Single crystal growth and oxide ion conductivity of apatite-type rare-earth silicates

Ionic conductivity of apatite-type rare-earth silicates prepared by mechanical milling

“…However, the impact of the RE cations on the electrical properties of this type of materials have been investigated to a much lesser extent with contradic- tory results being published for single crystal and polycrystalline samples. Thus, whereas experimental data collected by using the first seem to suggest that conductivity is relatively independent of the rare-earth element involved [4], data obtained by using polycrystalline samples point at increasing conductivity and decreasing activation energy as the RE size increases [11,[23][24][25]. According to [4], these differences between results obtained for both types of samples should be attributed to increasing difficulties in obtaining single phase polycrystalline materials as the RE atomic number increases; i.e.…”

“…In addition, there are two available positions for the RE 3+ cations, a 7-coordinated site (6h (x, y, 1/4) according to the Wyckoff notation) located at the periphery of the same tunnel containing the mobile oxygen atoms and a 9-coordinated position (4f, (1/3, 2/3, z)) at the centre of a smaller tunnel running also parallel to the c-axis. Correspondingly, conductivity in single crystal apatite-type silicates, have been found to be anisotropic with values measured in a direction parallel to the c-axis in the hexagonal lattice higher than those observed perpendicular to the same axis [4]. A wide range of chemical substitutions as well as non-stoichiometry (cation vacancies and/or oxygen excess) are possible in such a crystal network both having an important effect on the electrical properties of this type of materials [5][6][7].…”

Oxygen-17 nuclear magnetic resonance measurements on apatite-type lanthanum silicate (La9.33(SiO4)6O2)

“…Apatite-type Ln silicates, Ln9.33+2x/3(SiO4)6O2+x, (Ln: rare earth elements) and related materials are promising oxide-ion conductors and are used as electrolytes in high-temperature solid oxide fuel cells because of their high conductivity, which is comparable or superior to that of yttria-doped zirconia [1][2][3][4][5][6][7]. The high conductivity was first observed by one of the authors [1,2].…”

“…In the study of single crystal Nd-Si-O apatite, the conductivity along the c-axis was found to be greater than that perpendicular to the c-axis; however, the difference between them decreases with increasing temperature, suggesting that the oxide ions migrate along the 001 direction. Furthermore, the migration perpendicular to the c-axis becomes significant at elevated temperatures [3,5]. The maximum entropy method (MEM) has been used to fit the neutron diffraction patterns and to investigate the conduction mechanism.…”

In situ X-ray diffraction studies of electroceramics

“…Two very recent reviews by Kharton et al [31] and Higuchi et al [63] to 1450 • C shown in Fig. 17 [64].…”