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 A2Mo2O7 and A2Mo3O10 (A = K, Rb, Cs)

Abstract 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 lyonsite-related 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 A2O–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 A2Mo2O7 and A2Mo3O10 (A = K, Rb, Cs) was measured and it was found that its highest values are achieved for Cs2Mo2O7 (σ = 1.2⋅10−4 S/cm at 460 °C). The electrical conductivity in the isostructural series A2Mo3O10 (A = K, Rb, Cs) increases from potassium trimolybdate to cesium trimolybdate and reaches σ = 3.1⋅10−5 S/cm at 460 °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 A2Mo2O7 and A2Mo3O10 (A = K, Rb, Cs), which show predominantly one-dimensional oxide-ion diffusion along the chains formed of molybdenum-centered polyhedra.