Zirconium tungstate hydroxide hydrate revisited: Crystallization dependence on halide and hydronium ions

The formation of zirconium tungstate hydroxide hydrate, a precursor to the negative thermal expansion material cubic zirconium tungstate, shows a strong dependence on hydrothermal reaction conditions. It was found that not only the acid concentration, but also the acid counterion plays a significant role in the crystallization of ZrW2O7(OH)2·2H2O. High temperatures, high acid concentrations, and the presence of chloride or bromide ions promote the formation of well-crystallized ZrW2O7(OH)2·2H2O. For low acid concentrations, a new zirconium tungstate hydrate polymorph is observed, which transforms to tetragonal ZrW2O7(OH)2·2H2O at longer reaction times. A study of crystallization kinetics in hydrochloric acid is presented.