Development of new solid electrolyte for low-temperature operating SOFC

Phase relationships in the LaO1.5–SiO2–MgO quasi-ternary system at 1773 K were investigated by powder x-ray diffraction (XRD) analysis applying single- and multiple-phase Rietveld methods. Most of the formed phases satisfied the Gibbs’ phase rule, except for the samples containing LaO1.5 and a liquid phase at 1773 K. The detection of segregated MgO phases was difficult in the XRD profiles of the compositional samples around the oxyapatite single phase because the MgO peaks were weak and heavily overlapped by peaks from the oxyapatite and La(OH)3 phases. The solid solubility limit of MgO in oxyapatite was determined not only from the chemical composition of the oxyapatite phase, which was confirmed by XRD, but also from several phase boundary compositions among the two-phase and three-phase regions based on the Gibbs’ phase rule. Formation of a liquid phase at 1773 K was observed in a wide range of compositions and considered when constructing the phase diagram.

In this study, we determined the crystallographic nature and electrical transport properties of Nd9.20(SiO4)6O1.8 and (La0.46Nd0.54)9.33(SiO4)6O2, which are defect-containing oxyapatites. The intensity data measured by synchrotron powder X-ray diffraction were analyzed by a Rietveld method. From the total conductivity data, the oxygen partial pressure region where the oxygen ionic conductivity (σO2-) predominates was determined to narrow down owing to the substitution of neodymium ions. A comparison of various solid solutions under similar temperature conditions ranging from 873 K to 1273 K showed that the σO2- values were lowest for (La0.46Nd0.54)9.33(SiO4)6O2 samples. The activation energy of the oxygen ionic conductivity increased with an increasing neodymium content.

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. ©2014 The Ceramic Society of Japan. All rights reserved.

The distribution of magnesium ions at the lanthanum and silicon sites in MgO-doped lanthanum silicate oxyapatite as well as the concentration of neutral lanthanum vacancies were determined using densities and chemical compositions of the doped samples. On the basis of the density data, it was found that magnesium ions are substituted at the silicon site as well as the lanthanum sites in the oxyapatite phase. Owing to the existence of neutral lanthanum vacancies, it was difficult to evaluate the number of the oxygen ions present, which are related to the oxygen ion conductivity of the compound, from the chemical compositions of the samples alone. Further, it was found that the fact that the total conductivity of MgO-doped lanthanum silicate oxyapatite depends on the MgO concentration as well as that of other defects could not be explained on the basis of conventional defect chemistry.

Although lanthanum germanate oxyapatite (La–Ge–O) has shown good potential for use as a solid electrolyte in energy storage applications, its synthesis has been challenging by either solid- or solution-state methods. In this study, a new synthesis of La–Ge–O was developed through a coprecipitation technique, in which a highly concentrated homogeneous aqueous solution of La and Ge was prepared from aqueous ammonium germanate and lanthanum nitrate solutions with the addition of dilute nitric acid. Several precipitates were formed by pH manipulation, including an amorphous material obtained at pH > 3. Compared to the individual precipitation behaviors of the parent compounds, the amorphous precipitate was formed only from the aqueous two-component mixture, and appeared to contain both metals. This material was transformed into crystalline mixtures upon heating at 1273 K. The crystalline phases were La2Ge3O9 and hexagonal-type GeO2 when the precipitate was formed below pH 8, and the La–Ge–O and La2Ge2O7 phases when the precipitate was formed around pH 8. Product formation from the coprecipitate was discussed based on X-ray diffraction and thermal analyses. The improved availability of La–Ge–O will allow more extensive investigations of its useful properties.

The discovery of oxygen-ion conductivity and changes in the crystal-structure model of nondoped lanthanum silicate oxyapatites are reviewed from the researches which led to the discovery to current work. The oxygen-ion conductivity of lanthanoid silicate oxyapatites was discovered by Nakayama et al., during development of new oxide lithium-ion conductors. Although samples with compositions of LiRESiO4 (RE = La, Nd, Sm, Gd, Dy) were initially reported to be single phases with the same crystal structure, the accurate crystal structure of LiRESiO4 was not clarified until later. The crystal structure determined from X-ray diffraction patterns of LiLnSiO4 was revised as oxyapatite by another group. The chemical composition of the crystal phases in LiRESiO4 was also revised to RE10Si6O25 and/or RE9.33Si6O24. The discovery of oxygen-ion conductivity in these materials was confirmed when researchers noted that samples of RE10Si6O25, specifically, the samples without lithium, are electrically conducting. Furthermore, very high oxygen-ion conductivity in the direction parallel to c-axis was discovered after single crystals of RE9.33(SiO4)6O2 (RE = Nd, Pr, and Sm) were successfully grown. The crystal structure and defect models were also altered after the discovery of oxygen-ion conductivity. Although numerous reports related to the electrical conductivity of La9.33+x(SiO4)6O2+3x/2 ceramics have appeared in the literature, clear dependences of the total conductivity on the cation nonstoichiometry (x) as well as the sintering temperature are still unclear because of large discrepancies in the reported data. Furthermore, the composition region, where the lanthanum silicate oxyapatite single phase is formed, is also still unclear because of the inconsistencies in the reported results.