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First-Principles Calculations of Phase Transition in CaTiO_3 under Negative Static Pressure
Hiroki Moriwake,Craig A. J. Fisher,Akihide Kuwabara,Tetsuya Tohei,Isao Tanaka 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.59 No.31
The relative enthalpy among different structure of CaTiO_3 under negative static pressure is studied by means of first-principles calculations. At zero pressure, the lowest energy structure calculated for CaTiO_3 was the same as that found experimentally at low temperatures, namely the Pnma orthorhombic phase. However, below -8.5 GPa, the symmetry of the lowest energy structure changes from non-ferrroelectric Pnma to ferroelectric P4mm, corresponding to a ferroelectric phase transition under negative static pressure. The P4mm structure was, however, found to be dynamically unstable by lattice dynamics calculation. This implies formation of lower symmetry structure under negative hydrostatic pressure.
Two competing soft modes and an unusual phase transition in the stuffed tridymite-type oxideBaAl2O4
Ishii, Y.,Mori, S.,Nakahira, Y.,Moriyoshi, C.,Park, J.,Kim, B. G.,Moriwake, H.,Taniguchi, H.,Kuroiwa, Y. American Physical Society 2016 Physical Review B Vol.93 No.13
<P>We investigated the structural phase transition of BaAl2O4, which has a network structure with corner-sharing AlO4 tetrahedra, via synchrotron x-ray thermal diffuse scattering measurements and first-principles calculations. BaAl2O4 shows the structural phase transition at T-C = 451.4 K from the P6(3)22 parent crystal structure to the low-temperature superstructure with a cell volume of 2a x 2b x c. This phase transition is unusual, in which two energetically competing phonon modes at M and K points soften simultaneously. When approaching T-C from above, the K-point mode appears first. However, this K-point mode is overcome by the later-developed M-point mode. The thermal diffuse scattering intensities from both modes increase sharply at T-C; therefore, both modes soften simultaneously. The first-principles calculations demonstrate that the M-point mode is electrostatically more preferable than the K-point mode and determines the eventual low-temperature structure, although these two modes are competing energetically. This competition is characteristic of BaAl2O4, which is ascribed to the structurally flexible network structure of this compound.</P>