Phonon frequencies of tetragonally strained PbTiO3 from first principles

First-principles total-energy calculations based on density functional theory within the local-density approximation were employed to investigate transverse and longitudinal optical phonon mode frequencies of ferroelectric PbTiO3 as a function of the aspect ratio of the tetragonal unit cell. Systematic frequency shifts concern practically all polar-mode frequencies. Variation of c/a ratio from 1 to 1.13 is also accompanied by considerable phonon eigenvector changes. Results can be compared with optical spectroscopy investigations of strained PbTiO3 epitaxial thin films.