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

Rapid, exothermic metathesis (exchange) reactions between ZrCl4 and Na2O re sult in crystalline ZrO2 in seconds. Thermal analysis and in situ reaction temperature measurements show that the reaction initiates near the sublimat ion point of ZrCl4, rapidly reaches temperatures above 1350 degreesC, and g enerates a molten NaCl flux for a few seconds. Powder X-ray diffraction dem onstrates 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 stab ilized products can withstand annealing to 1300 degreesC, unlike the unstab ilized product, which reverts to the thermodynamic room-temperature monocli nic form on heating to 900 degreesC. The analogous reaction between HfCl4 a nd Na2O results in crystalline HfO2. The addition of CaO to this reaction a lso enhances cubic phase formation.