Compression behavior and phase transition of β-Si₃N₄ under high pressure

The compressibility and pressure-induced phase transition of β-Si₃N₄ 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-Si₃N₄ remained stable under high pressure up to 37 GPa. Upon increasing pressure, β-Si₃N₄ transformed to δ-Si₃N₄ at about 41 GPa. The initial β-Si₃N₄ was recovered as the pressure was released to ambient pressure, implying that the observed pressure-induced phase transformation was reversible. The pressure-volume data of β-Si₃N₄ was fitted by the third-order Birch-Murnaghan equation of state, which yielded a bulk modulus K₀=273(2) GPa with its pressure derivative K'₀=4 (fixed) and K₀=278(2) GPa with K'₀=5. Furthermore, the compressibility of the unit cell axes (a and c-axes) for the β-Si₃N₄ demonstrated an anisotropic property with increasing pressure.