Rapid, energetic metathesis routes to crystalline metastable phases of zirconium and hafnium dioxide

Rapid, exothermic metathesis (exchange) reactions between ZrCl4 and Na2O result in crystalline ZrO2 in seconds. Thermal analysis and in situ reaction temperature measurements show that the reaction initiates near the sublimation point of ZrCl4, rapidly reaches temperatures above 1350 °C, and generates a molten NaCl flux for a few seconds. Powder X-ray diffraction demonstrates that the product is a mixture of room-temperature monoclinic and high-temperature cubic ZrO2 phases. The addition of cubic phase stabilizers such as Ca, Y, or Ce oxides or their chlorides to the reaction results in an increase in cubic phase formation up to nearly 70%. These partially stabilized products can withstand annealing to 1300 °C, unlike the unstabilized product, which reverts to the thermodynamic room-temperature monoclinic form on heating to 900 °C. The analogous reaction between HfCl4 and Na2O results in crystalline HfO2. The addition of CaO to this reaction also enhances cubic phase formation.