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MÖSSBAUER STUDIES OF SYSTEM Sn1-xGexTe
V. Fano, E. Maniezzi, I. Ortalli
To cite this version:
V. Fano, E. Maniezzi, I. Ortalli. MÖSSBAUER STUDIES OF SYSTEM Sn1-xGexTe. Journal de Physique Colloques, 1980, 41 (C1), pp.C1-325-C1-326. �10.1051/jphyscol:19801120�. �jpa-00219603�
JOURNAL DE PHYSIQUE Colloque C1, suppl&ment au n O 1, Tome 41, janvier 1980, page C1-325
F~SSBAUER STUD1 ES OF SYSTEM SnlmXGe,Te V.
an of
E. Maniezzi and I. ~ r t a l l i *I n s t i t u t e of Physics - U n i v e r s i t y and GNSM - Parma ( I t a l y ) .
+
Also MASPEC Laboratory.*
~ a t t e d r a Fisica - Medicine Faculty.GeTe and SnTe form a complete series of solid solutions('). At room temperature, GeTe has a rhomboedral (C:~-R~.) structure, i.e. a distorted NaCl structure('), which transforms to the NaCl (Bl) structure at 670 K. At room temperature, SnTe has the NaCl structure and the phase transition at low temperature critically depends on the carriers concentrati~n(~,~). In a complete ly miscible pseudo-binary alloy, such as Snl-xGexTe, the cation sublattice is occu- pied by Sn and Ge atoms, at random. Howe- ver, it is very important to know exactly the minimum value of x allowing the system to be considered a true pseudo-binary alloy.
In this paper the behaviour of the Mossbauer parameters and the associated phenomena, at very low concentration of Ge in SnlVxGexTe (xc 0.1)
,
are considered with special attention.Introduction of Ge atoms in SnTe ma- trix, both, at high concentrations (pseudo- binary alloy) and at micro-concentrations
(SnTe doped with Ge), does not substantial- ly modify the electrical properties, but significantly modifies the structural pro- perties. In fact, experiments with X-rays ( 4) have shown a continuous transformation Erom cubic to rhomboedral phase, while the tran- sition temperature T c increases with GeTe content. But this behaviour has only been observed for SnTe concentration up to 90%.
For higher concentrations of SnTe, the trag sition temperatures have been inferred by extrapolation by Muldawer et a1.(4) under the assumption that the undoped SnTe tran- sition temperature is 0°K. On the basis of these and our previous results relevant to undoped ~ n ~ e " ) we have been led to study
the Snl-,GeXTe system for 0 c x < O . l in or-
der to analyze its behaviour at very low Ge concentration.
The procedures for sample prepara- tion and for collection and analysis of Mossbauer spectra have been described pre- viously in (6). When taking the tin tellu- ride homogeneity region into account (71
,
we considered the following compositions:
(Sn49. 7Ge. 1STe50.3)
,
(Sn49. 7Ge.3Te50.3)'
(Sn49. 7Ge. gTeso. 3) and (Sn4a. 7Gei, 6Tes0. 3). The absorbers were prepared from powders obtained by crushing single-crystals of the alloy, which were then submitted to annealing at 40O0C. All absorbers had the same quantity of compound (68 mgr.).
In Fig. 1 the values of
r
-experimen- tal (full width at half maximum of the ex- perimentally observed resonance line) ver- sus temperature for our four different compositions are given. The structural phase transition temperatures (Ts) versus Ge concentration are given in Fig. 2. In Fig. 3 the recoil free fractions (fa) ver- sus temperature for the same compositions are reported. Finally in Fig. 4 we give the Mossbauer temperatures(0,
) which follow from the slopes of the (straight)lines o f Fig. 3.
-- -
Fig. 1 : Temperature de- pendence of observed half- widths for:
A) Sn49.7Ge.15Te50.3' B, Sn49. 7Ge. 3oTe50. 3'
Sn49. 7Ge. 6oTe50. 3 and Sn48. 7Ge1. 6Te50. 3'
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801120
JOURNAL DE PHYSIQUE
Fig. 2: Structural phase transition temperature versus Ge concentration. Note that accordhg to the extrapolated dashed line, a temperature of about 250 K would be expected for x=1.6.
Fig.4: Ge concentr. dependence of the bl6ssbauer temperature On
.
a present results, I, data of Rigamonti et al.i9).
As one can see from the results of Fig.1, the behaviour of the half-width as a function of the x concentration of Ge is the same as long as x is not too high, i.e.
in the three cases A),B) and C) where it is found a change of slope at a given va- lue of T. On the other hand, for sufficien tly low concentration of Ge, the behaviour
of Fs is similar to that of undoped
~ n T e ( ~ ) which presents a structural phase transition at 160 K. However this change of slope disappears for higher values of x as shown by curve D) in Fig. 1, which,
in our opinion, is due to the fact that the compound has
.
different structural characteristics at these concentrations.This agrees with both the results of Fig.2, which tend to indicate a transition tempera ture of about 250K at x=1.6, and the beha- viour of the Mossbauer temperatures (Q,)
as a function of x. In fact, as shown in Fig.4, the values of 0 , are x-independent within the experimental errors for x + 0.6
and change according to the results of Ri- gamonti et al. when x is increased
(xa1.6).
The explanation of this behaviour fol lows from the analysis of the consents of Ge in rapport to total number of cationic atoms (Sn+Ge) at stdchiometric ratio in hg mogeneity region(7). In fact, in correspog dence of the first three concentrations, the Mossbauer parameters are approximately those of undoped SnTe, just because the content of Ge maintains the SnTe compound inside the homogeneity region. For high concentration of Ge, the staichiometric ra- tio in SnTe is found outside of the homo- geneity region [(sn+~e) :Te=l:l
]
which means that the SnTe compound forms with Ge a ternary system.To conclude, the studies performed on these compounds, have revealed a new inte- resting application of the Mossbauer Spec- troscopy, by showing how it is possible to obtain the minimum value of doping element concentration at which the system becomes a true pseudo-binary alloy. In particular, in our case, we conclude that for
Snl-xGexTe, this happens when x is within the values .988 and .968.
References
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(1958) 817./2/ Bierly, J.P., Erhldawer, L. and B e c k , O., Acta Metall. 11 (1963) 447.
/3/ Iizumi,M., W g w M , Y . , Komatsu-b-abara,K.F. and KSto ,Y., J. F%ys .Sot‘. Japan
38
(1975) 443./4/ Kobayashi,~x. I;, Kato,Y., Katayama,Y. and Ko- 19atsubara,K., Phys.Rev.Lett.
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(1976) 772./5/ Fano,V., Fedeli,G. and Ortalli,I., Solid State Commun.22 (1977) 467.
/6/ Fano ,V.=d Ortalli, I., J.Chem.Phys.g(l974) 5017.
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/8/ Eamonti,A. Petrini,G.
