Mixed Ionic-Electronic Conductivity in K3W2N5 and the Nonconductive K2WN2O

We report on the synthesis of the novel alkali tungsten nitride K3W2N5 and the alkali tungsten oxonitride K2WN2O. Both compounds were synthesized in bulk using the ammonothermal method at 873 K with autogenous pressures between 100 and 175 MPa. The compounds feature zweier chains, ∞1[WNON2/2]2– in K2WN2O and zweier double chains, ∞1[W2N2N6/2]3– in K3W2N5. Temperature-dependent PXRD data revealed an anisotropic thermal expansion along the direction of the anionic chains in both compounds. The compounds are semiconductors with band gaps between 2.14 and 2.24 eV determined by UV/Vis measurements and supported by DFT calculations. Electrochemical impedance spectroscopy measurements revealed K2WN2O to be an insulator and mixed ionic-electronic conductivity in K3W2N5, which makes K3W2N5 the first tungsten nitride-based ionic conductor.