Prediction of a Stable Post-Post-Perovskite Structure from First   Principles
  A novel stable crystallographic structure is discovered in a variety of ABO3, ABF3 and A2O3 compounds (including materials of geological relevance, prototypes of multiferroics, exhibiting strong spin-orbit effects, etc...), via the use of first principles. This novel structure appears under hydrostatic pressure, and is the first "post-post-perovskite" phase to be found. It provides a successful solution to experimental puzzles in important systems, and is characterized by one-dimensional chains linked by group of two via edge-sharing oxygen/fluorine octahedra. Such unprecedented 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 ~ 10 eV being direct or indirect in nature, which emphasizes its "universality" and its potential to have striking, e.g., electrical or transport phenomena. 
Changsong Xu, Bin Xu, Yurong Yang, Huafeng Dong, A. R. Oganov,   Shanying Wang, Wenhui Duan, Binglin Gu and L. Bellaiche
cond-mat.mtrl-sci