The modulated structure and frequency upconversion properties of CaLa2(MoO4)4:Ho3+/Yb3+ phosphors prepared by microwave synthesis

Abstract

CaLa_2−x(MoO₄)₄:Ho³⁺/Yb³⁺ phosphors with the doping concentrations of Ho³⁺ and Yb³⁺ (x = Ho³⁺ + Yb³⁺, Ho³⁺ = 0.05; Yb³⁺ = 0.35, 0.40, 0.45 and 0.50) have been successfully synthesized by the microwave sol–gel method. The modulated and averaged crystal structures of CaLa_2−x(MoO₄)₄:Ho³⁺/Yb³⁺ molybdates have been found by the Rietveld method, and the upconversion photoluminescence properties have been investigated. The synthesized particles, being formed after the heat-treatment at 900 °C for 16 h, showed a highly crystallized state. Under the excitation at 980 nm, CaLa_2−x(MoO₄)₄:Ho³⁺/Yb³⁺ particles exhibited strong 545 and 655 nm emission bands in the green and red regions. When the Yb³⁺ : Ho³⁺ ratios are 9 : 1 and 10 : 1, the UC intensity of CaLa_1.5(MoO₄)₄:Yb_0.45/Ho_0.05 and CaLa_1.45(MoO₄)₄:Yb_0.50/Ho_0.05 particles is the highest for different bands. The CIE coordinates calculated for CaLa_2−x(MoO₄)₄:Ho³⁺/Yb³⁺ phosphors are related to the yellow color field. The Raman spectrum of undoped CaLa₂(MoO₄)₄ has revealed about 13 narrow lines. The strongest band observed at 906 cm⁻¹ was assigned to the ν₁ symmetric stretching vibration of MoO₄ tetrahedra. The spectra of the samples doped with Ho and Yb, as obtained under the 514.5 nm excitation, were dominated by Ho³⁺ luminescence over the wavenumber range of >700 cm⁻¹ preventing the recording of the Raman spectra.