Synthesis, characterization and thermal behaviour of new copper and rare-earth metal tungstates

Three series of new copper and rare-earth metal tungstates with the formulas: CuRE ₂W₂O₁₀ (RE = Nd, Sm, Eu) and Cu₃ RE ₂W₄O₁₈ (RE = Sm, Eu or RE = Dy, Ho, Er) were synthesized by the solid-state reaction method. The CuRE ₂W₂O₁₀ and Cu₃ RE ₂W₄O₁₈ (RE = Dy, Ho, Er) compounds crystallize in the monoclinic system. The Cu₃ RE ₂W₄O₁₈ phases with the large rare-earth ions crystallize in the triclinic system. All obtained compounds melt incongruently below 1273 K. The anion lattice of the Cu₃ RE ₂W₄O₁₈ phases is built from isolated groups of octahedra (W₄O₁₆)^8?, while CuRE ₂W₂O₁₀ from WO₆ octahedra forming structural elements (W₂O₉)^6?]^∞. The EPR spectra of analyzed compounds consisted of an intense line originating generally from the rare-earth ions and a weak, narrow line from Cu²⁺ separate centers appearing only on the surface of the grains. The absence of bulk copper in the EPR spectrum is probably due to a very short relaxation time of the Cu²⁺ subsystem.