Towards atomistic understanding of polymorphism in the solvothermal synthesis of $ZrO_2$ nanoparticles

Varying atomic short-range order is correlated with the ratio of the monoclinic (m) to tetragonal (t) phase in ZrO$_2$ 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$_2$, or more gradually into mixed m- and t-ZrO$_2$ with a concurrent increase of the shortest Zr-Zr distance. In the hydrothermal case, the structural similarity of the amorphous intermediate and m-ZrO$_2$ favors the formation of almost phase-pure m-ZrO$_2$ 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$_2$ phase formation on the atomic scale and in this way provides a major step towards understanding polymorphism of ZrO$_2$ beyond empirical approaches