Morphotropy and Temperature-Driven Polymorphism in A₂Th(AsO₄)₂ (A = Li, Na, K, Rb, Cs) Series

A new alkaline thorium arsenate family was obtained and systematically investigated. The structures of A₂Th­(AsO₄)₂ (A = Li, Na, K, Rb, Cs) were determined from single crystal X-ray diffraction data. Li₂Th­(AsO₄)₂ and either isostructural K₂Th­(AsO₄)₂ and Rb₂Th­(AsO₄)₂ crystallize in the monoclinic crystal system. Na₂Th­(AsO₄)₂ and Cs₂Th­(AsO₄)₂ crystallize in the orthorhombic and tetragonal crystal systems, respectively. Li₂Th­(AsO₄)₂ consists of [Th­(AsO₄)₂]^2– layers with Li atoms in the interlayer space. The rest of the compounds are based on 3D frameworks. Differences in local environments of ThO₈ coordination polyhedra are described in relation to the symmetry. Despite different local environments of ThO₈ coordination polyhedra and different structural symmetry, underlying nets of A₂Th­(AsO₄)₂ (A = Na, K, Rb, Cs) were shown to be the same. Single-crystal and powder Raman spectra were measured, and bands are assigned. DSC measurements showed phase transitions in K₂Th­(AsO₄)₂ and Rb₂Th­(AsO₄)₂, which were studied using high-temperature powder X-ray diffraction (HT-PXRD). The data of HT-PXRD demonstrates two high-temperature polymorphic modification of K₂Th­(AsO₄)₂ and only one for the isotypic Rb₂Th­(AsO₄)₂. The phase transitions in both K and Rb phases are reversible