Structural,morphological and optical properties of new Eu-doped and vacancied lead molybdato-tungstates

A new tetragonal, scheelite-type Pb_1–3x?_xEu_2x(MoO₄)_1–3x(WO₄)_3x (0 < x ≤ 0.1970 and ? denotes A-site vacancies) solid solution was synthesized via solid state reaction method. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) results confirm the formation of single, tetragonal scheelite-type phases (space group I4₁/a) with the average crystallite size in the range of ～20–100 μm. Substitution of Pb²⁺ with Eu³⁺ is relatively easy despite the large difference in ionic radii, and the formation of vacancies is necessary to compensate the excess positive charge in PbMoO₄ framework. A change in lattice parameters (both a and c as well as lattice parameter ratio c/a) and progressive deformation of MoO₄ and WO₄ tetrahedra with increasing Eu concentration are observed. Thermal stability of Eu-doped materials strongly depends on the concentration of Eu³⁺ ions. The Pb_1–3x?_xEu_2x(MoO₄)_1–3x(WO₄)_3x solid solution for x = 0.0098 shows the highest melting point (1057 °C) which is slightly higher than that of pure PbMoO₄ (1040 °C). The UV-vis diffuse reflectance spectroscopy (DRS) and the Tauc plots were used to extrapolate the optical indirect band gap (E_g) of doped materials. Eu-doped ceramics are insulators (E_g > 3 eV) and their band gap nonlinearly decreases with increasing dopant concentration.