Coexistence mechanism of Eu²⁺/Eu³⁺ ions in YAl₃(BO₃)₄: Crystal structure, luminescence property, and substitution defects

Abstract

The coexistence mechanism of Eu 2+ and Eu 3+ ions in YAl 3 (BO 3 ) 4 host under different reducing conditions is investigated and confirmed in great detail, which can be described by three aspects as following. First, Eu 3+ is protected by the layered structure of YAl 3 (BO 3 ) 4 . Second, the number of the interstitial defects ((Al) i˙˙˙, (Y) i˙˙˙) increases with increasing Eu 2+ to maintain the charge balance of the system. And these defects can capture free electrons which are used for the reduction of Eu 3+ . Finally, free holes become more and more with the number of nonequivalent substitution defects (Eu Y ') increasing, which make the 5d electrons of Eu 2+ easily escape to the conduction band and Eu 2+ convert to Eu 3+ . This work will be of great significance to research coexistence of multiple valence ions in the structure of borate and defect motion with nonequivalent substitution.presents polysynthetic twin which is composed of a number of parallel single crystals. The parallel layer structure is beneficial for the movement of electrons and defects. 26 Many possibilities for the luminescence of activators are created since its unique structure. Recently, Prokhorov et al. reported an interesting phenomenon about EuAl 3 (BO 3 ) 4 crystals. The X-ray irradiation can modify the color of the EuAl 3 (BO 3 ) 4 crystal, and annealing of the crystal at 600°C can lead to the restoration of the original color. It was concluded that Eu 2+ ion located at the Eu 3+ site in the EuAl 3 (BO 3 ) 4 crystal lattice. 27 However, when Eu concentration is low, Eu 3+ is always existed in YAB no matter how strong the reduction atmosphere is. Generally, the reasons for the coexistence of multivalent ions are imbalance of the charge, defects, and crystal structure. 28 For further exploration, Eu 2+ /Eu 3+ -doped YAl 3 (BO 3 ) 4 phosphors are synthesized in CO or H 2 atmospheres. The structure and morphological characteristics are studied by the X-ray diffraction (XRD), the field emission scanning electron microscope (SEM), and diameter analyzer. The luminescence properties and luminescence decay curves are investigated in detail. Meanwhile, the energy transfer process and behavior of intrinsic and substitution defects are further analyzed. The model of charge compensation is established. The surface chemical compositions of YAB:Eu are determined by X-ray Photoelectron Spectroscopy (XPS). Moreover, the influence of crystal structure and defect motion on incomplete reduction is also explored.