Ball milling-induced pyrochlore-to-tungsten bronze phase transition in RbNbWO6

A set of Bragg peaks consistent with a hexagonal Bravais lattice was observed in the x-ray powder diffraction pattern of cubic pyrochlore rubidium tungstoniobate (RbNbWO6) subjected to high-energy ball milling. The calculated lattice parameters for this hexagonal phase are similar to those of compounds with tungsten bronze structure. In fact, the powder pattern of the hexagonal phase could be refined with a structural model based on the tungsten bronze structure. The hexagonal phase produced by high-energy ball milling of RbNbWO6 transforms back to the pyrochlore structure upon heating to 773 K in air. A similar phase was obtained by ball milling the mixture RbNbWO6 + WO3, but, in this case, the stoichiometric hexagonal tungsten bronze compound thus obtained remained stable up to 1273 K.