Ba6Ge25:: low-temperature Ge-Ge bond breaking during temperature-induced structure transformation

In order to find the optimal conditions for sample preparation of the binary germanide Ba6Ge25, the germanium-rich part of the Ba-Ge phase diagram was redetermined by means of metallography, X-ray powder diffraction and differential thermal analysis. The temperature behavior of cubic Ba6Ge25 was investigated both on polycrystalline samples and single crystals. The temperature dependence of the lattice parameter exhibits two anomalies at about 180 and 230K, respectively, which are caused by a structure transformation in two steps with hysteresis. Powder (T = 10-295 K) and single-crystal (T = 95-295 K) X-ray diffraction studies confirm that the symmetry of Ba6Ge25 (space group P4(1)32) remains unchanged within the entire temperature range. A reconstructive behavior of the structural transformation is observed, involving Ge-Ge bond breaking and barium cation displacements. Some Ge4 type atoms (similar to 28%) are so significantly displaced during cooling that Ge4-Ge6 bonds break and new three-bonded (3b)Ge- species (electron acceptors) are formed. Consequently, the number of charge carriers is reduced, affecting the physical properties. The reversible bond breaking involved in this process is a typical characteristic of a solid-state chemical reaction. (c) 2004 Elsevier Inc. All rights reserved.