Apatite-type lanthanum silicates of general formula La9.33+2x/3(SiO4)6O2+x have appeared recently as a new promising class of oxide ion conductors with potential applications as electrolytes for solid oxide fuel cells (SOFCs). They have been shown to demonstrate relatively high oxide ion conductivity at moderate temperatures as well as at low oxygen partial pressures. In this paper, the diffusion pathways and the conduction mechanism of oxide ions in these phases are reinvestigated. This is done by means of atomic scale computer modeling techniques with both semiempirical and bond valence methods. Our results support that oxide ion conduction along the c-axis proceeds by an interstitial mechanism. They also support the presence of interstitial sites located within the conduction channel. However, contrarily to recent research, it is shown that the channel oxide ions are involved in the conduction process by a push−pull type mechanism. This mechanism brings into play a cooperative movement of both two adjacent interstitial oxide ions forming a complex defect and the channel oxide ions. This complex defect is shown to move along the c-axis via a nonlinear pathway different from the conduction path proposed in literature to date. The calculated migration energy of this mechanism is found to be equal to 0.32 eV, which compares well with activation energy measured along the c-axis for Nd9.33(SiO4)6O2 single crystals.

No abstract

We have successfully synthesized the highly c-axis-oriented polycrystals of apatite-type lanthanum silicate by the reactive diffusion technique. When the La 2 SiO 5 /La 2 Si 2 O 7 diffusion couples were isothermally heated at 1773−1873 K for 5−100 h, the apatite polycrystals were readily produced in the form of a layer at the interfacial boundaries. The annealed couples were characterized using optical microscopy, micro-Raman spectroscopy, X-ray diffractometry, and electron probe microanalysis. The product layers were composed of the highly c-axis-oriented prismatic crystallites, with their elongation directions being almost parallel to the diffusion direction. The formation of the apatite layer was controlled by volume diffusion, the overall reaction of which is described by (10 + 6x)La 2 SiO 5 + (4−3x)La 2 Si 2 O 7 → 3La 9.33+2x (SiO 4 ) 6 O 2+3x (0.01 ≤ x ≤ 0.13). The apatite layer formed at 1873 K was characterized by the steady decrease of the x-value along the diffusion direction from 0.13 at the La 2 SiO 5 /apatite interface to 0.01 at the apatite/La 2 Si 2 O 7 interface. We have also prepared sandwich-type La 2 Si 2 O 7 /La 2 SiO 5 /La 2 Si 2 O 7 diffusion couples and heated them at 1873 K for 100 h. The annealed couple was mechanically processed, and the thin-plate electrolyte consisting of the highly c-axis-oriented polycrystal was obtained. The oxide-ion conductivity was determined from the impedance spectroscopy data at 573−973 K, which steadily increased from 2.4 × 10 −3 S/cm to 2.39 × 10 −2 S/cm with increasing temperature. The empirical activation energy of conduction was 0.35 eV, which compares well with the calculated migration energy of 0.32 eV in a previous study.

Despite numerous prior studies, important questions about the Japanese color lexicon persist, particularly about the number of Japanese basic color terms and their deployment across color space. Here, 57 native Japanese speakers provided monolexemic terms for 320 chromatic and 10 achromatic Munsell color samples. Through k-means cluster analysis we revealed 16 statistically distinct Japanese chromatic categories. These included eight chromatic basic color terms (aka/red, ki/yellow, midori/green, ao/blue, pink, orange, cha/brown, and murasaki/purple) plus eight additional terms: mizu ("water")/light blue, hada ("skin tone")/peach, kon ("indigo")/dark blue, matcha ("green tea")/yellow-green, enji/maroon, oudo ("sand or mud")/mustard, yamabuki ("globeflower")/gold, and cream. Of these additional terms, mizu was used by 98% of informants, and emerged as a strong candidate for a 12th Japanese basic color term. Japanese and American English color-naming systems were broadly similar, except for color categories in one language (mizu, kon, teal, lavender, magenta, lime) that had no equivalent in the other. Our analysis revealed two statistically distinct Japanese motifs (or color-naming systems), which differed mainly in the extension of mizu across our color palette. Comparison of the present data with an earlier study by Uchikawa & Boynton (1987) suggests that some changes in the Japanese color lexicon have occurred over the last 30 years.

We have prepared the highly c-axis-oriented polycrystalline material of apatite-type La 9.50 Si 6 O 26.25 by isothermal heating of the sandwich-type La 2 SiO 5 /La 2 Si 2 O 7 / La 2 SiO 5 diffusion couple at 1873 K for 50 h. The resulting polycrystal was characterized using optical microscopy, X-ray diffractometry, and impedance spectroscopy. The annealed couple was mechanically processed, and the thin-plate electrolyte consisting of the textured polycrystal was obtained. The oxide-ion conductivity along the c-axis steadily increased from 2.0 × 10 −2 S/cm to 7.9 × 10 −2 S/cm with increasing temperature from 723 to 1073 K. The conductivity of this material was, at 723−973 K, about 2.5 times higher than that of the c-axis-oriented apatite polycrystal of La 9.33 Si 6 O 26 . These two materials have the identical activation energy of conduction (0.35 eV), and hence the conduction mechanism must be the same. Both crystal structures of La 9.50 Si 6 O 26.58 and La 9.33 Si 6 O 26 at ambient temperature (space group P6 3 /m) showed the appreciable positional disordering of O atoms (12i site) that are bonded to Si atoms, together with the anharmonic displacements of La atoms (4f and 6h sites). The former structure is further characterized by the positional disordering of channel oxide ions (2a and 4e sites) as well as the presence of interstitial oxide ions (6h site), which would contribute to the higher conductivity along the c-axis.

We investigated the crystallographic structure of FePS3 with a layered structure using transmission electron microscopy and powder X-ray diffraction. We found that FePS3 forms a rotational twin structure with the common axis along the c*-axis. The high-resolution transmission electron microscopy images revealed that the twin boundaries were positioned at the van der Waals gaps between the layers. The narrow bands of dark contrast were observed in the bright-field transmission electron microscopy images below the antiferromagnetic transition temperature, TN ≈ 120 K. Low-temperature X-ray diffraction showed a lattice distortion; the a- and b-axes shortened and lengthened, respectively, as the temperature decreased below TN. We propose that the narrow bands of dark contrast observed in the bright-field transmission electron microscopy images are caused by the directional lattice distortion with respect to each micro-twin variant in the antiferromagnetic phase.

No abstract

No abstract