The effect of crystal equilibrium condition on the lattice dynamical model parameters of a body centred cubic metal

Using barium, for which results that compare favourably with experiment have been obtained, the effect on the lattice dynamical model parameters of a body centred cubic metal of explicitly imposing the crystal equilibrium condition on various ion-ion coupling schemes when used with the modified electron-ion interaction scheme of Bhatia has been studied. When the equilibrium condition is not used, the calculated model parameters lose their physical significance with the result that at equilibrium, the sum of the ionic and electronic pressures is not zero. This inconsistency is removed when the equilibrium condition is explicitly imposed on the models. It is not meaningful to compare the two results with and without the equilibrium condition imposed, not only because the imposition of the equilibrium condition reduces the number of independent parameters of the models by one, but also because the results apply to different states of the crystal. Also the comparison of calculated model parameters and measured quantities such as phonon frequencies make sense only when both are determined for the same state of the crystal. This condition is satisfied only when the equilibrium condition, which has unfortunately been ignored in most lattice dynamical studies, is explicitly imposed on lattice dynamical models.