Anharmonic phonon spectra of all four phases of BaTiO₃

The standard first-principles approach for calculating lattice dynamics in crystals is based on the harmonic approximation, which expands the crystal potential to second-order. However, higher-order (anharmonic) contributions are often important, especially in materials relevant for modern electronic, optical, thermal, and energy storage applications. In such materials, the harmonic approximation yields very poor predictions; therefore, advanced computational techniques that can incorporate anharmonic effects are essential for efficient material screening for advanced device applications. We apply two such computational methods, compressive sensing lattice dynamics (CSLD) and quantum self-consistent ab intio lattice dynamics (QSCAILD), to all four phases (cubic, tetragonal, orthorhombic, and rhombohedral) of perovskite BaTiO₃, a highly anharmonic crystal with promising device applications, and calculate the phonon dispersions and static dielectric constants as functions of temperature. Finally, we compare our predictions to calculations done in the harmonic approximation, and to experiment.