Abstract
A series of Mn2+-doped and Eu3+-co-doped calcium molybdato-tungstates, i.e., Ca1−3x−yMny⌷xEu2x(MoO4)1−3x(WO4)3x (0 < x ≤ 0.2222 when y = 0.0200 and 0 < y ≤ 0.0667 when x = 0.1667, ⌷ represents vacancy) materials were successfully synthesized via high-temperature annealing. XRD results confirmed the formation of single, tetragonal scheelite-type phases (space group I41/a). A change in both lattice constants (a and c), lattice parameter ratio c/a and progressive deformation of MoO4/WO4 tetrahedra with increasing Eu3+ as well as Mn2+ contents were observed. The melting point of doped materials is lower than the melting point of pure matrix, i.e., CaMoO4. New materials exhibit strong absorption in the UV range. They are insulators with the optical direct band gap (Eg) higher than 3.50 eV. The Eg values nonlinearly change with increasing dopants concentrations. EPR measurements allowed to establish the nature of magnetic interactions among Mn2+ ions. Additionally, EPR spectra were sensitive on both parameters: Mn2+ and Eu3+ concentration.
Highlights
Metal molybdates and tungstates were proved to be excellent host lattice of luminescent materials due to their high thermal stability and chemical tolerance
XRD analysis shows the powder diffraction patterns of Mn2?-doped and Eu3?-co-doped calcium molybdatotungstates consisted of diffraction lines which can be attributed to scheelite-type framework
I.e., initial reactants, oxides (Eu2O3, MnO, CaO, MoO3 and WO3), and other europium tungstates or molybdates were observed with increasing Eu3? concentration only up to x = 0.2222 (40.00 mol% of Eu2(WO4)3 in initial MnMoO4/Eu2(WO4)3/CaMoO4 mixtures when y = constant)
Summary
Metal molybdates and tungstates were proved to be excellent host lattice of luminescent materials due to their high thermal stability and chemical tolerance. These compounds show many different types of structures, i.e., scheelite (I41/a, No 88), pseudo-scheelite (Pnma, No 62), wolframite (P2/c, No 13), zircon (I41/amd, No 141) and fergusonite (C2/c, No 15) [1,2,3,4,5,6,7]. Divalent manganese ion plays an important role in some inorganic phosphors. It can produce a broad emission due to the transition from 4T1 to 6A1 level. Simultaneously doping the host material with Eu3? and Mn2? ions allows to energy transfer from Mn2? to Eu3? ions and improve the luminescent efficiency of doped samples [15, 16]
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