Dipole-active optical phonons in YTiO3: ellipsometry study and lattice-dynamics calculations
Abstract
The anisotropic complex dielectric response was accurately extracted from spectroscopic ellipsometry measurements at phonon frequencies for the three principal crystallographic directions of an orthorhombic (Pbnm) YTiO3 single crystal. We identify all twenty five infrared-active phonon modes allowed by symmetry, 7B1u, 9B2u, and 9B3u, polarized along the c-, b-, and a-axis, respectively. From a classical dispersion analysis of the complex dielectric functions ε(ω) and their inverses -1/ε(ω) we define the resonant frequencies, widths, and oscillator strengths of the transverse (TO) and longitudinal (LO) phonon modes. We calculate eigenfrequencies and eigenvectors of B1u, B2u, and B3u normal modes and suggest assignments of the TO phonon modes observed in our ellipsometry spectra by comparing their frequencies and oscillator strengths with those resulting from the present lattice-dynamics study. Based on these assignments, we estimate dynamical effective charges of the atoms in the YTiO3 lattice. We find that, in general, the dynamical effective charges in YTiO3 lattice are typical for a family of perovskite oxides. By contrast to a ferroelectric BaTiO3, the dynamical effective charge of oxygen related to a displacement along the c-axis does not show the anomalously large value. At the same time, the dynamical effective charges of Y and ab-plane oxygen exhibit anisotropy, indicating strong hybridization along the a-axis.