Structural investigation of the A-site vacancy in scheelites and the luminescence behavior of two continuous solid solutions A1–1.5xEux□0.5xWO4 and A0.64–0.5yEu0.24Liy□0.12–0.5yWO4 (A = Ca, Sr; □ = vacancy)

Abstract

Scheelite compounds with Eu³⁺ substitution are well-known red-phosphors. We prepared and performed a detailed structural characterization of A_1–1.5xEu_x□_0.5xWO₄ and A_0.64–0.5yEu_0.24Li_y□_0.12–0.5yWO₄ (A = Ca, Sr; □ = vacancy) to confirm the A-site vacancy mechanism for charge balance when bivalent A cations were substituted by Eu³⁺. All compounds crystallize in I4₁/a with a disordered arrangement of A²⁺, Eu³⁺, □ at the A-site. The title compounds are all good red phosphors with a high R/O ratio (∼10), indicating that Eu³⁺ is located at a significantly distorted cavity. A_1–1.5xEu_x□_0.5xWO₄ shows a saturation phenomenon at a high doping level, x = 0.20. With the incorporation of Li⁺, the emission intensity was generally enhanced compared to the Li⁺-free samples, moreover, an increase of the Li⁺ content reduces the content of vacancies, resulting in further increase of the luminescence intensity.