Observation of Sb2S3-type post-post-perovskite in NaFeF3. Implications for ABX3 and A2X3 systems at ultrahigh pressure

We describe the structures and transformations that lead to the crystallization of a post-post-perovskite of Sb2S3 type in a GdFeO3-type fluoroperovskite system at high-pressure conditions, through use of large-volume powder and single-crystal x-ray diffraction techniques. The results of this analysis gives unique access to the relative crystallographic orientations of all the polymorphs encountered (GdFeO3 type, CaIrO3 type and Sb2S3 type). We use this information to extend this description to include other calculated and observed forms that are competitive in ABX 3 and A 2 X 3 stoichiometries (e.g . α-Gd2S3 and Be3N2 types) and provide substantial information on inter-relationships between these structures. Such information is critical to the interpretation of transition mechanisms, predicting transition sequences and to the expression of directional properties in those transformed structures. The transformation from CaIrO3 type to Sb2S3 type is group-subgroup, from Cmcm with fc 2 a , to Pnma c 5, with no observable volume change, but considerable change to the morphology of the lattice. There is a concomitant increase in coordination and average bond length compared to the post-perovskite form of NaFeF3.