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 $B19\mathrm{}$ phase will undergo a low-temperature phase transition to a monoclinic ${B19\mathrm{}}^{\prime }$ ground state. Within a soft-mode framework, we relate the $B19\mathrm{}$ structure to the cubic $B2\mathrm{}$ structure, observed at high temperature, and the ${B19\mathrm{}}^{\prime }$ structure to $B19\mathrm{}$ via phonon modes strongly coupled to strain. In contrast to NiTi, the $B19\mathrm{}$ 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.

• Received 2 July 2002

DOI:https://doi.org/10.1103/PhysRevB.67.024101