Understanding the Charge State of Eu ion in Antiperovskites: The Role of Intrinsic Defect

Abstract

The valence state of the europium ion essentially determines its luminescence in solids. In this work, we focus on the stability of charge states of the europium ion in the anti-perovskite compound, Sr 3 AlO 4 F. The previous research showed that only Sr 3 AlO 4 F:Eu 3+ was synthesized even under the H 2 /Ar atmosphere. To understand this phenomenon, here we performed a systematic study from topological crystallography, bond valence sum (BVS) and ab-initio calculations. The obtained results indicated that the F site of Sr 3 AlO 4 F suffered from the unstable problem, and can be easily replaced by oxygen, which played a determinant effect on the charge state of Eu ion in the host. Experimental synthesis was then conducted following these theoretical analyses. Specially, with H 2 /Ar atmosphere, the graphite crucible was used to delete the excess of oxygen in the reaction system. Under this reaction condition, Sr 3 AlO 4 F:Eu 2+ was firstly synthesized. The existence of Eu 2+ ion was confirmed by the X-ray photoelectron spectroscopy and photoluminescence measurement. This work demonstrates the critical role of defect engineering in stabilizing Eu valence states and provides a novel strategy for the design of high-performance phosphors.