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 fired in air at constant temperatures in the range from 100 to 1400ºC for 3h. The lowest temperature at which a pure oxide is obtained varies with composition. Whereas Y₂O₃ has always the Mn₂O₃ type cubic structure (cI80), pure R oxides have also other crystal structures. Therefore, formation of a single phase as a solid solution of yttria and a rare earth oxide is not certain, and is dependent on the xerogel composition and the temperature of annealing. A special case of dissimilarity is the attempt to alloy together Y(III) oxide with an R(IV) oxide. Solid solutions containing Y(III) and R(III) have chemical formula (R_xY_1-x)₂O₃, and those containing Y(III) and R(IV) have formula (R_xY_1-x)₂O_3+x. The latter compositions require a peculiar type of cubic structure, based on the Mn₂O₃ structure. The binary oxides of Y(III) and R(III) afford ideal solid solutions where the unit cell parameter obeys Vegard's law. In the case of Y(III)-R(IV) oxides, the alloys are not ideal binary solutions and the unit cell parameters are not a linear function of the atomic ratio x = R/(R+Y). The cell parameters usually are larger for alloys fired at lower temperatures. 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 relatively small. The XRD results are confirmed by TEM observations. The magnetic properties are strongly dependent on the particle size. Nanocrystalline Sm(III) oxide and the SmYO₃ solid solution are paramagnetic; however, superparamagnetism is also sometimes detected

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