  The phase transition behaviour of PbTiO3 and BaTiO3 under (111) epitaxial
strain is investigated using density-functional theory calculations. From
tensile strains of +0.015 to compressive strains of -0.015, PbTiO3 undergoes
phase transitions from C2 through two Cm phases and then to R3m. The total
polarisation is found to be almost independent of strain. For the same range of
strains BaTiO3 undergoes phase transitions from a single Cm phase, through R3m
and then to R-3m. In this case the application of compressive strain inhibits
and then completely suppresses the polarisation on transition to the non-polar
R-3m phase.
