Despite this, the strong similarities in hydrogen bonding lead to similar intra-planar molecular positions (Figure ). A comparison between molecules within each plane using a root-meansquared deviation (RMSD) gave 0.275 Å (Figure . asymmetric peak broadening (see Supplementary Material). This strongly suggests that other phases are present in the bulk xanthine powder, in addition to planar defects. Considering the purported chemical purity of the sample, our interpretation is that the analysed powder contains other solid-state forms of xanthine, which is the subject of further work . This would also explain the non-ideal Rietveld refinement of the XRPD data.