A new series of long-range ordered metastable phases in the system M–Sb–Te (M = Ge, Ag)

Abstract

New phases with the general formula (MTe)_n(Sb₂Te₃)(Sb₂)₄ (n = 0, 1, 2, ⋯; M = Ge, Ag) have been synthesized by quenching stoichiometric melts of the pure elements and subsequent annealing. These phases represent a combination of the well-known homologous series (Sb₂Te₃)_m(Sb₂)_k and (GeTe)_n(Sb₂Te₃)_m, including substitution with Ag. Their layered crystal structures contain rocksalt-type building blocks (MTe)_n(Sb₂Te₃) alternating with antimony slabs which represent sections of the A7 structure type of elementary antimony. These blocks, which are stacked along the threefold axis of the trigonal crystals, vary in size according to the composition. Powder diffraction patterns allow predictions concerning the symmetry and periodicity of the stacking sequence. Single-crystal structure refinements of the members Ge_2−xSb₂₊xTe₅·Sb₈ (x = 0.43; Rm, a = 4.258(1) Å, c = 97.23(2), R = 4.38%) and Ag_xSb_3−xTe₄·Sb₈ (x = 0.24; Pm1, a = 4.282(1) Å, c = 28.638(5), R = 5.38%) reveal completely ordered superstructures with extremely long periodicities containing slabs similar to those in Ge₂Sb₂Te₅ and GeSb₂Te₄, respectively, alternating with four antimony layers. According to the ternary phase diagram the phases are metastable but thermal analyses do not evidence phase separation or structural phase transformations before incongruent melting, which indicates that the structures are kinetically very stable. Although the phases are valence compounds, temperature dependence of resistance shows metallic like behaviour in the range of 300–10 K, which probably indicates degenerate semiconductivity.