Prediction of a stable post-post-perovskite structure from first principles

A novel stable crystal structure is discovered in a variety of $AB{O}_3,AB{F}_3$, and $A_2{O}_3$ compounds, via the use of first principles. This novel structure appears under hydrostatic pressure, and can be considered to be a post-post-perovskite phase. It provides a successful solution to experimental puzzles in important systems and is characterized by one-dimensional channels linked by a group of two via edge-sharing oxygen/fluorine octahedra. Such organization automatically results in anisotropic elastic properties and new magnetic arrangements. Depending on the system of choice, this post-post-perovskite structure also possesses electronic band gaps ranging from zero to $\simeq 10\mathrm{eV}$ being direct or indirect in nature.

Figure 1

Pressure range of stability of the Pv-$Pnma$, pPv-$Cmcm$, and ppPv-$Pnma$ phases in the ${\mathrm{ABX}}_3$ and $A_2{O}_3$ materials under study. (a) and (b) report materials possessing or missing, respectively, the presently discovered ppPv-$Pnma$ structure for pressure up to 120 GPa. Numbers in these panels indicate the decrease in volume (in percentage) at the transitions.

Figure 2

Pressure dependence of the enthalpy of the Pv-$Pnma$, pPv-$Cmcm$, and ppPv-$Pnma$ phases of ${\mathrm{NaMgF}}_3$ (a), along with the schematization of (b) the Pv-$Pnma$, (c) pPv-$Cmcm$, and [(d) and (e)] ppPv-$Pnma$ crystal structures. Note that the enthalpy of the Pv-$Pnma$ phase has been set to be zero for any pressure in (a), and that the inset in (a) displays the behavior of the volume vs pressure in the Pv-$Pnma$, pPv-$Cmcm$, and ppPv-$Pnma$ phases (in the pressure ranges they have the lowest enthalpy).

Figure 3

Magnetic ground state of the ppPv-$Pnma$ phase of ${\mathrm{GdFeO}}_3$ under 60 GPa. (a) The strength of the magnetic interactions and the resulting magnetic ordering between Fe ions, (b) the coupling coefficients associated with Fe and Gd magnetic interactions as well as the spin pattern adopted by these two types of ions.