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

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 calcul ations (LDA and GGA). The unit cell dimension and the ambient temperature b ulk modulus and its pressure derivative are determined to be V-0 = 8.29(+/- 0.03) Angstrom (3)/atom, K-0 = 308(+/-5) GPa and K'(0) = 4+/-(0.2), in exce llent agreement with theoretical calculations within the LDA and GGA. The c alculated sh;ear modulus is two to three times those of corresponding oxide spinels, and there is a substantial Cauchy violation, indicating a materia l with strong covalent bonding that is likely to be extremely hard.