Topotactic, pressure-driven, diffusion-less phase transition of layered CsCoO₂ to a stuffed cristobalite-type...

CsCoO₂, featuring a two-dimensional layered architecture of edge- and vertex-linked CoO₄ tetrahedra, is subjected to a temperature-driven reversible second-order phase transformation (α → β) at 100 K, which corresponds to a structural relaxation with concurrent tilting and breathing modes of edge-sharing CoO₄ tetrahedra. In the present investigation, it was found that pressure induces a phase transition, which encompasses a dramatic change in the connectivity of the tetrahedra. At 923 K and 2 GPa, β-CsCoO₂ undergoes a first-order phase transition to a new quenchable high-pressure polymorph, γ-CsCoO₂. It is built up of a three-dimensional cristobalite-type network of vertex-sharing CoO₄ tetrahedra. According to a Rietveld refinement of high-resolution powder diffraction data, the new high-pressure polymorph γ-CsCoO₂ crystallizes in the tetragonal space group I4₁/amd:2 (Z = 4) with the lattice constants a = 5.8711 (1) and c = 8.3214 (2) Å, corresponding to a shrinkage in volume by 5.7% compared with the ambient-temperature and atmospheric pressure β-CsCoO₂ polymorph. The pressure-induced transition (β → γ) is reversible; γ-CsCoO₂ stays metastable under ambient conditions, but transforms back to the β-CsCoO₂ structure upon heating to 573 K. The transformation pathway revealed is remarkable in that it is topotactic, as is demonstrated through a clean displacive transformation track between the two phases that employs the symmetry of their common subgroup Pb2₁a (alternative setting of space group No. 29 that matches the conventional β-phase cell).