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RAYLEIGH SCATTERING OF MÖSSBAUER RADIATION ON VAPOUR-QUENCHED Te
F. Wagner, F. Litterst, G. Kalvius
To cite this version:
F. Wagner, F. Litterst, G. Kalvius. RAYLEIGH SCATTERING OF MÖSSBAUER RADIATION ON VAPOUR-QUENCHED Te. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-140-C2-141.
�10.1051/jphyscol:1979248�. �jpa-00218646�
JOURNAL DE PHYSIQUE
Colloque C2, supplt!ment au no
3,Tome
40,mars 1979,
page C2-140RAYLEI GH SCATTERING OF MOSSBAUER RADIATION ON VAPOUR-QUENCHED
~ e "F.M. Wagner, F.J. Litterst and G.M. Kalvius
Physik Department
der
Technischen Universitat Miinchen, 0-8046 Garching, Germany.Rdsum6.- Du tellure non cristallin a dtd pr6pard par ddposition d'un faisceau moldculaire sur un substrat de mylar maintenu 2 80K. La structure amorphe est fortement influencde par 1'6paisseur de l'dchantillon. Des dchantillons minces (510~A) ont St6 chauffds 2 partir de 80K 1 une vitesse de
"15K/jour. A 245K on observe une diminution brusque de 11intensit6 de diffusion Ra leigh dlastique.
Ceci corresponbl 1 une augmentation de l'amplitude moyenne de vibration atomique &> qui passe de -0.15A 2 -0.32A. AprZs avoir maintenu l'dchantillon pendant 10 heures aux alentours de 2no.K54
retrouve la valeur initiale de
m>.
Les spectres de RX montrent que cette variation de d<x2> est li6e 3 un &arrangement structural. Dans le mzme intervalle de tempdrature, des dchantillons dpais cristallisent et ne prdsentent que de faibles anomalies de =>.Abstract.- Non-crystalline Te was prepared by deposition from a molecular beam onto a mylar substra- te kept at 80K. The amorphous structure is strongly influenced by sample thickness. Thin samples (=105A) were heated from 80K at a rate of "15K/day. At 245K a sudden decrease in the intensity of elastic Rayleighscattering wasoobserved.oThis corresponds to an increase in the mean atomic vibra- tion amplitude /<xZ> from -0.15A to -0.32A. After keeping the samples for ten hours near 245K the initial value of => was recovered. The X-ray pattern of the sample show that this change in -
> is connected with a structural re-arran ement. In the same temperature range thick samples undergo crystallization where anomalies in
&
are only weakly present.Rayleigh scattering of M6ssbauer radiation has pro- ven to be a sensitive tool for studying lattice dy- namical properties both in crystalline and non-crys- talline materials. The basic theoretical and expe- rimental outlines are described by oiconnor /I/.
This method offers the important advantage that the scatterer must not necessarily contain a M6ssbauer isotope. From the dependence of the relative inten- sities of the elastic and the inelastic scattering on sample temperature and scattering angle one ob- tains detailed information on the Debye-Waller fac- tor; from the width of the scattered resonance line one deduces diffusional properties.
In non-crystalline materials like supercooled orga- nic liquids the interest has concentrated on tracing the softening of these glasses from a decrease of the Debye-Waller factdr in the regime some ten Kelvin above the glass transition point 121. The non-equilibrium thermodynamic properties in the non- crystalline state are still not very well known in detail. A prominent experimental handicap is the poor reproducibility of data caused by minor diffe- rences in the thermal history of otherwise identical samples. For example, it has been shown recently /3/
that ofganic glasses doped with molecules containing iron as a Mijssbauer probe, show drastic anomalies in
7-
*
Work supported by the Deutsche Forschungsgemein- schaf t.the recoilfree fraction depending on different ther- mal treatments of the samples. These phenomena are caused by structural relaxation processes between different metastable states near the glass transi- tion. The present experiments investigate the struc- tural re-arrangement in amorphous semi-conductors which do not contain additional impurities of Mzss- bauer resonant atoms.
Samples of amorphous Te have been prepared by quen- ching the material from a molecular beam onto a 80K cold mylar substrate. The speed of deposition was typically "5nm/s. The vacuum in the cryostat was kept torr. The experimental set-up for Ray- leigh scattering is similar to the one described in 121. As gamma ray source we used lOOmCi of 5 7 ~ o in Rh. The typical counting rate for a scattering angle of 2$30° and an acceptance angle of -28" was about 3cts/s. Background corrections were carried out on the basis of countrates measured both off resonance and in resonance using an absorber of stainless steel (-54% absorption) first positioned between the source and scatterer and then switched to a position between scatterer and detector. In addition a small correction was included accounting for the background for resonant Rayleigh scattering from the sample holder and the substrate. The Debye- Waller factor was then obtained from the ratio of background corrected resonantly to total scattered
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979248
i n t e n s i t y . The i n f l u e n c e of Bragg peaks can be ne- g l e c t e d due t o t h e wide a n g l e of acceptance i n our s c a t t e r i n g geometry. One may then simply s e t f - e - ~ 2 <x2 >
w i t h Q b e i n g t h e s i z e of t h e s c a t t e r i n g v e c t o r . We found t h e r o o t mean s q u a r e displacement of Te atoms a t 80K f o r
thin
amorphous samples ( 5 10'1) t o be 0 . 0 d . The sample temperature was then in-creased i n s m a l l s t e p s w i t h an average h e a t i n g r a t e of -I5K/day. A smooth i n c r e a s e of
&&
t o - 0 . 1 5 i a t 230K was seen. When r e a c h i n g 245K a d r a s t i c i n c r e a s e of t h e i n e l a s t i c t o g e t h e r w i t h a d e c r e a s e of t h e e l a s t i c i n t e n s i t y occurs. The v a l u e of => r e a c h e s -0.32i. A f t e r about t e n hours t h e o r i g i n a l v a l u e of0
t h e rms displacement (-0.15A) i s recovered. X-ray d i f f r a c t i o n p a t t e r n s taken i n s i t u show t h a t now t h e i n i t i a l l y broad l i q u i d - l i k e peaks have sharpened.
However, complete t r a n s f o r m a t i o n t o t h e u s u a l t r i g o - n a l c r y s t a l l i n e phase d i d n o t t a k e p l a c e . F u r t h e r g r a d u a l h e a t i n g t o 320K followed by c o o l i n g t o 80K does n o t r e s u l t i n f u r t h e r s t r u c t u r a l changes. Crys- t a l l i z a t i o n can only b e achieved by a rapid tempera- t u r e i n c r e a s e from 80K t o room temperatures.
Thick samples, even when f r e s h l y p r e p a r e d , show X-
-
r a y d i f f r a c t i o n p a t t e r n s a t low temperatures s i m i l a r t o those found i n t h i n samples a f t e r t h e a n n e a l i n g s t e p n e a r 250K. Between 250 and 260K a g r a d u a l crys- t a l l i z a t i o n i s observed i n t h i c k samples. This pro- c e s s t a k e s p l a c e w i t h o u t l a r g e changes i n t h e Debye- Waller f a c t o r .
From e l e c t r o n d i f f r a c t i o n d a t a /4/ i t i s known t h a t t h e n o n - c r y s t a l l i n e s t r u c t u r e of vapour-quenched Te c o n s i s t s of s h o r t , d i s t o r t e d c h a i n s e a c h c o n t a i n i n g o n l y few molecular u n i t s . The p r e s e n t d a t a show, t h a t a d e f i n e d s t r u c t u r a l change i s p r e s e n t a l r e a d y below t h e c r y s t a l l i z a t i o n p o i n t (-300K i n c a s e of f a s t a n n e a l i n g ) . We assume t h a t i n t h e temperature r a n g e of about 2453 t h e slowly annealed samples have a t t a i n e d a m e t a s t a b l e s t r u c t u r e which undergoes s t a b i l i z a t i o n on t h e time s c a l e of s e v e r a l hours.
Due t o t h e s h o r t c h a i n s t h e quenched sample c o n t a i n s o r i g i n a l l y a r e l a t i v e l y l a r g e number of molecules s i t u a t e d a t t h e end of c h a i n s . For t h e s e molecules a d d i t i o n a l degrees of freedom of motion a r e a c t i v a - t e d a t t h e a n n e a l i n g p o i n t . This l e a d s t o a f a s t re-arrangement and t o n o t i c e a b l e growth i n l e n g h t of c h a i n s . The rms displacement reaches a t t h i s tem- p e r a t u r e a v a l u e which corresponds t o about 1/10 of t h e n e a r e s t neighbour s e p a r a t i o n and i s t h u s compa- r a b l e t o t h e v a l u e s found n e a r t h e m e l t i n g p o i n t . F i n a l c r y s t a l l i z a t i o n i s accomplished b y a g r a d u a l
coalescence of t h e longer c h a i n s i n t o t h e t h r e e f o l d s p i r a l c h a i n s c h a r a c t e r i s t i c f o r c r y s t a l l i n e Te.
This p r o c e s s i s obviously connected w i t h f a r l e s s d r a m a t i c l o c a l re-arrangement.
References
/ I / O'Connor, D.A., Proc. I n t . Conf. Mgssbauer
Spectroscopy, Cracow 1975, Vol. 2 , p. 369 f f .
/ 2 / Champeney, D.C. and Sedgwick, D.F., J. Phys. C L
(1972) 1903.
/ 3 / L i t t e r s t , F . J . , Ramisch, R. and K a l v i u s , G . X . , J . Non-Cryst. S o l i d s
24
(1977) 19.141 Ichikawa, T., Phys. S t a t u s S o l i d i (b)
