Incommensurately modulated structure and spectroscopic properties of CaGd2(MoO4)4:Ho3+/Yb3+ phosphors for upconversion applications

Abstract

CaGd2(MoO4)4:Ho3+/Yb3+ phosphors doped by Ho3+ and Yb3+ (Ho3+ = 0 and 0.05, and Yb3+ = 0, 0.35, 0.40, 0.45 and 0.50) were successfully synthesized by the microwave sol-gel method. The synthesized particles, being formed after heat-treatment at 900 °C for 16 h, showed a well crystallized morphology. All compounds are (3 + 2)D incommensurately modulated with superspace group I41/a(α,β,0)00(-β,α,0)00. It was found that parameter (α2 + β2)1/2 is proportional to cell parameter a for all studied compositions and, therefore, modulation vector k is the same for all known CaRE2(MoO4)4 compounds. The modulation vector invariance is a specific and valuable feature of this type of the structure. Under the excitation at 980 nm, the doped particles exhibited the yellow emission composed of green (545-nm) and red (655-nm) emission bands due to frequency upconversion (UC). The pump power dependence and CIE chromaticity of the UC emission were evaluated. The shape of UC bands in CaGd2(MoO4)4:Ho3+/Yb3+ is dependent on the Yb content due to the influence of the crystal field affecting a holmium ion. 13 Raman-active modes of the CaGd2(MoO4)4 lattice were identified via a comparison of experimental Raman spectra and the lattice dynamics simulation results. Four additional Raman lines were found in the region of stretching vibrations and, at least, two additional modes are present in the bending mode region. These additional modes are ascribed to incommensurate crystal lattice modulation. Luminescence bands of Ho ions are severely broadened due to a statistical disorder in the CaGd2−xYby (MoO4)4 lattice.