Luminescent properties and energy transfer of Sm³⁺ doped Sr₂CaMo_1-xW_xO₆ as a potential phosphor for white LEDs

A series of Sm³⁺ doped Sr₂CaMo_1-xW_xO₆ phosphors were synthesized through solid state reaction process and their luminescent properties have been investigated. They presented excitation bands in UV, near UV and blue regions, originating from charge transfer transitions within Mo(W)O₆ or Sm³⁺ f-f transitions. Concentration quenching still occurs among MoO₆ groups even when Mo concentration is very low, however, if there is no Mo content in Sr₂CaMo_1-xW_xO₆ phosphors, absorption in near UV region cannot happen. It was found there is a big difference between the calculated optical band gap of Sr₂CaMo_0.02W_0.98O₆: 2% Sm³⁺ and that of Sr₂CaMo_0.01W_0.99O₆: 2% Sm³⁺. Band structure and density of state of Sr₂CaMo_0.5W_0.5O₆ were examined to explore the origination of charge transfer transition. Site occupation of Sm³⁺ in host lattice was discussed based on Raman spectra and group theory analysis. There are two kinds of luminescence centers in Sm³⁺ doped Sr₂CaMo_1-xW_xO₆ (x > 0) phosphors. When 98% Mo were substituted by W, the excitation band at 350 nm by monitoring 608 nm emission in Sr₂CaMo_0.02W_0.98O₆: 2%Sm³⁺ is about 8.5 times stronger than that in Sr₂CaMoO₆: 2%Sm³⁺. Energy transfer process was also discussed considering the overlap between the emission spectrum of Mo(W)O₆ groups and the absorption spectrum of Sm³⁺, as well as the atom arrays of Mo⁶⁺(W⁶⁺), O^2- and Sm³⁺.