A composite structure disclosed includes a base (X) and a layer (Y). The layer (Y) includes a mixture of a metal oxide (A), a phosphorus compound (B), and a compound (L a ) (silicon compound). Examples of the phosphorus compound (B) and the compound (L a ) include a compound containing a site capable of reacting with the metal oxide (A). When the number of moles of metal atoms (M) derived from the metal oxide (A) is denoted by N M and the number of moles of Si atoms derived from the compound (L a ) is denoted by N Si , 0.01 ≤ N Si /N M ≤ 0.30 is satisfied. When the number of moles of phosphorus atoms derived from the phosphorus compound (B) is denoted by Np, 0.8 ≤ N M /N P ≤ 4.5 is satisfied.