Influence of inversion defects and Cr–Cr pairs on the photoluminescent performance of ZnAl₂O₄ crystals

ZnAl₂O₄:Cr³⁺ crystals were synthesized by using sol–gel method at 1000 °C. The results showed that with the concentration of Cr³⁺ in spinel lattices increasing, the x-ray diffraction peaks shifted toward lower diffraction angles, and the grain size revealed a rapid increase and became invariable when the Cr³⁺ dopant concentration was above 0.7 mol%. All the samples showed a series of emission peaks ranging from 660 to 730 nm assigned to spin-forbidden ²E_g→⁴A_2g transition of Cr³⁺ ions in spinel lattices. The intensity of emission peaks revealed a strong dependence on the Cr³⁺ concentration. An additional emission peak at 740 nm was present when the Cr³⁺ ions concentration was above 0.5%. All emission peaks were quenched when the Cr³⁺ concentration reaches 1.3 mol%. The results of x-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) suggested that the Cr³⁺ ions occupied the octahedral sites in spinel lattices, and with the Cr³⁺ concentration increasing, the exchange interaction and coupling interaction between Cr–Cr were enhanced and the antisite defects related to Zn in octahedral sites increased.