A composite structure disclosed includes a base (X) and a layer (Y) stacked on the base (X). The layer (Y) includes a reaction product (R). The reaction product (R) is a reaction product formed by a reaction at least between a metal oxide (A) and a phosphorus compound (B). A peak for a binding energy of an oxygen-atom 1s orbital observed by X-ray photoelectron spectroscopy of the layer (Y) is located at 532.0 eV or higher, and the peak has a half width of less than 2.0 eV