Monolithic t-ZrO2 (tetragonal structure) nanopowder is synthesized with an amorphous ZrO(OH)(2)(.)xH(2)O polymer precursor. The H2O molecules impart the structure and promote reconstructive thermal decomposition of the structure to t-ZrO2 nanoparticles on heating at temperatures as low as 200degreesC. A prevalent endothermic heat output in the dissociation process controls the local temperature in exothermic nucleation and growth of various groups of the reaction species so that it is self-controlled in high-energy nanoparticles. Crystallites are, on average, d = 8 nm diameter, and they have a high value of Gibbs free energy or lattice volume V-o = 0.06770 nm(3). The excess volume decreases to V-o = 0.06705 nm(3) if the reaction temperature is increased to >200degreesC, i.e., approximately the bulk value of 0.06681 nm(3); there is a minor increase to d = 12 nm at 600degreesC. Many oxygen vacancies in the thin surface space-charge layers seem to support the stability of small particles in this particular polymorph. A pure ni-ZrO2 (monoclinic structure) appears with d = 22 nm at temperature as low as 800degreesC. The results are analyzed using X-ray diffractometry, microstructure, infrared spectroscopy, and thermal studies of the polymer precursor and derivative t-ZrO2 nanoparticles.