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

TL;DR

This study uses first-principles calculations to explore the structural energetics of PdTi and PtTi, predicting phase transitions and relating structures to high-temperature phases through phonon modes.

Contribution

It provides the first-principles prediction of a low-temperature phase transition in PdTi and PtTi, linking structural changes to phonon modes and strain effects.

Findings

Predicted low-temperature transition from B19 to B19' phase.

Related B19 structure to high-temperature B2 phase via soft phonon modes.

B19 structure is very close to hcp, suggesting transition mechanisms similar to bcc-hcp.

Abstract

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.