A nanometer niobium carbide/carbon nanotube reinforced diamond composite and a preparation method thereof, belonging to the field of materials science. The nanometer niobium carbide/carbon nanotube reinforced diamond composite is composed of nanometer niobium carbide/carbon nanotube composite powders, matrix powders and diamond grains, wherein the nanometer niobium carbide/carbon nanotube composite powders are the composites of nanometer niobium carbide which are evenly distributed in the surface defects and interior of the carbon nanotube, the nanometer niobium carbide/carbon nanotube reinforced diamond composite is prepared by mixing the nanometer niobium carbide/carbon nanotube composite powders, matrix powders and diamond grains uniformly and sintering with a hot pressing technique.

A nanometer niobium carbide/carbon nanotube reinforced diamond composite and a preparation method thereof, belonging to the field of materials science. The nanometer niobium carbide/carbon nanotube reinforced diamond composite is composed of nanometer niobium carbide/carbon nanotube composite powders, matrix powders and diamond grains, wherein the nanometer niobium carbide/carbon nanotube composite powders are the composites of nanometer niobium carbide which are evenly distributed in the surface defects and interior of the carbon nanotube, the nanometer niobium carbide/carbon nanotube reinforced diamond composite is prepared by mixing the nanometer niobium carbide/carbon nanotube composite powders, matrix powders and diamond grains uniformly and sintering with a hot pressing technique.

Provided is a complex oxide that has a space group I-43d, has a high hydrogen content, contains almost no impurity phase, exhibits almost no aluminum substitution in the structure thereof, and is suitable for proton conductivity. This complex oxide is represented by a chemical formula Li.sub.7-x-yH.sub.xLa.sub.3Zr.sub.2-yM.sub.yO.sub.12 (M represents Ta and/or Nb, 3.2<x7y, and 0.25<y<2) and is a single phase of a garnet type structure belonging to a cubic system, and the crystal structure thereof is a space group I-43d.

A lithium niobate solution is provided containing lithium niobate having a molar ratio of lithium and niobium Li/Nb of 0.8 or more and 2.0 or less and ammonium ions, and the particle size (D50) of the lithium niobate in the lithium niobate solution according to dynamic light scattering is 100 nm or less. A method for producing a lithium niobate solution is also provided and has a step of producing an acidic niobium solution containing niobium, a step of obtaining a precipitate slurry containing niobium by inverse neutralization by adding the acidic niobium solution to ammonia water and a step of obtaining a lithium niobate solution by maintaining a mixture of the precipitate slurry containing niobium and lithium hydroxide at 20? C. to 100? C. with stirring.

In an embodiment, an adsorbent that absorbs a Group V element compound and contains at least one amino acid selected from basic amino acids, acidic amino acids, and polar uncharged amino acids, or a salt of the amino acid.

Provided is a complex oxide that has a space group I-43d, has a high hydrogen content, contains almost no impurity phase, exhibits almost no aluminum substitution in the structure thereof, and is suitable for proton conductivity. This complex oxide is represented by a chemical formula Li.sub.7-x-yH.sub.xLa.sub.3Zr.sub.2-yM.sub.yO.sub.12 (M represents Ta and/or Nb, 3.2<x7-y, and 0.25<y<2) and is a single phase of a garnet type structure belonging to a cubic system, and the crystal structure thereof is a space group I-43d.