MÖSSBAUER EFFECT STUDY OF THE Sn0.9Ge0.1Te SYSTEM

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MÖSSBAUER EFFECT STUDY OF THE Sn0.9Ge0.1Te SYSTEM

V. Fano, E. Maniezzi, I. Ortalli, N. Burriesci, M. Petrera

To cite this version:

V. Fano, E. Maniezzi, I. Ortalli, N. Burriesci, M. Petrera. MÖSSBAUER EFFECT STUDY OF THE Sn0.9Ge0.1Te SYSTEM. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-624-C2-626.

�10.1051/jphyscol:19792217�. �jpa-00218597�

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JOURNAL DE PHYSIQUE Colloque C 2 , suppl&ment au n o 3, Tome 40, mars 1979, page C2-624

MOSSBAUER EFFECT

STUDY

OF

THE

SnO. gGeo. l T e

SYSTEM

V. ~ano*, E. Maniezzi, I. ~rtalli'", N. ~urriesci+ and M. ~etrera+

Physics I n s t i t u t e , University and GNSM, Puma, I t a l y

+~iissbauer Luboratory, Daegani Research I n s t i t u t e , Novara, I t a l y

R l s d . - La correlation entre les parametres Mijssbauer et l'apparition de modes mous dans le spectre de phonons, lils 3 la prlsence d'une transition de phase dans les matgriaux ferro6lectriques. est rediscutee 1 la lumilre des d6saccords observes dans la littsrature pour le system Sno.gGe~.~Te.

Dans ce travail on n'observe pas de variation anormale en fonction de la tempgrature entre 80 et 300K pour la fraction sans recul et pour la conductivitl Blectrique, mais seulement pour

r

explrimental.

Abstract.- The relation between the Gssbauer parameters and the appearance of soft modes in the phonon spectrum related with the phase transition for ferroelectric materials is rediscussed in the light of the discrepancies existing in the literature about the Sno.sGeo.lTe system. In the present work no anomalous temperature dependence of the recoil-free fraction and also of the electrical conductivity is observed between 80 and 300 K, but only in linewidth.

1. Introduction.- X-rays diffraction measurements have shown that the intermetallic ternary GeTe-SnTe system forms a continuous series of solid solutions which present a cubic to rhombohedra1 phase transi-

tion at a composition-dependent temperature T /1,2/.

The compounds of this system are considered to be- long to the class of ferroelectric materials of the displacive type.

SnTe and d e ~ e were therefor9 assumed to be ferroelectric, because they show the same anomalous temperature dependence of the transverse optical branch as SrTiO3 and BaTiOa respectively, which are known to be ferroelectric.

Moreover the existence of a ferroelectric phase for ionic diatomic crystals cannot be excluded in principle 13-71.

With respect to the SnxGel-xTe solid solutions, the phonon dispersion curves obtained from neutron diffraction studies for the composition x = 0.935 /8/, show that the TO modes frequency decreases going to lower temperatures, whereas the short wavelength modes frequency is practically temperature- independent. It is therefore concluded that the phase transition in SnxGe,-xTe is ferroelectric of the displacive type and takes place as consequence of the crystal instability due to the large fluctua- tions introduced by the TO mode for vanishing fre- quencies.

The correlation between ferroelectric phase transition and anomalv of the ~ h o n o n sDectrum (soft

+ Also MASPEC-CNR Laboratory, P a m , Italy.

++ Present address : Physics Institute. University- Ferrara, Italy.

mode) suggests that this transition could be asso- ciated to an anomaly of the Debye-Waller factor and therefore of the temperature-dependence of the recoil-free fraction f.

Several authors 19-131 have studied the reduc- tion in f at the transition temperature and there is not general agreement between their results.

In the specific case of Sno.goGeo.loTe discrepancies are found between different Mijssbauer analysis.

sis.

A remarkable decrease of f (%25%) was observed by Chekin et al. 1141 and by Rigamonti 1151 at the temperature of about 120 K, accompanied by a slight increase of the linewidth for lower temperatures.

However similar experiments done by Knauer I161 did not confirm these results. The problem of the effect of the soft modes on the Miissbauer parameters is therefore not yet completely clarifyed for the SnTe- GeTe system at low GeTe concentrations. Recently it was demonstrated that also SnTe can either show or not show an anomalous temperature dependence of the recoil-free fraction f, depending on the carrier concentration 1171.

2. Experimental.- The procedures for collecting and analysing the Miissbauer spectra have been described previously 1181. The experiments were performed using constant acceleration spectrometers connected with multichannel analysers operated in the time mode. As a critical-edge filter for the tin K

a X-rays a 0.065 m Pd foil was placed between the

~ a " ~ ~ ~ n 0 3 source and the absorbers, whose temperature could be varied in the range 77 to 300 K (t0.5 K).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792217

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The Sno.goGeo.loTe single crystal was grown by direct reaction of the ultrapure (99.999 X ) compo- nents as described in /18/ and shows a p-type conduc tivity. The carrier concentration, as determined by Hall effect measurements, is 7.2 x 10" ~ m - ~ at room temperature and 1.0 x 1021 cm-3 at liquid nitrogen temperature.

We carried out three series of measurements on different absorbers taken from the central part of the same crystal. The absorbers were prepared by the powders obtained after crushing the chalcogenide crystal and by pressing them either into plexiglass sample holders or between to thin mylar foils.

3. Results and discussion.- The absorption peak area values obtained from the four series of ~sssbauer measurements are reported in figure 1 as a function of temperature in a semilogaritmic plot.

Fig. 1 : Area under the ' ~ n Miissbauer resonance curves versus temperature of three powder samples obtained from the same Sno.soGeo.loTe single crystal.

In the thin absorber approximation this curve dif- fers only for a constant from the recoil-free fraction f. No appreciable sistematic anomaly could be found in the range 110 to 130 K, where the cubic to rhombohedra1 phase transition /I/ and a decrease in the f value /14,15/ were observed by other authors.

This result is in agreement with that reported in 1161 and gives support to the debate concerning the observation through the f Sssbauer parameter of phase transitions eventually assisted by soft mode phonons

.

Also the electrical conductivity a, reported in figure 2 together with the carrier concentration p, does not show any kink in the curve which repre- sents its temperature dependence between 77 and 300 K.

Our opinion is that the carrier concentration

probably should be taken into account for the expla- nation of the phenomena connected with the ferroelectric phase transition of the low GeTe content compounds of the SnTe-GeTe system.

It has been demonstrated through resistivity- versus-temperature measurements that for SnTe the transition temperature T can be shifted from about

100 K to near 0 K by simple varying the carrier concentration between

lo2'

^and

loz1

^{~ m - ~}^/19/.

Fig. 2 : Electrical conductivity u and carrier concentration p versus temperature of the Sno.goGeo.loTe single crystal.

Such variations are easily obtained in the usual preparations as a consequence of crystal defects, impurities and small non-stoichiometries. It is therefore possible that the relatively high carrier concentration reduces T via the electron

-

^TO-phonon

interaction /19,20/. Some authors suggest namely that it can be still possible to observe no anomaly in the Msssbauer parameters near the transition temperature for a given sample, because of unfavorable ratios between the density of states of the anomalous and the other phonon modes /16,21/.

The anomalous phonon mode is usually considered to be originated from a softening of the TO mode at the center of the Brillouin zone /14,15/. However the observation of soft acoustical modes near the boundary of the Brilloiin zone has also been reported 122-24/.

Keune /21/ and Knauer /16/ have demonstrated that the contribution to f of the TO soft modes at long wavelength is very small.

On the other hand, in terms of the Kristofel- Konsin model /25/ the perturbation of the band ener- gies, which causes the dependence of Tc from the carrier density /20/, can also alter the density of states of the various phonon modes and therefore either appreciably influence or not the temperature

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JOURNAL DE PHYSIQUE

dependence of f.

We are thus led to the conclusion that the observation of an influence of the ferroelectric phase transition on the Mgssbauer parameters depends strongly from the crystal and absorber preparation and then from the carrier density.

The investigation should be carried out near the proper transition temperature, determined in an independent way; it will be successful only in the case when for that particular density of carriers the number of soft modes responsable for the crystal instability at the transition is relatively large, so as to produce a relative variation of some per- cent in the f value.

From the results of the present work the existence of a phase transition near 120 K for Sno.90 Geo,loTe is not confirmed by recoilless fraction measurements.

Namely as a further indication the temperature dependence of the ~gssbauer resonance linewidth is reported in figure 3. The curves show a continuous variation about 130 K for these measurements, espe- cially for one series.

Fig. 3 : Linewidth of the '19sn Gssbauer resonance curves versus temperature. Same samples as in figure 1.

This sort of anomalous behaviour might be due to the quadrupole splitting caused by phase transition, because recent experiments of the elastic cons- tants and the specific heat in Sno.glGeo.ogTe show the ferroelectric phase transition at 130 K 126,271.

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Acknowledgment.- We are indebted to Professor I. Hatta for valuable discussions.