Molten salt synthesis of K4Nb6O17, K2Nb4O11 and KNb3O8 crystals with needle- or plate-like morphology

Abstract

Here we report on molten salt synthesis of four different compounds in the K₂O–Na₂O–Nb₂O₅ system. The three compounds K₄Nb₆O₁₇, K₂Nb₄O₁₁ and KNb₃O₈, with non-cubic crystal structures, were prepared as single crystalline particles with large aspect ratios. K₄Nb₆O₁₇ was prepared with a plate-like morphology, K₂Nb₄O₁₁ a fibre-like and finally KNb₃O₈ had a plate like morphology. K_xNa_1−xNbO₃, with a cubic perovskite crystal structure at the synthesis conditions, was only obtained as cube-shaped crystals, which became larger with increasing synthesis temperature. A systematic study of the influence of the processing conditions on the morphology of the product compounds was conducted. The study demonstrated that the morphology of the product phase was controlled by the crystal structure and the connectivity of the NbO₆-octahedra of the crystal structures. Finally, the crystal structure of K₂Nb₄O₁₁ was determined to be a tungsten bronze type structure with space groupP4/mbm.