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.