1 2 ]T 2 tilting modes together with rocksalt M-site order (R - 2 ) was sufficient to produce the observed P2/n (2 × 6 × 4) structure. These modes are therefore likely the primary order parameters, with the R - 5 (Γ + 4 ) mode being a secondary order parameter. The A-site order can only be described by secondary order parameters arising from all three tilts, with any pairwise combination being insufficient, which suggests that it is the final distribution of anions ordering produced by the complete octahedral tilt pattern which is responsible for the observed ordering. tals of compound 4. We were again able to determine its structure using SCXD, which revealed that it was also a vacancy ordered double perovskite, in space group Pn 3, a (2 × 2 × 2) supercell of the Pm 3m aristotype [Fig. ]. However, 4 contains only half the number Gua + cations anticipated and refinement of the occupancies showed that this structure contained a significant fraction of M-site vacancies ( The observed space group of Pn 3 is that expected for the a + a + a + tilt sequence, and indeed analysis using ISODISTORT confirmed this tilt sequence is adopted by compound 4. This tilt sequence is well known for other perovskites with 1:3 A-site cation ratio. In addition each Gua + cation is disordered over four positions. Our single crystal diffraction measurements are consistent with both static and dynamic disorder, but the absence of any A-site order at 120K, well-below typical ordering temperatures for Gua + containing molecular perovskites, suggests that this disorder is static.