Phase Formation, Polymorphism, Optical Properties, and Conductivity of Nd₂WO₆-Based Compounds and Solid Solutions

The phase formation of neodymium tungstate Nd₂WO₆ from mechanically activated oxides was studied in a wide temperature range: 25–1600°C. Conditions for the formation of various polymorphic modifications were determined: low-temperature orthorhombic β-Nd₂WO₆ and δ-Nd₂WO₆ (P2₁2₁2₁ (no. 19)); high-temperature monoclinic Nd₂WO₆ (C12/c1 (no. 15)). Optical absorption spectra were studied for polymorphic ceramics with a nominal composition of Nd₂WO₆. Differences in the spectra of δ-Nd₂WO₆ and monoclinic Nd₂WO₆ were detected. Both modifications (δ-Nd₂WO₆ and monoclinic Nd₂WO₆) showed proton conductivity with activation energies of 1.05 and 1.06 eV, respectively. For the Ca-containing solid solution with a monoclinic structure (Nd \(_{{1 - x}}\) Ca_x)₂WO \(_{{6-\delta }}\) (x = 0.01), whose overall conductivity increases compared to that of pure monoclinic Nd₂WO₆, hole conductivity predominates in air.