X-ray study of the nGeTe•mBi2Te3 mixed layered tetradymite-like compounds

Single crystals and powders of the ternary mixed layered compounds Ge3Bi2Te6 (phase A), GeBi2Te4 (phase B) and GeBi4Te7 (phase C) have been investigated by X-ray diffraction analysis. The dependencies of the a and c/N parameters of the hexagonal lattices (N is the number of layers per unit cell) on the cation-to-anion ratio for the ternary phases and the GeTe and Bi2Te3 binary compounds have been analyzed. These dependencies were considered to be very close to linear. Based on those results, the lattice parameters were also calculated for other members of the (GeTe)(n)(Bi2Te3)(m) homologous series in the quasi-binary GeTe-Bi2Te3 system for which experimental data were not reported. The GeBi2Te4 and Ge1.5Bi3.5Te5 (D) crystal structures were determined using an automatic diffractometer "Syntex P (1) over bar". Results indicated that Ge and Bi atoms are statistically distributed in octahedral interstices to form mixed-cation layers. In addition to the observed disorder in the cation sublattice, anti-structure defects were also discovered in these compounds. (C) 1998 Elsevier Science S.A.