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The compressibility and thermal expansion of the cubic silicon nitride (c− Si 3 N 4) phase have been investigated by performing in situ x-ray powder-diffraction measurements using synchrotron radiation, complemented with computer simulations by means of first-principles calculations. The bulk compressibility of the c− Si 3 N 4 phase originates from the average of both Si-N tetrahedral and octahedral compressibilities where the octahedral polyhedra are less compressible than the tetrahedral ones. The origin of the unit cell expansion is revealed to be due to the increase of the octahedral Si-N and NN bond lengths with temperature, while the lengths for the tetrahedral Si-N and NN bonds remain almost unchanged in the temperature range 295–1075 K.