SOLID STATE SYNTHESIS OF MULTICOMPONENT RARE-EARTH OXIDE CERAMICS

Phase formation in multicomponent rare-earth oxides is determined by a combination of composition, sintering atmosphere, and cooling rate. Polycrystalline ceramics comprising various combinations of Ce, Gd, La, Nd, Pr, Sm, and Y oxides in equiatomic proportions were synthesized by solid-state sintering. The effects of composition (type and number of cations), sintering atmosphere (oxidizing, inert, and reducing), and cooling rate on phase formation were investigated. Single cubic or monoclinic phase compositions were obtained with a slow cooling of 3.3 ºC/min, indicating that rare-earth oxides follow a different phase stabilization process than that of transition metal high-entropy oxides. In an oxidizing atmosphere, both Ce and Pr induce the formation of a cubic phase, while only Ce plays that role in an inert or reducing atmosphere. Samples without Ce or Pr develop a single monoclinic phase. The phases formed at initial synthesis may be converted to a different one, when the ceramics are annealed in an atmosphere different than the original sintering atmosphere. Additionally, phase evolution of a five-cation composition was studied as a function of sintering temperature. The binary oxides used as raw materials completely dissolve into a single cubic structure at 1450ºC in air.