Structural Study of (GeTe)1–x[(Ag2Te)0.5333(Bi2Te3)0.4667]xSolid Solutions

Abstract

The reversible cubic ↔ rhombohedral (O h5 ↔ C 5V3) phase transition of GeTe‐rich (GeTe)1–x[(Ag2Te)0.5333(Bi2Te3)0.4667]xsolid solutions is studied by means of high temperature X‐ray analysis. Temperature and composition changes of the rhombohedral lattice parameters are found. They prove the displacive nature of the solid solution phase transition. With increasing amount of heterovalent Ag and Bi atoms replacing Ge cations the phase transition temperature Tc decreases.

Thermoelectric Power of Some Compositions of GeTe-Rich (GeTe)1—x(AgBiTe2)2 Solid Solution

“…As the VB structures of GeTe-AgBiTe, SS have not been investigated until now, to explain the observed temperature dependence of its transport coefficients we supposed that as in the analogous system GeTe-AgSbTe, [29] the rhombohedral SS retain not only the crystal structure [13] but also the band structure of GeTe. C extrema of the first BZ of the cubic lattice in the ratios 1 : 3 (L) and 6: 6 (C) due to the O;-C:, phase transition in accordance with the experimental results about the lattice parameters [13].…”

“…In the phase transition region ( T = T,) with T, = 700 K for GeTe [l] and T, x 420 K for SS (GeTe),,,,(AgBiTe,),., [13] no anomalous changes in the thermoelectric power are observed. At T, , , > T, S undergoes a maximum and after this it decreases.…”

“…(131 The model can be extended to involve the case of the band inversion transition (the position of the bands in SnTe and GeTe are inverted to the bands in PbTe) [21] if E, = E(L;) -E(Li) and (13) is rewritten as This model was called the modified Bangert model. strains by an amount [22] It is well known that in a cubic crystal the band edge at the n-th valley shifts with the…”

“…The position of the Fermi level is derived from the experimental value of the thermoelectric power supposing that the two-band Kane model for narrow-gap semiconductors especially for IV-VI compounds[15,34] is valid and taking into account the band nonparabolicity with the modified Fermi-Dirac integrals[35]. The energetic positions of the T, valley and the three L, valleys in the rhombohedral phase (0 5 x 5 0.23) are determined from the experimental results of the SS interaxial angle[13]…”