The effect of local structure on the luminescence of Eu2+ in ternary phosphate solid solutions by cationic heterovalent substitution and their application in white LEDs

Abstract

In this work, using β-Ca₃(PO₄)₂ as a structural model, a series of [Ca₉Na_3xY_1−x(PO₄)₇ (CNYPO-I, 0 ≤ x ≤ 1/2) ← Ca₃(PO₄)₂ → Ca_9+yNa_3/2−y/2Y_(1−y)/2(PO₄)₇ (CNYPO-II, 0 ≤ y ≤ 1)] phosphors were prepared using a cationic heterovalent substitution approach. The substitution of Y³⁺ and Na⁺ for Ca²⁺ led to a redistribution of Eu²⁺ within the lattice and was accompanied by changes to the corresponding luminescence behavior. As the x and y values increased, the vacancies in the structure were gradually occupied by Na⁺ to form Na(4) sites that are conducive to Eu²⁺ occupation. When y = 1, the vacancies were completely occupied, and the Eu²⁺ emission peaks from Ca(1/2) and Ca(3) sites showed a significant blue shift. The underlying mechanism of that shift is discussed. The CNYPO-I (x = 0) sample displayed the best thermal stability; the effects of vacancies, thermal ionization and cross-over mechanism on thermal quenching are also discussed. As x and y increase, the thermal stability of the samples gradually decreased. Eu²⁺ and Mn²⁺ co-doped CNYPO-I (0 ≤ x ≤ 1/2) and CNYPO-II (0 ≤ y ≤ 1) phosphors achieved full coverage of the visible light region (400–800 nm). These white LEDs have relatively high color-rendering indexes (>80) and adjustable color temperatures from 2854–7000 K. This work provides an effective strategy for designing novel luminescent materials and realizing multi-site adjustment of activator ions.