Energy transfer in Eu³⁺ doped scheelites: use as thermographic phosphor

In this paper the luminescence of the scheelite-based CaGd₂(₁-x)Eu₂x(WO₄)₄ solid solutions is investigated as a function of the Eu content and temperature. All phosphors show intense red luminescence due to the ⁵D₀ - ⁷F₂ transition in Eu³⁺, along with other transitions from the ⁵D₁ and ⁵D₀ excited states. For high Eu³⁺ concentrations the intensity ratio of the emission originating from the ⁵D₁ and ⁵D₀ levels has a non-conventional temperature dependence, which could be explained by a phonon-assisted cross-relaxation process. It is demonstrated that this intensity ratio can be used as a measure of temperature with high spatial resolution, allowing the use of these scheelites as thermographic phosphor. The main disadvantage of many thermographic phosphors, a decreasing signal for increasing temperature, is absent.