Molybdates and tungstates of the alluaudite family: crystal chemistry, composition, and ionic mobility

The work reviews the structure, non-stoichiometry, and ionic mobility of molybdates, tungstates, and other compounds crystallizing in the structure type of alluaudite (Na, Ca)(Fe, Mn, Mg) 3 (PO 4 ) 3 with the general Moore′s crystal chemical formula X (2) X (1) M (1) M (2) 2 ( T O 4 ) 3 , where X are large cations Na + , Ca 2+ , K + , Pb 2+ , etc., with the coordination number 8; M are octahedral cations, T = P, As, V, S, Mo, W. Using this formula and the corresponding site occupancies, possible limits of double molybdate and tungstate compositions of the alluaudite family are determined. Various types of distortions (superstructures) of alluaudite are considered; several groups of phases with different symmetries, numbers of anions in the unit cell, and vector relations with the unit cell of the original alluaudite structure are distinguished. It is shown that chains of partially defective positions X (2) and X (1) aligned along axis c play a key role in the transport of sodium cations in the alluaudite type phases. Phosphates and sulfates with alluaudite structure exhibit mainly 1D transport of sodium ions; however, calculations of the bond-valence sum maps, NMR data, and ab initio calculations show that 2D transport in the (100) plane is possible in complex molybdates and tungstates due to the transport of Na + ions between X (2)– X (2) and X (1)– X (1) channels through the bridging site M (1). It is shown that the family of alluaudite-related (pseudo)orthorhombic triple molybdates Na 10 Cs 4 M 5 (MoO 4 ) 12 ( M = Mn, Co) and Na 25 Cs 8 R 5 (MoO 4 ) 24 ( R = Fe, Sc, In) also exhibits 2D diffusion of sodium ions via successive zigzag ion hoppings and that 3D transport may appear at elevated temperatures.