Ab initio lattice dynamics calculation of vibrational density of states and Raman active modes of the olivine mineral Ni₂SiO₄

We present results of ab initio lattice dynamics calculations for the olivine mineral Ni₂SiO₄ using first-principles improved approaches within the Kohn-Sham formulation of density functional theory. Dispersion relationships, vibrational density of states and Raman shifts have been evaluated using the direct method. The calculations are compared with data from inelastic neutron scattering and Raman spectroscopy experiments. Non-spin-polarized calculations lead to clear structural instabilities and discrepancies with the measurements. Magnetic effects are taken into account by including first spin-polarization and then also a Hubbard term to describe correctly the strongly correlated character of the 3d electrons of Ni ²⁺ ions. Results of these models are in excellent agreement with the observations, indicating that the structural stability and dynamics of Ni ₂SiO₄ depend strongly on the magnetic interactions, spin-lattice coupling and electron correlation. These calculations will be used to help separate phonon and spin contributions in on-going studies of magnetic excitations.