Towards better analogues for MgSiO3 post-perovskite: NaCoF3 and NaNiF3, two new recoverable fluoride post-perovskites

We present details of two new quenchable post-perovskite-structured NaBF3 compounds, where B is Co or Ni. Both post-perovskites are readily synthesised in the multi-anvil-press pressure range and have unit cells with axial ratios similar to MgSiO3 post-perovskite (NaCoF3a = 3.07035(7) Å, b = 10.1291(3) Å, c = 7.4664(1) Å, V = 232.20(1) Å3; NaNiF3a = 3.0247(2) Å, b = 10.0543(7) Å, c = 7.3989(3) Å, V = 225.01(3) Å3). For NaCoF3, we have synthesised post-perovskite single crystals and determined the phase diagram: the perovskite-post-perovskite transition occurs at 18 GPa and 700 °C, with a Clapeyron slope of 15.5 MPa/K. On the basis of the above factors, combined with (1) the fact that ABF3 compositions also have stable perovskite phases and (2) the relatively small differences in the mass ratios of their constituent atoms, we suggest that these systems might prove to be better analogues for MgSiO3 post-perovskite than the currently used CaBO3 (where B is Ir, Pt, Rh, Ru) compositions.

► Fluoride post-perovskites as analogues for MgSiO3.
► Two new quenchable NaBF3 post-perovskites (B = Co, Ni).
► Easily synthesised at 16–20 GPa.
► Single crystals of the post-perovskites readily grown.
► ABF3 post-perovskites might be better analogues than the popular CaBO3 compounds.