Compression behavior and phase transition of β -Si ₃ N ₄ under high pressure

Abstract

The compressibility and pressure-induced phase transition of β -Si 3 N 4 were investigated by using an angle dispersive x-ray diffraction technique in a diamond anvil cell at room temperature. Rietveld refinements of the x-ray powder diffraction data verified that the hexagonal structure (with space group P63/m, Z = 2 formulas per unit cell) β -Si 3 N 4 remained stable under high pressure up to 37 GPa. Upon increasing pressure, β -Si 3 N 4 transformed to δ -Si 3 N 4 at about 41 GPa. The initial β -Si 3 N 4 was recovered as the pressure was released to ambient pressure, implying that the observed pressureinduced phase transformation was reversible. The pressure-volume data of β -Si 3 N 4 was fitted by the third-order Birch-Murnaghan equation of state, which yielded a bulk modulus K 0 = 273(2) GPa with its pressure derivative K 0 = 4 (fixed) and K 0 = 278(2) GPa with K 0 = 5. Furthermore, the compressibility of the unit cell axes (a and c-axes) for the β -Si 3 N 4 demonstrated an anisotropic property with increasing pressure.