Unlocking the key to persistent luminescence with X-ray absorption spectroscopy: a local structure investigation of Cr-substituted spinel-type phosphors

Abstract

Developing new persistent luminescent phosphors, a unique class of inorganic materials that can produce a visible light emission lasting minutes to hours requires improving our understanding of their fundamental structure–property relationships. Research has shown that one of the most critical components governing persistent luminescence is the existence of lattice defects in a material. Specifically, vacancies and anti-site defects that coincide with substitution of the luminescent center, e.g., Eu²⁺ or Cr³⁺, are generally considered essential to generate the ultra-long luminescent lifetimes. This research solidifies the connection between defects and the remarkable optical properties. The persistent luminescent compound Zn(Ga_1−xAl_x)₂O₄ (x = 0–1), which adopts a spinel-type structure, is investigated by examining the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) at the Cr K and Zn K edges. This investigation reveals a structural distortion of the octahedrally coordinated main group metal site concurrent with increasing Al³⁺ content. Moreover, these results suggest there is a dependence between the local crystallographic distortions, the presence of defects, and a material's persistent luminescence. In combination, this work provides an avenue to understand the connection between the structure–defect–property relationships that govern the properties of many functional inorganic materials.