to exist in a rock salt structure (space group F m 3m, ICSD entry # 290603), where no cation order was observed (see Fig. )) The sample synthesized by A-lab with the composition Mg 3 NiO 4 was claimed to be a successful synthesis in space group P m 3m. However, the provided structure file, which can be found in the Supplemental Information of the A-lab paper and is shown in Fig. (c), has disordered cations, hence the CIF used for fitting by A-Lab has the space group F m 3m. The powder x-ray can be relatively well indexed with space group F m 3m (as shown in the supplemental files in the paper and also in Fig. (c)) but lacks additional peaks (e.g. the 100 peaks around 21.1°) that would appear in the original proposed space group P m 3m (see Fig. ). In fact, the powder pattern and relevant systematic absence conditions of h + k, h + l, and k + l = 2n agrees very well with the known compound MgNiO 2 as shown in Fig. , however the peaks appear at slightly different diffraction angles, which may suggest a doped material or a solid solution. MgO-NiO solid solutions are well studied as catalytic materials, and solid solutions can be formed across the composition range. and (metastable) rock salt NiO (4.1718 Å), Vegard's law places the composition of a solid solution with the lattice parameter 4.20311 Å at exactly Mg 3 NiO 4 , in line with the expected composition from the synthesis recipe. Note that in the image of the refinement provided in the A-lab paper, the indexed peaks (tick marks) do not line up with the diffraction peaks, thus we believe there has been an error in producing the image in this case. We conclude that the synthesised compound is actually a member of the MgO-NiO solid solution series, with disordered cations, that has been studied for many years, and not the cation ordered material predicted by Materials Project. Nonetheless, this does not mean that the proposed ordered material cannot be synthesized. However, it will require a different synthetic route to potentially stabilize Mg 3 NiO 4 with ordered cations. Here, the clear distinction between the PXRD of the ordered and disordered material allows for easy identification of the former.