Cation and anion ordering in Sr2Si7Al3ON13 phosphors

Abstract

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.