Equations of state of α-SiC (6H) and βʹ-Mg2Si1.1 from single-crystal X-ray diffraction data and novel high-pressure magnesium silicide Mg2Si7

Abstract

AbstractSiC and (Fe, Mg)-silicide are candidate phases forming under reducing conditions in the Earth and planetary interiors. However, structural studies of SiC and Mg2Si at high pressure and their thermal stability are presently lacking. In this work, we applied single-crystal X-ray diffraction in a diamond anvil cell at high pressure and determined the equations of state of α-SiC (6H) and βʹ-Mg2Si1.1 up to 60 and 40 GPa, respectively, yielding bulk moduli of 226.0(4) and 56(1) GPa. We also report the formation of a novel orthorhombic Mg2Si7 phase upon laser heating βʹ-Mg2Si1.1 at ~ 45 GPa and 2000 °C [Pbam, a = 7.16(1) Å, b = 12.490(3) Å, c = 2.6545(3) Å, V = 237.5(3) Å3]. The structure of this compound contains layers formed by irregular 12-member silicon rings, which are arranged in channels filled with both Mg and Si atoms. No signs of the Mg2Si7 phase were detected upon releasing the pressure in the DAC, which suggests that this phase is unstable under ambient conditions.