BRILLOUIN SCATTERING OBSERVATION OF PHONON SCATTERING BY INHOMOGENEITIES IN PHASE SEPARATED GLASSES

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BRILLOUIN SCATTERING OBSERVATION OF PHONON SCATTERING BY INHOMOGENEITIES

IN PHASE SEPARATED GLASSES

J. Pelous, A. Essabouri, R. Vacher

To cite this version:

J. Pelous, A. Essabouri, R. Vacher. BRILLOUIN SCATTERING OBSERVATION OF PHONON

SCATTERING BY INHOMOGENEITIES IN PHASE SEPARATED GLASSES. Journal de Physique

Colloques, 1982, 43 (C9), pp.C9-549-C9-551. �10.1051/jphyscol:19829110�. �jpa-00222415�

JOURNAL DE PHYSIQUE

Colloque C9, supplement au n°!2, Tome 42, decembre 1982 page C9-549

BRILLOUIN SCATTERING OBSERVATION OF PHONON SCATTERING BY INHOMOGENEITIES IN PHASE SEPARATED GLASSES

J. Pelous, A. Essabouri and R. Vacher

Laboratoire de Spectrométrie Rayleigh-Brillouin, ERA 460, Université des Sciences & Techniques du Languedoc, 34060 Montpellier Cedex, France

Résumé.- Nous avons étudié par diffusion Brillouin l'atténuation et la vitesse des ondes hypersoniques dans un verre formé en trempant une solu- tion aqueuse de LiCC (contenu d'électrolyte : 10 moles pour cent).

Ce matériau a été utilisé parce que la séparation de phase peut y être facilement réalisée par des traitements thermiques : nous l'avons détectée en mesurant l'intensité de la diffusion Rayleigh. Nous avons observé une variation des propriétés élastiques avec le traitement thermique.

Les différences observées entre les échantillons sont indépendantes de la température et paraissent liées à la diffusion des phonons acoustiques par les inhomogénéités structurales.

Abstract.- We have studied by Brillouin scattering the attenuation and velocity of hypersonic waves in a glass formed by quenching an aqueous solution of LiC£ (molar electrolyte content : 10 %). This material has been used because the phase separation in this material can be easily controlled by heat treatments and detected by measuring the Rayleigh scat- tering intensity. The elastic properties appear to depend on heat treatment.

The differences observed for various samples are independent of temperature and seems likely to be due

to scattering of acoustic phonons by structural inhomogeneity.

Fluctuations of the macroscopic physical properties as refractive index, density, elastic constants are known for long time to scatter energy of light and acoustic waves in inhomogeneous materials. Such fluctuations are present in glas- ses, and can be easily detected even in perfectly transparent materials from elas- tic Rayleigh scattering of light. Microscopic fluctuations are also present in glasses and have been proposed as a possible scattering mechanism for phonons ! 1 ) . However, all the existing Brillouin scattering measurements in glasses have demons- trated that the structural scattering of high-frequency phonons, if it must exists in theory is a very small effect with is at maximum of the same order as the expe- rimental uncertainty (2). As the amplitude of the fluctuations must be a determi- ning factor on this process, we have studied the glass forming mixture LiC£ : H

0 , ,in which a phase separation has been detected in elastic neutron scattering experiment for appropriate electrolyte concentration (3). Recent studies (4) have demonstrated that an homogeneous nucleation take place in this glass, leading to the appearance of cubic ice crystals in the vitreous matrix. The glass transition of this mixture is around 140 K , near the crystallization temperature, and the process of crystallization can be easily controlled by varying the heat treatments.

The increase of the intensity of Rayleigh scattering can be used as a test of the increase of the size of domains.

In order to obtain a glass, the solution containing 10 I LiC£ in water was first rapidly quenched into the same cryostat used for light scattering measu- rements. Cracks related to internal stresses were then present in the material, but some regions of the sample were transparent enough to allow performing Brillouin scattering measurements. After this first study, the sample was put a short time at a temperature of 138 K , 2 K below the glass transition temperature, in order to relax the stresses. A perfectly transparent sample was then obtained, on which

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19829110

C9-550 JOURNAL DE PHYSIQUE

Brillouin s c a t t e r i n g measurements were performed. We found very small differences between the quenched and the relaxed samples. The sample was then held f o r a few hours a t 140 K : the phase separation then appeared and was detected by observing the Rayleigh s c a t t e r i n g of l i g h t . A t the end of the thermal treatment, the inten- s i t y of t h e Rayleigh l i n e was about 1000 times t h a t i n t h e relaxed sample. Bowever,

the sample was transparent enough t o keep a good d e f i n i t i o n of the wave vector of the incident and s c a t t e r e d l i g h t and t o be sure t h a t a p a r a s i t i c broadening due t o a spreading of l i g h t was not present.

F i g . 1 - Frequency s h i f t of t h e

- B r i l l o u i n l i n e i n 10 %

L ~ C L - H ~ O g l a s s . + u n t r e a t e d sample

phase s e p a r a t e d sample.

F i g . 2 - Linewidth o f t h e B r i l -

- l o u i n l i n e i n 10 % Lick-Hz0 g l a s s . + u n t r e a t e d sample

p h a s e s e p a r a t e d sample.

The meaa~rements on the phase separated glass a r e compared t o the o r i g i n a l sample i n Figs 1 and 2. The velocity v of acoustic waves i s related t o the measu- red frequency s h i f t v~ by vg = (2nv/Xo) sin(f3/2) where n i s the r e f r a c t i v e in- dex, X o the wavelengrh of incident l i g h t i n vacuum and 8 the s c a t t e r i n g angle.

The attenuation a can be deduced from the observed half-width a t half-maximum r

of the Brillouin l i n e from the r e l a t i o n : r = av/47~ . The most s t r i k i n g feature is

a difference i n the attenuation between t h e two sample : t h i s difference remains

constant between 10 K and the phase t r a n s i t i o n . The amplitude of the-excess a t t e -

nuation of 20 GHz longitudinal acoustic phonons is of about 700 cm-', which i s

well above experimental uncertainty.

A glass containing large density o r concentration fluctuations, which a r e r e l a t e d t o the i n t e n s i t y of the Rayleigh s c a t t e r i n g of l i g h t , can be thought t o exhibit c o r r e l a t i v e l y large fluctuations of e l a s t i c constants, and therefore t o pre- s e n t , the best conditions f o r the observation of t h e phonon s c a t t e r i n g by s t a t i c inhomogeneities. A constant excess attenuation between 10 K and the glass t r a n s i - t i o n seems very d i f f i c u l t t o explain by dynamical e f f e c t s . I t seems therefore l i k e l y t h a t t h e difference observed between the two samples i s due t o s t a t i c s t r u c t u r a l ef- f e c t s .

On the other hand, t h i s contribution i s small, even i n a very inhomogeneous glass. The order of magnitude of our r e s u l t indicates t h a t t h i s e f f e c t nust be ne- g l i g i b l e f o r more homogeneous glasses.

( 1 ) D. hlALTON, Phys. Rev. B16 (1977) 3723.

( 2 ) R. VACHER, H. SUSSNER, S. HUNKLINGER, Phys. Rev. B Z (1980) 5850 ; R. VACHER, J. PELOUS, H. SUSSNER, M. SCHMIDT, S. HIJNKLINGER i n "Condensed

Mater Physics" J.T. Devreese e t a2 Eds (Plenum New York 1981) Vo1.4 p. 229.

( 3 ) J. DUPUY, P. CHIEUX, C. F E W U , J.F. JAL and C.A. ANGELL, Nature (1981).

( 4 ) A. AOUIZERAT, J.F. JAL, J. DUPW, P. CHIEUX, A. WRIm, Paper a t t h i s

conference.