorption bands at 78 K.

ing [8]. (b) Raman spectra (scattered light is unpolarized).

DISCUSSION

Questiotl. N . T I I V I . Question. - U . M . GRASSANO.

Can you explain the reason why the cross-section Your results seem to support our hypothesis on increases in going from isolated impurities to aggre- the presence of colloids in NaCl crystals showing

gates ? second harmonic generation. Indeed anysotropic

defects or surface boundaries between the colloid R C ' I , ~ : - L. TAUKEI-. and the perfect crystal may provide sites without The increase of the scattering cross-section can be inversion symmetry, where second harmonic gene- explained if the clusters have a size smaller than the ration can take place.

wavelength of the laser line.

Do you have any estimate of how far apart the F- centers are in these loose aggregates ? If the centers are near enough together, there should be a decrease in the spin-orbit interaction because each center will have fewer host ions in its neighborhood.

Reply. - L. TAUREL.

We estimate the mean distance between two F- centers between two and three interionic distances. Yes I agree, but distortions of the lattice swamp this effect. Moreover, this initial state of aggregation may occur in preferential sites as dislocations.

( I ) Colloids in LiF have been prepared by ionic implantation

(D. Durand, Dkpartement de Physique des MatBriaux, Lyon).

Reply.

-

L. TAUREL. 1 agree with your comment.

Question.

-

P . A. FLEURY.

In either the Raman or Rayleigh scattering in Region I11 (large colloids) have you attempted to observe the angular dependence for the scattered intensity ?

Reply. - L. TAUREL.

STUDY OF AGGREGATION OF F-CENTERS IN NaCl BY RAMAN SCATTERING C6-471

References

[I] GHOMI, M., RZEPKA, E. and TAUREL, L., Phys. Status Solidi (b) [5] SAVOSTIANOVA, M., Z. Phys. 64 (1930) 262.

92 (1979) 447. [6] SCHULMAN, J. H. and COMPTON, W. D., Color centers in solids,

[2] BUISSON, J. P., LEFRANT, S., GHOMI, M. and TAUREL, L., chap. IX (Pergamon Press, New York) 1962.

Phys. Rev. B 18 (1978) 885. [7] RZEPKA, E., LEFRANT, S. atid TAUREL, L., Solid Stute Cominun.

[3] MARADUDIN, A. A., Solid Sfate Phys. 19 (1 966) 1 . 30 (1979) 801.

[4] S B I ~ ~ , H. and WOLF, H., in : Physics of Color centers, Ed. 181 KAWABATA,A. a n d K u ~ o , R . , J. Phys. Soc. Japun21(1966) 1765.