Redox-Mediated Stabilization in Zinc Molybdenum Nitrides

Abstract

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, ZnMoN and ZnMoN, and to analyze their bonding environment. Experiments show that Zn-Mo-N alloys can form in broad composition range from ZnMoN to ZnMoN in the wurtzite-derived structure, accommodating very large off-stoichiometry. Interestingly, the measured wurtzite-derived structure of the alloys is metastable for the ZnMoN stoichiometry, in contrast to the ZnMoN stoichiometry, where ordered wurtzite is predicted to be the ground state. The formation of ZnMoN-ZnMoN alloy with wurtzite-derived crystal structure is enabled by the concomitant ability of Mo to change oxidation state from +VI in ZnMoN to +IV in ZnMoN, 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 ZnMoN and ZnMoN compounds. The smooth change in the Mo oxidation state between ZnMoN and ZnMoN stoichiometries leads to a continuous change in optoelectronic properties-from resistive and semitransparent ZnMoN to conductive and absorptive ZnMoN. 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.

Assessing High-Throughput Descriptors for Prediction of Transparent Conductors

“…, as shown recently in the space of metastable nitride semiconductors 77,78. These effects may correlate with a negative ∆E non−eq and effectively reduce E exp .…”

A map of the inorganic ternary metal nitrides

“…48,49 Thin-film synthesis from these precursors can form remarkably metastable nitrides, such as SnTi2N4 (metastable by 200 meV/atom), 50 or ZnMoN2 in a wurtzite-derived structure (metastable by 160 meV/atom). 42 On Figure 1, we use orange boxes to highlight 95 spaces with metastable ternary nitrides predicted to be stabilizable under elevated nitrogen chemical potentials of ΔμN < +1 eV/N.…”

“…For example, varying the Zn/Mo ratio in a wurtzite-based Zn-Mo-N compound can modulate the molybdenum oxidation state from Mo 4+ to Mo 6+ , turning conductive ZnMoN2 into insulating Zn3MoN4, a wide-bandgap semiconductor. 42…”

Ternary nitride semiconductors in the rocksalt crystal structure

“…The ground state structure search was performed using the "kinetically limited minimization" approach, which is unconstrained and does not require prototypical structures from databases. 7 Seed structures are generated from random lattice vectors and atomic positions, subject to geometric constraints to avoid extreme cell shapes, and to observe minimal interatomic distances (2.8 Å for cation-cation and anion-anion pairs, 1.9 Å for cation-anion pairs). New trial structures are generated by the random displacement of one atom between 1.0 and 5.0 Å while maintaining the minimal distances.…”

“…4 In these compounds, the open d-shell of the transition metal leads to metallic behavior; some of them also exhibit superconducting transitions approaching 20 K. 5 While some rocksalt transition metal nitrides (e.g. ScN) are narrow indirect gap semiconductors 6 , and metallic wurtzite ZnMoN2 was recently reported 7 , these are rare exceptions to the more general trends.…”