Cation-Dependent Structural Evolution in A2Th(TVO4)2 (A = Li, Na, K, Rb, Cs; T = P and As) Series

A new alkali thorium phosphate family, A(2)Th(PO4)(2)(A = Li, Na, K, Rb, Cs), has been synthesized and systematically investigated. The structural evolution of this new family was compared with that of the previously reported arsenic based family A(2)Th(AsO4)(2) (A = Li, Na, K, Rb, Cs). Similar compositions of the phosphorus- and arsenic-containing series allowed their direct comparison in order to reveal the influence of the size and nature of either monovalent or pnictogen cations on the crystal structure and related properties. Detailed crystal chemical analyses suggest that two main driving forces are in charge for the structural evolution, i.e.,the size of alkali cations as well as the local environment of the thorium polyhedra and the TO4 (T = P, As) tetrahedra. Raman spectra were measured for all studied phases and bands have been assigned. The density functional theory calculations revealed that the local environment of the PO4 tetrahedra may be responsible for shifts of PO4 vibrational bands observed in Li-based phases.