Since the space groups Cmcm and Cmc2 1 share the same translational symmetry, potential experimental differences lie only in the diffraction intensities, while there are no supplementary reflections that could be detected. The same is not true for the transition from Cmcm to P 2 1 /c. Notably, a in Cmcm becomes the c lattice parameter in P 2 1 /m and is doubled during the klassengleiche symmetry descent from P 2 1 /m to P 2 /c, Figure . This doubling has the structural consequence that the atoms can shift away from the (004) plane alternatingly and would hence form an antipolar structure, in which the dipoles cancel each other out. Furthermore, the doubling would lead to supplementary reflections that can be observed in the odd hk1, hk3, hk5 etc. layers when indexing the diffraction data in this doubled lattice. Diffraction spots are clearly observed in the hk0 and hk2 planes, but there is no indication of Bragg diffraction in the hk1 and hk3 planes, Figure (a), suggesting that this ordering does not occur at room temperature. This analysis is independent of the cation swapping, indicating that the ordered P 2 1 /c structure (athermal DFT ground state) is not present at room temperature.