Stable cubic ZrO2 up to a temperature of 1173 K has been synthesized b
y a chemical precipitation technique. Such a stability appears to be d
riven by particle size. The critical value below which cubic ZrO2 has
been found to be stable is 17 nm. The x-ray diffraction pattern of suc
h ultrafine cubic particles is similar to that obtained by stabilizati
on of ZrO2 by the addition of 20 mole % CaO.