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.