Preferential occupancy of Eu³⁺ and energy transfer in Eu³⁺ doped Sr₂V₂O₇, Sr₉Gd(VO₄)₇ and Sr₂V₂O₇/Sr₉Gd(VO₄)₇ phosphors

Abstract

The vanadate-based phosphors Sr₂V₂O₇:Eu³⁺ (SV:Eu³⁺), Sr₉Gd(VO₄)₇:Eu³⁺ (SGV:Eu³⁺) and Sr₉Gd(VO₄)₇/Sr₂V₂O₇:Eu³⁺ (SGV/SV:Eu³⁺) were obtained by solid-state reaction. The bond-energy method was used to investigate the site occupancy preference of Eu³⁺ based on the bond valence model. By comparing the change of bond energy when the Eu³⁺ ions are incorporated into the different Sr, V or Gd sites, we observed that Eu³⁺ doped in SV, SGV or SV/SGV would preferentially occupy the smaller energy variation sites, <i>i.e.</i>, Sr4, Gd and Gd sites, respectively. The crystal structures of SGV and SV, the photoluminescence properties of SGV:Eu³⁺, SV, SGV/SV and SGV/SV:Eu, as well as their possible energy transfer mechanisms are proposed. Interesting tunable colours (including warm-white emission) of SGV/SV:Eu³⁺ can be obtained through changing the concentration of Eu³⁺ or changing the relative quantities of SGV to SV by increasing the calcination temperature. Its excitation bands consist of two types of O^2- → V⁵⁺ charge transfer (CT) bands with the peaks at about 325 and 350 nm respectively, as well as f-f transitions of Eu³⁺. The obtained warm-white emission consists of a broad photoluminescence band centred at about 530 nm, which originates from the O^2- → V⁵⁺ CT of SV, and a sharp characteristic spectrum (⁵D₀-⁷F₂) at about 615 and 621 nm.