Cation and anion ordering in Sr2Si7Al3ON13 phosphors

A series of photoluminescent Ce3+ doped samples with compositions close to Sr2Si7Al3ON13:Ce have been studied by neutron powder diffraction to determine the Si4+/Al3+ and N3−/O2− site ordering. Contrary to a commonly held assumption that the edge sharing tetrahedral sites in this structure are occupied exclusively by Al3+, we find a partial occupancy of Al3+ on these site but also an unexpected preference for Al3+ to occupy 2 other tetrahedral sites which are only corner sharing. From the crystal structures and local structures, as determined by pair distribution function (PDF) analysis, we also find evidence for alternating Si–Al site ordering within the edge sharing chains as well as dimerization of the Si4+ and Al3+ cations within these chains. The O2− are found to be partially ordered onto 2 of the anion sites, although small amounts of O2− are found on other sites as well. The cation and anion ordering found by neutron diffraction is supported by theoretical calculations. Understanding cation and anion ordering is essential for optimizing the photoluminescence properties of this promising class of phosphor materials.