This invention relates to partially ordered and ordered oxynitride perovskites of the general formula ABO2N that are polar insulators. A comprises one or more cations or set of cations that sit in sites derived from the A-site in the perovskite structure. B comprises one or more cations or set of cations that sit in sites derived from the B-site in the perovskite structure. C comprises oxygen, O, with optionally some nitrogen, N, and D comprises N, with optionally some O. The total valence of the cations A+B is equal to the total valence of the anions 2 C+D. Also disclosed are methods of producing such oxynitride perovskites and uses of such oxynitride perovskites.