Highly crystalline pure perovskite-type LaNbON2powders were synthesized in supercritical ammonia using sodium hydroxide as an oxygen source. Additionally, doping LaNbON2with cations of lower valence than that of the parent cation was performed to inhibit reduction of Nb5+. Various characterization methods indicated that crystallinity, particle morphology, and absorption edge of the product, that is, the factors possibly affecting photocatalytic activity, were not significantly changed by the doping of a lower-valence cation. Nevertheless, the doped LaNbON2synthesized using the ammonothermal method evolved hydrogen, suggesting that this type of doping decreases the formation of reduced niobium species and consequently enhances the photocatalytic activity of LaNbON2. In case of doped LaNbON2synthesized using conventional method, no hydrogen evolution was observed. This difference is probably due to the higher crystallinity of ammonothermally synthesized LaNbON2. Therefore, we successfully produced LaNbON2with improved potential for photocatalytic activity for hydrogen evolution under visible light irradiation using ammonothermal synthesis and lower-valence cation doping.

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The preparation of La5Ti2CuS5O7 single crystallites was attempted in various alkali metal halide molten salts used as flux materials. The average size of La5Ti2CuS5O7 crystallites produced was found to correlate with La5Ti2CuS5O7 solubility in the flux materials. For elongated La5Ti2CuS5O7 crystallites, CsCl melted at 1273 K was an optimal flux material. Needlelike crystallites with an average diameter of 0.8 ± 0.2 µm and an average length of 109.7 ± 19.2 µm were obtained after treatment for ∼48 h. Transmission electron microscopy revealed the monocrystalline nature of the needlelike La5Ti2CuS5O7 crystallites with a specific orientation. The quality of the crystallites of La5Ti2CuS5O7 and similar oxysulfide materials indicates their promising future application as transparent conductors, low-dimensional magnetic materials, and photocatalysts.

Hydroxyapatite (Ca 10 (PO 4) 6 (OH) 2 ; HAp) is widely used as a biomaterial due to its high biocompatibility. However, biological apatite present in bone and teeth contains various ions in the HAp crystal structure. Thus, biological apatite has many strains and defects, which may impart high osteoconductivity to apatite. To clarify the effects of the strain and/or defects in the HAp crystal structure on the bioactivity, nitrogen-doped (N-doped) HAp ceramics were fabricated by heating pure HAp ceramics. This N-doped method is well-known as a technique to modify the chemical structure at the surface. Some properties of the N-doped HAp ceramics were examined for optimization of the heating temperature. N-doped HAp ceramics fabricated by heating at 850°C in an NH 3 atmosphere had N 2 O molecules in the crystal structure. Therefore, the N-doped HAp ceramics fabricated by the N-doping method have a rare moleculesubstitution structure. In conclusion, we have developed the N-doped HAp ceramics which have a rare molecule-substitution structure.