High pressure-high temperature synthesis and elasticity of the cubic nitride spinel γ-Si3N4

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Abstract
The compressional behaviour of a new dense form of silicon nitride with the cubic spinel structure is studied by energy dispersive x-ray diffraction, following in situ synthesis from the low pressure form by laser heating in the diamond anvil cell, combined with theoretical density functional calculations (LDA and GGA). The unit cell dimension and the ambient temperature bulk modulus and its pressure derivative are determined to be V0 = 8.29(±0.03) Å3/atom, K0 = 308(±5) GPa and K'0 = 4±(0.2), in excellent agreement with theoretical calculations within the LDA and GGA. The calculated shear modulus is two to three times those of corresponding oxide spinels, and there is a substantial Cauchy violation, indicating a material with strong covalent bonding that is likely to be extremely hard.