The physical properties of the post-perovskite phase of MgSiO3 control the dynamics of the lower thermal boundary layer of the Earth¿s mantle, but many of these properties cannot be measured at the extreme pressures (120 GPa) of stabilisation of post-perovskite MgSiO3. The best chance of constraining them is to combine measurements on low-pressure isostructural analogues of different chemical composition with ab initio simulations of both the analogue and natural systems. We have recently developed a set of ABF3 analogues with properties much more similar to MgSiO3 than those of the oxides that are currently used and have just been awarded a NERC grant to investigate them. The NaCoF3 analogue magnetically orders at low-T. We propose to determine the magnetic structure of NaCoF3 and the thermal evolution (4 < T < 300 K) of its crystal structure to ground-truth our ab initio simulations.