Our fit necessitated four phases: SnO 2 , ZrO 2 (which are both starting materials), BaSnO 3 (present in 44 wt %, a known perovskite phase ICSD 188149) and BaSn 0.5 Zr 0.5 O 3 (present in 21 wt %, another known perovskite phase ICSD 43137). Thus in our view the XRD pattern provided is best explained as originating from a mixture of staring materials and known perovskite phases. We note the model used by the authors in their refinement of Ba 2 SnZrO 6 , (published as a CIF file) contains disordered Zr and Sn ions on the B site of the perovskite. This contrasts with the Materials Project entry of the stated material, which has ordered Sn and Zr ions, and consequently, a doubling of the lattice parameter. The small difference in x-ray scattering factor between Sn and Zr means that simulated patterns of the ordered Ba 2 SnZrO 6 phase show only very small changes compared with the disordered phase. The analysis of Ba 2 ZrSnO 6 suffers thus errors one (poor fit), two (inconsistency in predicted and reported structure) and three (no evidence of cation order). the nature of the ordering does not allow new reflections that are absent in the disordered structure. Similarly to previous examples, for this particular compound, due to the electron densities of the constituent atoms, the intensity difference is very small between ordered and disordered phase. Therefore in our opinion, to distinguish the ordered from the disordered phase by PXRD will be exceptionally challenging, but if it is to be attempted then very careful measurement of the peak intensities and explicit comparison with the ordered and disordered models need to be made. When we turn to the diffraction pattern presented in the A-lab paper, we find the pattern for MgTi 2 NiO 6 can be adequately fitted with a model of the known ilmenite phase Ni 0.5 Mg 0.5 TiO 3 and a small impurity (2 wt%) of NiO. This again, is an example of error three. It may be that exceptionally careful analysis of the diffraction intensities can provide evidence that the new, ordered phase has been made.