Relationships between luminescence properties and polyhedron distortion in Ca9−x−y−zMgxSryBazCe(PO4)7:Eu2+,Mn2+

Abstract

Generally, the luminescence properties of phosphors can be tuned by cations substitutions (Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺); however, the underlying mechanism is not clear. The luminescence properties are influenced by many factors, such as symmetry of the ligand polyhedron and the bond length between the luminescent centre and its ligand. Therefore, in this research, the phosphor Ca_{9−x−y−z}Mg_xSr_yBa_zCe(PO₄)₇:Eu²⁺,Mn²⁺ was selected as the research object, and the reasons are investigated. Detailed data regarding the crystal space structure is acquired through the Rietveld refinement and used to systematically analyze the change of the two factors. It was found that the two factors are sometimes synergetic and sometimes antagonistic to the spectrum shift. The change of the luminescence properties is the comprehensive result of the two factors. It was found that the influence produced by the change of symmetry of the polyhedron is stronger than that by the change of bond length between the luminescence centre and its ligand. When changing the doping concentration of Sr²⁺ and Ba²⁺, the chromaticity coordinates of (Ca,Sr,Ba)_8.78Ce(PO₄)₇:Eu²⁺,Mn²⁺ could change from cold white light to warm white light, and there is a larger distribution range in the white light region in the CIE 1931 chromaticity diagram. Cations substitutions tuning the luminescence thermal stability are also observed. When partial Ca²⁺ are substituted by other cations, the luminescence thermal stability of the luminescent centre was improved to some degree. It is expected that these methods can be generalized to analyze the changes in optical and other properties that are sensitive to local coordination environments for luminescent materials.