Structures of stable and metastable Ge₂Bi₂Te ₅, an intermetallic compound in a GeTe-Bi₂Te₃ pseudobinary system

Ge2Bi2Te5 in the GeTe-Bi2Te3 pseudobinary system has two single-crystalline phases: a metastable phase with an NaCl-type structure and a stable phase with a nine-layer trigonal structure. In the metastable phase, the structure consists, in the hexagonal notation, of infinitely alternating stacks of Te and Ge/Bi layers at equal intervals along the c axis. On the other hand, in the stable phase those two layers are stacked alternately nine times to form an NaCl block. The blocks are then piled to construct a nine-layered trigonal structure with cubic close-packed stacking. Both ends of each block are covered with Te layers, contrary to the infinite alternation of Ge/Bi and Te layers in the structure of the metastable phase. The Ge/Bi layers in the metastable phase contain as much as 20 at. % vacancies; on the other hand, those in the stable phase are filled with atoms. These two crystalline phases in Ge2Bi2Te5 have identical atomic configurations to the two corresponding phases found in Ge2Sb2Te5.