Stability and Physical Properties of Metastable Ba-Si Clathrates

In the present study, we have investigated the stability of different Ba-Si clathrates with pressure and temperature using DFT calculations and studied the stability of type I Ba₈Si₄₆ and type IX Ba₂₄Si₁₀₀ clathrates using high pressure─high temperature synthesis technique, calorimetry, and diffraction experiments. When increasing pressure, the type I Ba₈Si₄₆ clathrate and BaSi₆ become more stable. In good qualitative agreement with experiments, the type IX Ba₂₄Si₁₀₀ clathrate becomes stable at a pressure of 1-2 GPa thanks to the pressure and thermal effect of both electronic and vibrational contributions. One can notice that the presence of Ba in the cages of type IX clathrate increases significantly the stability and the mechanical properties of type IX clathrate. We have determined the P-T existence domain of type IX Ba₂₄Si₁₀₀ clathrate from ex situ experiments, which was confirmed by in situ synchrotron X-ray experiments. At room pressure and under an oxidizing atmosphere, the type I Ba₈Si₄₆ and the type IX Ba₂₄Si₁₀₀ clathrates are stable up to about 560 °C and up to about 600 °C, respectively. The thermoelectric properties of type IX Ba₂₄Si₁₀₀ are also reported.