Formation and characterization of nanocrystalline binary oxides of yttrium and rare earths metals

Binary oxides of yttrium (Y) and rare earths (R) are used for their varied chemical and physical (e.g., optical and dielectric) properties. Coprecipitated xerogels, which are mixtures of hydroxides and oxides of Y and R, were thermally annealed in air at constant temperatures in the range from 100 to 1400 °C for 3 h. The lowest temperature at which a pure oxide is obtained varies with composition. The binary oxides of Y(III) and R(III) afford ideal solid solutions with chemical formula (R_xY_1-x)₂O₃, where the unit cell parameters are a linear function of the atomic ratio x = R/(R + Y), obeying Vegard's law. However, the range of solubility was dependent on the composition and temperature of formation. In the case of binary Y(III)-R(IV) oxides, of general formula (R_xY_1-x)₂O_3+x, two ranges of solubility were clearly identified: in the system of Y-Pr oxides a solubility gap was found between these ranges, whereas in the case of Ce-Y oxides the solutions were found by XRD and TEM to be coherent, without a solubility gap. XRD line broadening analysis points to line broadening as being mainly due to small crystallite size, whereas the contribution of microstrain to line broadening is negligible. The magnetic properties are strongly dependent on the particle size. Nanocrystalline Sm(III) oxide and the SmYO₃ solid solution formed at 900 °C are paramagnetic; however, superparamagnetism was found for the same compounds with larger grain size, formed at 1200 °C, for both cubic and monoclinic structures.