Triple molybdates and tungstates scheelite structures: Effect of cations on structure, band-gap and photoluminescence properties

Abstract A series of eight compounds with general composition A′AEu(MO4)3 (where A′ = Li+ and Ag+; A = Ca2+ and Sr2+ and M = Mo6+ and W6+) have been synthesized by conventional solid state reaction route. These compounds are characterized by Powder X-ray Diffraction(PXRD), Fourier Transform Infrared (FTIR) spectroscopy and Raman spectroscopy. The Rietveld refinement of PXRD patterns of the compounds is carried out to find out the structural characteristics. These experimental techniques confirm the scheelite structure of compounds. The band-gap of the compounds was measured by UV-DRS method. The band-gaps in tungstate compounds are larger than those of molybdates. The photoluminescence properties of these compounds were elucidated recording excitation, emission and lifetime spectra. The effect of cations on photoluminescence properties of these compounds was studied. The silver containing compounds are shown to have superior photoluminescence when excited by 395 nm light (f-f transition), but when excited by charge transfer band (CTB) energy, these compounds show degradation of the matrix. The reason of this contrasting effect is due to the reduction of Ag+ to Ag0. The photoluminescence properties of molybdates are better in comparison to those of tungstates due to higher covalence of Eu–O bond in molybdates, as evidenced by Judd-Ofelt calculations.