Instability of the layered orthorhombic post-perovskite phase of SrTiO3 and other candidate orthorhombic phases under pressure.

While the tetragonal antiferro-electrically distorted (AFD) phase with space group I 4 / m c m is well known for SrTiO 3 to occur below 105 K, there are also some hints in the literature of an orthorhombic phase, either at the lower temperature or at high pressure. A previously proposed orthorhombic layered structure of SrTiO 3 , known as the post-perovskite or CaIrO 3 structure with space group C m c m is shown to have significantly higher energy than the cubic or tetragonal phase and to have its minimum volume at larger volume than cubic perovskite. The C m c m structure is thus ruled out. We also study an alternative P n m a phase obtained by two octahedral rotations about different axes. This phase is found to have slightly lower energy than the I 4 / m c m phase in spite of the fact that its parent, in-phase tilted P 4 / m b m phase is not found to occur. Our calculated enthalpies of formation show that the I 4 / m c m phase occurs at slightly higher volume than the cubic phase and has a negative transition pressure relative to the cubic phase, which suggests that it does not correspond to the high-pressure tetragonal phase. The enthalpy of the P n m a phase is almost indistinguishable from the I 4 / m c m phase. Alternative ferro-electric tetragonal and orthorhombic structures previously suggested in literature are discussed. [ABSTRACT FROM AUTHOR]