Characterization of blue-excited yellow phosphor (Y,Ca)6+x/3Si11(N,O)21:Ce by the bond valence sum model

Abstract

The new bright yellow phosphor (Y,Ca)_6+x/3Si₁₁(N,O)₂₁:Ce (CYSON) providing broad emission excited by blue LED is the product of extensive substitution (about 70% of Y) of Y³⁺ → Ca²⁺ and N³⁻ → O²⁻ in the Y₆Si₁₁ON₂₀:Ce parent host, which exhibits a weak emission excited by near UV-excitation only. Such a considerable difference is caused by particular distribution of substituting ions through the crystal lattice sites. For the first time for the title host, these intricate effects have been thoroughly studied in the present paper. We analyzed distribution of cations and their anion surrounding for each of the three Y sites. In addition, the local charge balance was also considered in detail by calculating the Brown's bond valence sum (BVS). The results suggest that the number of oxygen ligands is likely to be 0 at the Y₁ and Y₂ sites, 2–3 at the Ca₁ and Ca₂ sites, 0–1 at the Y₃ site, and 3–4 at the Ca₃ site (the subscript enumerates the inequivalent sites in the CYSON lattice). These data indicate that the Ca ions are likely to be coordinated by a greater number of the oxygen ions than the Y ions, which leads to the conclusion that the Y–N bond should be extensively substituted by the Ca–O bond. Therefore, agglomeration of an increased number of the oxygen ions far away from the Ca²⁺ ions is suppressed. It is suggested that as the bond lengths of the cation–anion pairs in CYSON are much larger than the sum, 2.28 Å, of the ionic radii of the Y³⁺ and O²⁻ ions (which yields a smaller BVS and a small local charge balance), the substituted Ca²⁺–O²⁻ pair contributes to the stabilization of electric charge distribution. This is the first detailed study of the structural properties of this new phosphor that allowed to identify most probable coordination around each and every cation site in this complicated structure with many inequivalent crystallographic positions.