Rudimental research progress of rare-earth silicate oxyapatites: their identification as a new compound until discovery of their oxygen ion conductivity

Abstract

Rudimental research progress of oxyapatite-type rare-earth silicates is reviewed based on the published papers mainly from 1959 to 1993 that have not yet been discussed in detail. The knowledge of oxyapatite-type rare-earth silicates significantly increased during this period. Chemical compounds of rare-earth oxides and silica were discovered around 1960. Because of the complex chemical composition of the oxyapatite phase, the composition was initially considered as 2RE2O3·3SiO2, which was called orthosilicate. “RE” is the rare-earth elements. Different compositions of 2RE2O3·3SiO2 have been proposed by crystal structure analysis based on the crystal chemistry and the leaping model. With respect to crystal structure analysis, knowledge has gradually improved step-by-step, including the implicit distinction between oxygen-stoichiometric apatite and oxygen-deficient apatite. Based on the published work, the rare-earth silicate oxyapatites are considered to have an apatite-like structure. Initially, application research focused on the optical properties of oxyapatite because rare-earth metals were constituent elements of the crystals, and on the use of oxyapatite as a stabilizer of unwanted radioactive waste produced in nuclear power reactors because oxyapatites can dissolve the actinide elements.