  Ten years have passed since the description of the perovskite to
post-perovskite transition in MgSiO3 and its' impact on the mineralogy and
rheology of the D" region at the base of the Earth's mantle. Much work has
explored the mechanisms operating during this transformation and their
influence on seismic response. In parallel, calculations in ABX3 systems have
identified potential structures for a denser post-post-perovskite phase
occurring at higher pressures. However, experiments have yet to elucidate any
higher pressure form, beyond the CaIrO3-type. Here we describe the structures
and transformations that lead to the crystallisation of a post-post-perovskite
of Sb2S3-type in a GdFeO3-type fluoroperovskite at high pressure conditions.
The use of single-crystal techniques gives unique access to the relative
crystallographic orientations of all polymorphs. We use this information to
extend this description to include other calculated and observed forms that are
competitive in ABX3 and A2X3 stoichiometries (e.g. Sb2S3, a-Gd2S3, Be3N2) and
provide substantial information on inter-relationships between these
structures, which is critical to the interpretation of transition mechanisms,
predicting transition sequences, to the expression of directional properties in
those transformed structures when assessing their suitability to modelling e.g.
their rheological response in deep-Earth and exoplanet interiors.
