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 rare-earth silicate oxyapatites: their identification as a new compound until discovery of their oxygen ion conductivity Kiyoshi KOBAYASHI³ and Yoshio SAKKA Materials Processing Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, JapanRudimental 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 2RE 2 O 3 ·3SiO 2 , which was called orthosilicate. "RE" is the rare-earth elements. Different compositions of 2RE 2 O 3 ·3SiO 2 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.

Research progress in nondoped lanthanoid silicate oxyapatites as new oxygen-ion conductors

“…As pointed out in our previous review, 5) the oxyapatite phase in many quasi-ternary systems were misidentified. 13) One reason for this misidentification was a lack of quasi-ternary phase diagrams that included rare-earth oxides and silica.…”

“…4) Even though almost no reports concerning point defects related to the oxygen and the disorder of oxygen distribution in lanthanoid silicate oxyapatites were published before 1995. 5) Furthermore, the discovery of high oxygen-ion conductivity has opened new research areas for many researchers. As a result, the development direction of the research field related to lanthanoid silicate oxyapatites has drastically changed since 1995.…”

“…As explained in our previous review, 5) which summarized the progress regarding the research related to rare-earth silicate oxyapatites starting from its identification as a new compound (1959) until the discovery of its oxygen-ion conductivity (1993), the basic knowledge of the crystal structure and applications of rare-earth silicate oxyapatites gradually improved from a state of uncertainty caused by a discrepancies in understanding of underlying physical and chemical properties to a consistent and reliable state. However, the knowledge acquired before 1993 was partially cast into doubt in 1995 because of the discovery of a new property, specifically, oxygen-ion conductivity.…”

Low-temperature formation of apatite structure of neodymium silicate in silica matrix

“…According to the experimental and theoretically predicted phase diagrams of the system Nd2O3-SiO2, three compounds of above neodymium silicates, their mixtures with Nd2O3 and SiO2 are present [5][6][7][8]. These neodymium silicates are formed at ratio of Nd2O3:SiO2 as 1:1 (Nd2SiO5), 7:9 (Nd9.33Si6O26), 1:2 (Nd2Si2O7) and coexist with Nd2O3 and SiO2 when other ratios.…”

“…Among them, neodymium silicates occupy a significant place. There are silicates -Nd2SiO5, Nd2Si2O7, Nd9.33Si6O26 and their solid solutions with other elements instead of Nd and Si [1][2][3][4][5].…”