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Fine single-domain and polydomain particles of tetragonal ZrO, were prepared by hydrothermal and heat treatment of ZrO,.nNO. The particles were characterized by X-ray diffraction, electron microscopy, NMR, mass spectrometry, and ir spectroscopy. The main impurity in the samples was 1 to 2 wt% OH-ions, most of which were concentrated on the particle surfaces or a t domain boundaries; some were also distributed in the lattice. Fine single-domain tetragonal particles were strain-free, but polydomain particles had large strains. The single-domain tetragonal particles were transformed much more easily than the polydomain particles by mechanical treatment. The stablization of metastable tetragonal ZrOI cannot be explained adequately by the surfaceenergy theory. An explanation based on the concept of a martensitic transformation may be more reasonable.

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The enthalpies of transformation of pure, well‐characterized samples of brookite and anatase to rutile were determined by solution calorimetry in a 3Na2O·4MoO3 melt at 971 ±2 K. The experiments gave the following results: brookite→rutile, ΔH°971= ‐0.17±0.09 kcal mol−1; anatase → rutile, ΔH°971= ‐0.78±0.20 kcal mol−1.