Time-of-flight neutron powder diffraction with milligram samples: the crystal structures of NaCoF₃ and NaNiF₃ post-perovskites

The crystal structures of the post-perovskite analogues NaCoF₃ and NaNiF₃ have been determined from time-of-flight neutron powder data collected with the Polaris diffractometer at the ISIS spallation neutron source from samples of mass ∼56 and ∼16 Mg, respectively. The results for NaNiF₃ are in good agreement with those from a single-crystal X-ray experiment using synchrotron radiation.