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 230 K, 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 P4₁32) 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 (∼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.