Towards atomistic understanding of polymorphism in the solvothermal synthesis of ZrO nanoparticles.

Varying atomic short-range order is correlated with the ratio of the monoclinic (m) to tetragonal (t) phase in ZrO nanoparticle formation by solvothermal methods. Reactions from Zr oxynitrate in supercritical methanol and Zr acetate in water (hydrothermal route) were studied in situ by X-ray total scattering. Irrespective of the Zr source and solvent, the structure of the precursor in solution consists of edge-shared tetramer chains. Upon heating, the nearest-neighbor Zr-O and Zr-Zr distances shorten initially while the medium-range connectivity is broken. Depending on the reaction conditions, the disordered intermediate transforms either rapidly into m-ZrO, or more gradually into mixed m- and t-ZrO with a concurrent increase of the shortest Zr-Zr distance. In the hydrothermal case, the structural similarity of the amorphous intermediate and m-ZrO favors the formation of almost phase-pure m-ZrO nanoparticles with a size of 5 nm, considerably smaller than the often-cited critical size below which the tetragonal is assumed to be favoured. Pair distribution function analysis thus unravels ZrO phase formation on the atomic scale and in this way provides a major step towards understanding polymorphism of ZrO beyond empirical approaches.