Ab-initio total energy calculation of α- and β-silicon nitride and the derivation of effective pair potentials with application to lattice dynamics

The ground state total energies of alpha- and beta-Si(3)N(4) crystals are calculated by an ab-initio method based on local density functional theory. The calculated bulk modulus and pressure coefficients for both crystals are in good agreement with recent experimental measurements. Interatomic pair potentials of the Buckingham form with an additional repulsive term are derived using ab-initio effective charges and total energy data. The effective pair potentials give excellent results on equilibrium lattice parameters, elastic constants, phonon spectra, and lattice specific heat for both crystals. The zone-center optical phonon modes are in good agreement with measured infrared and Raman spectra, The pair potentials for alpha- and beta-Al(2)O(3) are similar but not the same, despite their similarities in the local short-range order. Applications of the pair potentials to the simulation and modeling of surfaces and interfaces in Si(3)N(4) ceramics are discussed.