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

New phases with the general formula (MTe)(n)(Sb(2)Te(3))(Sb(2))(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 (Sb(2)Te(3))(m)(Sb(2))(k) and (GeTe)(n)(Sb(2)Te(3))(m), including substitution with Ag. Their layered crystal structures contain rocksalt-type building blocks (MTe)(n)(Sb(2)Te(3)) 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-x)Sb(2+x)Te(5).Sb(8) (x = 0.43; R3[combining macron]m, a = 4.258(1) A, c = 97.23(2), R = 4.38%) and Ag(x)Sb(3-x)Te(4).Sb(8) (x = 0.24; P3[combining macron]m1, a = 4.282(1) A, c = 28.638(5), R = 5.38%) reveal completely ordered superstructures with extremely long periodicities containing slabs similar to those in Ge(2)Sb(2)Te(5) and GeSb(2)Te(4), 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.