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We synthesize, for the first time, using sol−gel technique and without doping any trivalent impurities, monodispersed, spherical ZrO 2 particles in size range of ∼500−600 nm, exhibiting metastable tetragonal crystal structure at room temperature. It is revealed using high-resolution transmission electron microscopy that "hard-aggregates" forming tendency of ZrO 2 nanocrystallites of size ∼45 nm is responsible for stabilizing the high-temperature metastable tetragonal phase, at room temperature, within large sized (∼500−600 nm) undoped ZrO 2 particles.