Dipole-active optical phonons in YTiO_3: ellipsometry study and lattice-dynamics calculations

TL;DR

This study combines spectroscopic ellipsometry and lattice-dynamics calculations to identify and analyze all infrared-active phonon modes in orthorhombic YTiO_3, providing detailed insights into its lattice vibrations and effective charges.

Contribution

It presents a comprehensive experimental and theoretical analysis of phonon modes and effective charges in YTiO_3, including mode assignments and anisotropic charge behavior, which was not previously detailed.

Findings

All 25 infrared-active phonon modes were identified and assigned.

Dynamical effective charges are typical for perovskite oxides, with anisotropic features.

No anomalously large effective charge for oxygen displacement along the c-axis was observed.

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) YTiO_3 single crystal. We identify all twenty five infrared-active phonon modes allowed by symmetry, 7B_1u, 9B_2u, and 9B_3u, polarized along the c-, b-, and a-axis, respectively. From a classical dispersion analysis of the complex dielectric functions \tilde\epsilon(\omega) and their inverses -1/\tilde\epsilon(\omega) we define the resonant frequencies, widths, and oscillator strengths of the transverse (TO) and longitudinal (LO) 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 YTiO_3 lattice. We find that, in general, the dynamical effective charges in YTiO_3 lattice are typical for a family of perovskite oxides. By contrast to a ferroelectric 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 strong hybridization along the a-axis.