Zirconia-based particles, sols containing the zirconia-based particles, methods of making the sols and the zirconia-based particles, composites containing the zirconia-based particles in an organic matrix, and sintered bodies prepared from the zirconia-based particles are described. The zirconia-based particles are crystalline, have a primary particles size no greater than 100 nanometers, and are doped with a lanthanide element or with both a lanthanide element and yttrium.