Solution: insoluble in water, soluble in acids. Appearance and properties: light yellow crystalline powder. Under light conditions, the color of holmium oxide changes significantly. It is light yellow in sunlight and has a strong orange-red color under the tri-primary light source. It can hardly be distinguished from erbium oxide in the same light, which is related to its phosphorescent emission band. Holmium oxide has a band gap width of 5.3 eV and should therefore be colorless. The yellow color of holmium oxide is caused by a large number of lattice defects (such as oxygen vacancy) and internal conversion of HO3. It is used as an additive in steel and nonferrous alloys. FECr alloys typically contain 0.5-4% yttrium, which improves the oxidation resistance and ductility of these stainless steels; After adding some yttrium mixed rare earth, M26 alloy can replace some medium and strong aluminum alloy to be used in aircraft mechanical parts. A small amount of yttrium-rich rare earth can improve the conductivity of the alloy. The alloy has become the largest wire mill in China to adopt it. Adding yttrium to copper alloy can improve the conductivity and mechanical strength. Silicon nitride ceramic material containing yttrium 6% and aluminum 2% can be used for engine development parts: neodymium yttrium aluminum garnet laser beam power 400W, can be used for drilling, cutting and welding of large parts; For the electron microscope phosphor screen composed of y-al garnet single crystal, the phosphor screen has high brightness, low absorption of scattered light, high temperature resistance and mechanical wear. High yttrium structure alloy, can be used in aviation and other requirements of low density and high melting point occasions; High temperature proton conductive material doped with yttrium SrZrO3 is of great significance for the production of fuel cells, electrolytic cells and gas sensors requiring high hydrogen solubility. Used for nuclear reactor fuel diluent, permanent magnet material additive, electronic industry getter and other high temperature resistant spraying materials.