Orthorhombic distortion energy for monatomic fcc crystals

Abstract

The potential energy barrier and the geometry of the volume-conserving orthorhombic distortion for monatomic face-centered cubic (fcc) crystals in the absence of thermal motion have been investigated. It is shown that the orthorhombic transformation path can continuously transform a fcc crystal structure into six neighboring fcc crystal structures. Each of the new fcc lattices is equivalent to the original one but has a different orientation. The only difference of the six new fcc monatomic structures is that all the atoms change one pair of twelve atoms in the first coordination sphere into another pair and the structure has finite shear strains. The height of the potential energy barrier between the two neighboring stable fcc structures is calculated with the Morse and Mie two-body potentials under constant volume expansion or contraction. The barrier height is several times less than the energy corresponding to dilatations (the melting energy) where the lattice cohesion is lost. The total energy difference between body- and face-centered cubic crystal structures is equal to the minimum barrier height for large values of the effective radius of interatomic interaction. For small values of the radius, the minimum barrier height is less than this difference. The growth of the effective radius of the interatomic interaction decreases the height of the energy barrier. The height increases greatly with the volume contraction and decreases with the volume expansion of the fcc structures.