EPITAXIAL FREE-ENERGY AND STABILITY UNDER EPITAXIAL STRAIN

First-principles ground-state total-energy calculations show that tetr agonal crystals generally have two structures at which the energy is a minimum, which are appropriately called tetragonal phases in equilibr ium. The calculations also show that a small isotropic two-dimensional (epitaxial) strain in the basal plane of a tetragonal phase produces a first-order phase transition to another tetragonal phase. By definin g and calculating a special free energy for the states produced by epi taxial strain, the stability limits of each phase and the occurrence o f a first-order phase transition between them are clearly demonstrated . Epitaxially strained states and the epitaxial free energy are calcul ated for vanadium. The epitaxial free energy as a function of the epit axial stress for these strained states is shown to be similar to free- energy curves calculated for other first-order phase transitions which have analytic descriptions.