Structural characterization of amorphous hydrous Zr(IV) oxide and the transformation occurring under hydrothermal conditions

The impact of temperature on the transformation of ZrO$_2$(am, hyd) in aqueous systems was systematically investigated using a multi-method approach for solid phase characterization. Ageing at T = 80° C in NaOH and alkaline CaCl$_2$–Ca(OH)$_2$ solutions triggered the conversion of ZrO$_2$(am, hyd) to more crystalline, thermodynamically stable anhydrous oxides. Compared to the fresh amorphous solid and solids aged at T = 22 °C, an increase in crystallinity and particle size accompanied by a significant decrease in the amount of hydration water or hydroxide was observed. After equilibration for 4 or 10 months at T = 80° C, XRD measurements confirmed the predominance of monoclinic ZrO$_2$ with a particle size of 23–27 nm in NaOH systems, whereas tetragonal/cubic ZrO$_2$ with a particle size of 11–14 nm formed in CaCl$_2$–Ca(OH)$_2$ solutions. All solids aged at T = 22° C remained X-ray amorphous within the timeframe of this study. Rietveld refinement of the XRD patterns, EDX and EXAFS supported that the stabilization of a different crystal structure in CaCl$_2$–Ca(OH)$_2$ systems is caused by the incorporation of Ca$_{2+}$ in the ZrO$_2$ structure. ... mehrThe weight loss observed with TG-DTA was attributed to a decreasing amount of hydration water during the ageing at T = 80° C. Assuming a stoichiometry of ZrO$_2$·nH$_2$O, the fresh solid showed a weight loss corresponding to n = 2.1, which remained constant during the ageing at T = 22 °C. The solids aged at T = 80° C showed a smaller weight loss corresponding to n = 0.2–0.6. This behavior was qualitatively confirmed by IR-ATR. XPS analysis of the O1s line also showed a decrease of H$_2$O/OH$^-$ contributions and an increase of O$^{2−}$ contributions in the solid phases aged at T = 80° C. This study provides new insights for the understanding of the transformation of M(IV) amorphous, hydrous oxides into the thermodynamically stable anhydrous crystalline oxide phases. The process involves particle growth, increased ordering and loss of H$_2$O/OH$^-$ groups. In aqueous systems, the presence of other metal ions like Ca$^{2+}$ may promote the stabilization of different crystal structures, even if they are found at low concentrations.