Yu-Seong Seo, Jai Seok Ahn
We report our first principles investigations on phonons of three polytypes of BaTiO3 (BTO): paraelectric (PE) cubic Pm-3m and two ferroelectric (FE) phases, tetragonal P4mm and rhombohedral R3m. We reviewed the phonon frequencies calculated by using various exchange-correlation functionals, including density functional theory, Hartree-Fock approximation, and their hybrids. By calculating the phonon modes as a function of pressure, we demonstrated that theoretical trajectories of pressure-sensitive modes of FE phases showed softening and converged to the modes of PE phase at pressure below 20 GPa. These results on FE phases can be interpreted as phonon-precursors for symmetry change from low- to high-symmetry and partly as a theoretical explanation for the pressure-induced mode-coupling behaviors of Sood et al. [Phys. Rev. B 51, 8892 (1995)]. The pressure-induced stabilization of imaginary-frequency-modes was also demonstrated by calculating phonon modes for all the possible directions of phonon-propagation vectors. As pressure increases, the imaginary-frequency-mode of P4mm was transformed into a stable mode in an orderly sequence of R-X-Z-{\Gamma}.
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http://arxiv.org/abs/1305.1369
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