Anomalous upconversion emission of Eu3+–Yb3+–MoO6 in double perovskites induced by a laser

Abstract

The anomalous upconversion (UC) emission of Eu³⁺–Yb³⁺–MoO₆ in Sr₂Ca(Mo,W)O₆ double perovskites is presented in detail and the proper theoretical interpretation is proposed. The relative intensity ratios of the magnetic dipole transition ⁵D₀ → ⁷F₁ at ∼594 nm to the electric dipole transition ⁵D₀ → ⁷F₂ at ∼615 nm of Eu³⁺ are changed for both A-site (Sr, C₁ symmetry) and B-site (Ca, C_i symmetry) substitution by Eu³⁺ in Sr₂Ca(Mo,W)O₆ when comparing the UC emission spectra with the Stokes luminescence spectra, which may be due to the difference in energy transfer efficiency between Yb³⁺–Yb³⁺ to Eu³⁺ and (Mo,W)O₆ to Eu³⁺. Another remarkable phenomenon is the emission peak at ∼700 nm, ascribed to the ⁵D₀ → ⁷F₄ transition, which is observed in the UC spectra for the samples with Mo while it is absent in the Stokes luminescence spectra. Furthermore, this peak almost disappears for those samples without Mo in the UC spectra. Theoretical analysis and Raman spectra are utilized to interpret these phenomena, which can probably be accounted for by the polarizability variation of MoO₆ groups induced by a 976 nm laser beam.