Perspective view of the unit cell along b showing the proposed structure model of S124 as obtained from ADT data by δM recycling and by posterior crystal chemical interpretation (only bonds between atoms inside one unit cell are represented; the maximum bond length is 3.15 Å). Large (green) circles: Sr; small circles at the vertices of the P tetrahedra are: N (light blue) and O (red). Atom O1 is linked to one P and three Sr and was already identified by Sedlmaier et al. (2011). The structure propagates along b through the bridging N1, N2, N3, N4, O2, O3 atoms lying on a mirror plane and connecting each one to two mirror-related P tetrahedra (for clarity, symmetry-related bridging N,O have not been labelled). In the proposed simpler model, the bridging atoms not bound to Sr are assumed to be O atoms (O2, O3).