Change in the bulk modulus at the B1-B2 phase transition

Ab initio Hartree-Fock calculations for NaCl are used to derive the changes across the B1-B2 phase transition in both the bulk modulus (ΔK_T) and the transverse infrared frequency (Δν_TO). ΔK_T is found to be positive but very small, and Δν_TO negative and relatively large (≈20%). In contrast, using a relationship based on lattice dynamics with rigid ions and short-range repulsion only between nearest neighbors, Hofmeister [Phys. Rev. B 56, 5835 (1997)] has recently deduced from the observed drop in ν_TO, itself in good agreement with the Hartree-Fock calculations, that there should be a marked drop in K_T. We have, therefore, also calculated ΔK_T and Δν_TO for NaCl using lattice dynamics, with a shell model that incorporates short-range potentials between both first and second neighbors, obtaining good agreement with the Hartree-Fock calculations. This calls into question the use at high pressures of a semiempirical relation between infrared frequencies and K_T, based on a model which includes only nearest-neighbor short-range interactions. Similar calculations for CaO give a much larger increase in the bulk modulus across the phase boundary, still accompanying a dramatic drop in ν_TO.