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.