Instability of the layered orthorhombic post-perovskite phase of SrTiO$_3$ and other candidate orthorhombic phases under pressure

TL;DR

This study investigates the stability of various orthorhombic phases of SrTiO₃ under pressure, finding that some proposed structures are energetically unfavorable and unlikely to occur as high-pressure phases.

Contribution

The paper provides a detailed energetic analysis showing the instability of the layered orthorhombic post-perovskite phase of SrTiO₃ and explores alternative phases with implications for high-pressure behavior.

Findings

The layered post-perovskite structure has higher energy than cubic or tetragonal phases.

The $Pnma$ phase has slightly lower energy than the $I4/mcm$ phase.

Certain ferro-electric phases are not stable at zero temperature.

Abstract

While the tetragonal antiferro-electrically distorted (AFD) phase with space group $I4/mcm$ is well known for SrTiO$_3$ to occur below 105 K, there are also some hints in literature of an orthorhombic phase, either at lower temperature at high pressure. A previously proposed orthorhombic layered structure of SrTiO$_3$, known as the post-perovskite or CaIrO$_3$ structure with space group $Cmcm$ 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. It thus cannot correspond to a high-pressure phase of SrTiO$_3$. We also study an alternative $Pnma$ phase obtained by two octahedral rotations about different axes. This phase is found to have slightly lower energy than the $I4/mcm$ phase in spite of the fact that its parent, in-phase tilted $P4/mbm$ phase is not found to occur. Alternative ferro-electric tetragonal and orthorhombic structures are also considered. These are not found to occur in our zero temperature calculations but have been suggested in previous literature to occur at higher temperature.