Identification of metastable phases: face-centred cubic Ti

First-principles total-energy electronic structure calculations are used to find the epitaxial line of tetragonal Ti, i.e., the line of body-centred tetragonal (bct) structures produced by isotropic epitaxial strain on the (001) plane of tetragonal phases of Ti. An fcc phase and a higher-energy noncubic bct phase are found at minima of the total energy with respect to tetragonal deformations, but the bcc structure is found to be a saddle point of energy. The calculation yields the strained structures, the corresponding stresses, and the elastic stiffness coefficients on the epitaxial line. A segment of the epitaxial line between the two phases is shown to be inherently unstable, so unique meaning can be given to designating a structure as strained fcc Ti. The bulk structure of a film of Ti on Al{001} is shown to be strained fcc Ti, a new phase not on the phase diagram.