Redox-Mediated Stabilization in Zinc Molybdenum Nitrides.

We report on the theoretical prediction and experimental realization of new ternary zinc molybdenum nitride compounds. We used theory to identify previously unknown ternary compounds in the Zn-Mo-N systems, Zn3MoN4 and ZnMoN2, and to analyze their bonding environment. Experiments show that Zn-Mo-N alloys can form in broad composition range from Zn3MoN4 to ZnMoN2 in the wurtzite-derived structure, accommodating very large off-stoichiometry. Interestingly, the measured wurtzite-derived structure of the alloys is metastable for the ZnMoN2 stoichiometry, in contrast to the Zn3MoN4 stoichiometry, where ordered wurtzite is predicted to be the ground state. The formation of Zn3MoN4-ZnMoN2 alloy with wurtzite-derived crystal structure is enabled by the concomitant ability of Mo to change oxidation state from +VI in Zn3MoN4 to +IV in ZnMoN2, and the capability of Zn to contribute to the bonding states of both compounds, an effect that we define as "redox-mediated stabilization". The stabilization of Mo in both the +VI and +IV oxidation states is due to the intermediate electronegativity of Zn, which enables significant polar covalent bonding in both Zn3MoN4 and ZnMoN2 compounds. The smooth change in the Mo oxidation state between Zn3MoN4 and ZnMoN2 stoichiometries leads to a continuous change in optoelectronic properties-from resistive and semitransparent Zn3MoN4 to conductive and absorptive ZnMoN2. The reported redox-mediated stabilization in zinc molybdenum nitrides suggests there might be many undiscovered ternary compounds with one metal having an intermediate electronegativity, enabling significant covalent bonding, and another metal capable of accommodating multiple oxidation states, enabling stoichiometric flexibility.