Tracing the phase formation and stability of the clathrate phase BaGe5

Abstract

Thermal stability and phase formation of the clathrate phase BaGe5 and its formation from the clathrate-I phase Ba8Ge43 square(3) (square = vacancy in the crystal structure) have been investigated by microstructure analysis, differential scanning calorimetry (DSC), and powder X-ray diffraction (PXRD). The experiments confirm the peritectic formation of the high-temperature phase Ba8Ge43 square(3) from alpha-Ge and liquid at 810 degrees C, and the eutectic temperature of Ba8Ge43 square(3) and Ba6Ge25 at 808 degrees C. At T = 770 degrees C, Ba8Ge43 square(3) decomposes by a eutectoid reaction to Ba6Ge25 and alpha-Ge, and at T = 520 degrees C, the low-temperature phase BaGe5 forms in a peritectoid reaction from these two phases. BaGe5 was synthesized by decomposition of the high-temperature clathrate-I phase Ba8Ge43 square(3) at temperatures between 350 degrees C and 520 degrees C. Annealing and DSC experiments verify that BaGe5 is an equilibrium phase at ambient pressure. The formation of BaGe5 from Ba8Ge43 square(3) was traced through microstructure analysis, revealing microdomains with pleochroic behavior in polarized light.