Tetragonal states from epitaxial strain on metal films

The tetragonal states produced by isotropic pseudomorphic epitaxial strain in the (001) plane on a tetragonal phase of a crystal are calculated for V, Ti, Rb, Li, K, and Sr from first-principles electronic theory. It is shown that each metal has two tetragonal phases corresponding to minima of the total energy with respect to tetragonal deformations (and hence are equilibrium phases) and that the equilibrium phases are separated by a region of inherent instability. The equilibrium phase for any strained tetragonal state can thus be uniquely identified. Lattice constants and relative energies of the two phases and the saddle point between them are tabulated, as well as the tetragonal elastic constants of each phase.