Lattice dynamics and Raman spectrum of supertetragonal PbVO3

Lead vanadate PbVO3 is a polar crystal with a P4mm space group at ambient conditions. It is isostructural with the model soft-mode driven ferroelectric PbTiO3, but differs from it by the so-called 'supertetragonal' elongation of its unit cell. In this paper, we report a combined study of the lattice dynamics of PbVO3 by Raman spectroscopy at room temperature and first-principle calculations. All zone-center transverse optical (TO) phonon modes are identified by polarized, angle-dependent Raman spectroscopy and assigned as follows: E modes at 136, 269, 374 and 508 cm-1, A1 modes at 188, 429 and 874 cm-1 and B1 mode at 319 cm-1. The calculations confirm the experimental symmetry assignment and allow to obtain the longitudinal (LO) phonons wavenumbers. Besides, we analyze the mode eigenvectors in detail, in order to identify the atomic displacements associated with each mode and compare them with PbTiO3. In spite of their differences in chemistry and strain, the phonon eigenvectors are found to be remarkably comparable in both compounds. We discuss the position of the ferroelectric soft mode in PbVO3 as compared to PbTiO3. A sizeable splitting of the B1+E modes appears as a characteristic feature of supertetragonal phases. The peculiarity of the vanadyl V-O bond frequency in PbVO3 is also addressed.