Structural investigation of the A-site vacancy in scheelites and the luminescence behavior of two continuous solid solutions A(1-1.5x)Eu(x)square 0.5xWO4 and A(0.64-0.5y)Eu(0.24)Li(y)square 0.12-0.5yWO4 (A = Ca, Sr; square = vacancy)

Scheelite compounds with Eu3+ substitution are well-known red-phosphors. We prepared and performed a detailed structural characterization of A(1-1.5x)Eu(x)square 0.5xWO4 and A(0.64-0.5y)Eu(0.24)Li(y)square 0.12-0.5yWO4 (A = Ca, Sr; square = vacancy) to confirm the A-site vacancy mechanism for charge balance when bivalent A cations were substituted by Eu3+. All compounds crystallize in I4(1)/a with a disordered arrangement of A(2+), Eu3+, square at the A-site. The title compounds are all good red phosphors with a high R/O ratio (similar to 10), indicating that Eu3+ is located at a significantly distorted cavity. A(1-1.5x)Eu(x)square 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.