First-principles investigation of ferroelectricity in epitaxially strained Pb ₂TiO ₄

The structure and polarization of the as-yet hypothetical Ruddlesden-Popper compound Pb ₂TiO ₄ are investigated within density-functional theory. Zone center phonons of the high-symmetry K ₂NiF ₄-type reference structure, space group I4/mmm, were calculated. At the theoretical ground-state lattice constants, there is one unstable infrared-active phonon. This phonon freezes in to give the I2mm ferroelectric state. As a function of epitaxial strain, two additional ferroelectric phases are found, with space groups I4mm and F2mm at compressive and tensile strains, respectively.