  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.
