Crystallization and Transformation of Zirconia Under Hydrothermal Conditions

During constant‐rate heating to 350°C in concentrated NaOH solutions, cubic ZrO 2 crystallized at ∼120°C from hydrated amorphous ZrO 2 ; these One cubic ZrO 2 particles abruptly changed into needlelike monoclinic ZrO 2 single crystals at 300°C. Crystallization and phase transformation were studied by XRD, TEM, and EPMA. Cubic ZrO 2 appears to crystallize via collapse of the ZrO 2 −nH 2 O structure and subsequent slight rearrangement of the lattice. The abrupt formation of mono‐clinic ZrO 2 was considered to result when the very fine cubic ZrO 2 particles coagulated in a highly oriented fashion.