Phase diagram of the Ge-rich of the Ba-Ge system and characterisation of single-phase BaGe4

The Ba-Ge binary system has been investigated by several authors, but some uncertainties remain regarding phases with Ba/Ge <= 2. The goal of this work was to resolve the uncertainty about the current phase diagram of Ba-Ge by performing DTA, X-ray powder diffraction, metallographic and chemical analyses, and measurements of the electrical conductivity and viscosity. The experimental Ba-Ge phase diagram over the composition range of 50-100 at.% Ge was constructed from the cooling curves and single-phase BaGe4 grown by the Czochralski crystal pulling method was characterised. Semiconducting BaGe4 crystallised peritectically from the liquid phase near the eutectic. In the liquid state, the caloric effects were observed in the DTA curves at 1050 degrees C where there are no definite phase lines in the Ba-Ge phase diagram. These effects are confirmed by significant changes in the viscosity and electrical conductivity of a Ba-Ge alloy with eutectic composition at this temperature. A phenomenological model based on two different approaches, a phase approach and a chemical approach, is proposed to explain the isothermal liquid-liquid phase transition observed in the Ba-Ge system from the Ge side. Our results suggest that this transition is due to the peritectic reactions in the liquid phase. This reversible phase transition results in the formation of precursors of various metastable clathrate phases and is associated with sudden changes in the structure of Ba-Ge liquid alloys. Characteristics of both first-and second-order phase transitions are observed. Charge transfer appears to play an important role in this transition. (C) 2014 Elsevier B.V. All rights reserved.