Zirconia (ZrO₂) primarily exists in three different polymorphs at ambient pressure: monoclinic (room temperature-1175°C), tetragonal (1175-2370°C), and cubic (2370-2680°C). The high-temperature ZrO₂ phases are suitable for various industrial applications such as solid electrolytes in solid oxide fuel cells and sensors, as a catalyst/catalyst support, and as membranes and dispersed phase in composite materials. Traditionally, high temperature ZrO₂ phases have been stabilized at room temperature by doping bi and trivalent cations, such as Y, in the ZrO₂ lattice. The high-temperature cubic or tetragonal phases can also be stabilized at room temperature without doping, provided ZrO₂ is synthesized in its nanocrystalline form with a grain size lower than a critical value (about 20-30 nm). Five different ZrO₂ samples were studied, with the following compositions: sample 1 (pure ZrO₂, no doping), sample 2 (0.5% Y), sample 3 (1% Y), sample 4 (2% Y) and sample 5 (4% Y). The aim of this study is double-fold: on one side the standard crystallographic techniques were used to check for the presence of size-stabilised polymorphs and to evaluate the crystal size and the microstrains present, while, on the other side, a PDF study of size-stabilised zirconia would provide information on local distortions of the nano-particles, on their temperature evolution, and on the eventual presence of significant surface defects, together with their nature. Moreover, such a study provides the relationship of the presence of size-stabilised polymorphs with the actual size of the particles and their local distortions, supplying thus a major step in the understanding of the profound reasons for size-stabilisation effects. Only the two samples with the higher Y content showed no monoclinic zirconia at all. All the other samples showed small amounts of the monoclinic phase (no more than 10% wt of monoclinic ZrO₂). Moreover, the higher the Y content, the less distorted the structure: sample 4 and 5 could be fitted (in a classical Rietveld refinement) with the cubic structure (fluorite type), while the others were fitted with the tetragonal structure. Total scattering experiments produced well defined PDFs, where the monoclinic polymorph was easily distinguishable from the tetragonal one. As an add on to traditional crystallography, the PDF fits showed a misfit in the low R region (below about 10 Å), probably due to a distortion in the local structure, produced by the limited spatial coeherency of the nano-particles. The origin of the local distortion is still under study.

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