Structural, Thermal, and Optical Properties of Er3+-Doped Lead Tungstates and Lead Molybdate–Tungstates

New Er3+-doped lead tungstates and molybdate–tungstates with the chemical formulae Pb1−3x▯xEr2xWO4 (0 < x ≤ 0.0283) and Pb1-3x▯xEr2x(MoO4)1-3x(WO4)3x (0 < x ≤ 0.0536), where ▯ denotes a vacant crystallographic site, were successfully synthesized via a high-temperature solid-state reaction method using PbWO4 or PbMoO4 and Er2(WO4)3 as starting reactants. X-ray diffraction and scanning electron microscopy analyses confirmed the formation of single-phase, tetragonal scheelite-type materials (space group I41/a) with an average crystallite size ranging from approximately 50 to 100 μm. The Er3+-doped ceramic materials remained stable up to their melting points and did not undergo any polymorphic transformation. Their melting points were lower than those of the corresponding scheelite-type matrix, namely, PbWO4 and PbMoO4. UV–Vis–NIR diffuse reflectance measurements demonstrated that all Er3+-doped materials are insulating and exhibit indirect optical transitions. The incorporation of Er3+ leads to a reduction in the optical band gap energy, from 3.73 to 3.47 eV for the doped tungstates and from 3.17 to 3.01 eV for the doped molybdate–tungstates, accompanied by a simultaneous increase in the Urbach energy. The refractive indices estimated using seven different empirical models exhibit only a weak dependence on Er3+ concentration and remain relatively high (approximately 2.15–2.50) for both material series.