Dipole-active optical phonons inYTiO3: 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 $(Pbnm)$ ${\text{YTiO}}_{3}$ single crystal. We identify all 25 infrared-active phonon modes allowed by symmetry $7{B}_{1u}$, $9{B}_{2u}$, and $9{B}_{3u}$ polarized along the $c$, $b$, and $a$ axes, respectively. From a classical dispersion analysis of the complex dielectric functions $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{ϵ}(\ensuremath{\omega})$ and their inverses $\ensuremath{-}1/\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{ϵ}(\ensuremath{\omega})$, we define the resonant frequencies, widths, and oscillator strengths of the transverse-optical (TO) and longitudinal-optical phonon modes. We calculate eigenfrequencies and eigenvectors of ${B}_{1u}$, ${B}_{2u}$, and ${B}_{3u}$ 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 ${\text{YTiO}}_{3}$ lattice. We find that in general, the dynamical effective charges in ${\text{YTiO}}_{3}$ lattice are typical for a family of perovskite oxides. By contrast to a ferroelectric ${\text{BaTiO}}_{3}$, 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 a strong hybridization along the $a$ axis.