First-principles study of the structural energetics of PdTi and PtTi

The structural energetics of PdTi and PtTi have been studied using first-principles density-functional theory with pseudopotentials and a plane-wave basis. We predict that in both materials, the experimentally reported orthorhombic B19 phase will undergo a low-temperature phase transition to a monoclinic B19(') ground state. Within a soft-mode framework, we relate the B19 structure to the cubic B2 structure, observed at high temperature, and the B19(') structure to B19 via phonon modes strongly coupled to strain. In contrast to NiTi, the B19 structure is extremely close to hcp. We draw on the analogy to the bcc-hcp transition to suggest likely transition mechanisms in the present case.