Cuboidal bcc to fcc Transformation of Lennard-Jones Phases under High Pressure Derived from Exact Lattice Summations

Abstract

The stability of the body-centered cubic (bcc) compared with the face-centered cubic (fcc) phase at finite pressures is investigated through exact lattice summations using a general (a, b) Lennard-Jones potential (a > b > 3). At zero pressure, the bcc phase is unstable or, at best, metastable for unphysical low values of the exponents (a, b) of the Lennard-Jones potential. From Helmholtz free energy calculations, we demonstrate that the stability of the bcc phase decreases with increasing pressure, with the metastable phase persisting into the high-pressure range up to a high pressure limit at exponent a = 7.6603891 for the repulsive wall. The transition path is chosen to be of Bain type, connecting smoothly the two phases through a series of body-centered tetragonal (cuboidal) lattices.