Selection of alkali polymolybdates as fluxes for crystallization of double molybdates of alkali metals, zirconium or hafnium, revisited crystal structures of K2Mo2O7, K2Mo3O10, Rb2Mo3O10 and ionic conductivity of A(2)Mo(2)O(7) and A(2)Mo(3)O(10) (A = K, Rb, Cs)

Composition, thermal stability, crystal structure, and flux crystallization features of double molybdates of alkali metals with zirconium or hafnium are considered. The homogeneity ranges and crystal structures of lyonsiterelated Li2+4xM1- x (MoO4)(3) (M = Zr, Hf) were revised. The alkaline polymolybdates being the decomposition products of the double molybdates are shown to be the optimal fluxes for their crystallization. The areas of the ternary systems A(2)O-MoO3-ZrO2 (A = Li, Na, K, Rb, Cs) suitable for obtaining crystals of the double molybdates are outlined. The crystal structures of K2Mo2O7, K2Mo3O10 and Rb2Mo3O10 obtained as by-products in runs on crystallization of the double molybdates were refined to update the data of previous works. The electrical conductivity of A(2)Mo(2)O(7) and A(2)Mo(3)O(10) (A = K, Rb, Cs) was measured and it was found that its highest values are achieved for Cs2Mo2O7 (sigma = 1.2.10(-4) S/cm at 460 degrees C). The electrical conductivity in the isostructural series A(2)Mo(3)O(10) (A = K, Rb, Cs) increases from potassium trimolybdate to cesium trimolybdate and reaches sigma = 3.1.10(-5) S/cm at 460 degrees C for the latter. As a result, it was suggested that ionic conductivity of these polymolybdates is caused by the transfer of oxide ions. This was confirmed with our calculations of the bond valence based energy (BVE) landscapes for oxygen anions in the structures of A(2)Mo(2)O(7) and A(2)Mo(3)O(10) (A = K, Rb, Cs), which show predominantly one-dimensional oxide-ion diffusion along the chains formed of molybdenumcentered polyhedra.