Metal Nitrides Grown from Ca/Li Flux: Ca6Te3N2 and New Nitridoferrate(I) Ca6(LixFe1-x)Te2N3

Preparation of iron(IV) nitridoferrate Ca4FeN4 through azide-mediated oxidation under high-pressure conditions

Transition metal nitrides are an important class of materials with applications as abrasives, semiconductors, superconductors, Li-ion conductors, and thermoelectrics. However, high oxidation states are difficult to attain as the oxidative potential of dinitrogen is limited by its high thermodynamic stability and chemical inertness. Here we present a versatile synthesis route using azide-mediated oxidation under pressure that is used to prepare the highly oxidised ternary nitride Ca₄FeN₄ containing Fe⁴⁺ ions. This nitridometallate features trigonal-planar [FeN₃]⁵⁻ anions with low-spin Fe⁴⁺ and antiferromagnetic ordering below a Neel temperature of 25 K, which are characterised by neutron diffraction, ⁵⁷Fe-Mössbauer and magnetisation measurements. Azide-mediated high-pressure synthesis opens a way to the discovery of highly oxidised nitrides.

High-valent metal nitrides are difficult to stabilise due to the high thermodynamic stability and chemical inertness of N₂. Here, the authors employ a large volume press to prepare an iron(IV) nitridoferrate Ca₄Fe^IVN₄ from Fe₂N and Ca₃N₂ via azide-mediated oxidation under high pressure conditions.

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.

Low-Dimensional Nitridosilicates Grown from Ca/Li Flux: Void Metal Ca8In2SiN4 and Semiconductor Ca3SiN3H

Reactions of indium and silicon with lithium nitride in Ca/Li flux produce two new nitridosilicates: Ca₈In₂SiN₄ (orthorhombic, Ibam; a = 12.904(1) Å, b = 9.688(1) Å, c = 10.899(1) Å, Z = 4) and Ca₃SiN₃H (monoclinic, C2/c; a = 5.236(1) Å, b = 10.461(3) Å, c = 16.389(4) Å, β = 91.182(4)°, Z = 8). Ca₈In₂SiN₄ features isolated [SiN₄]^8- units and indium dimers surrounded by calcium atoms. Ca₃SiN₃H features infinite chains of corner-sharing SiN₄ tetrahedra and distorted edge-sharing H@Ca₆ octahedra. Optical properties and band structure calculations indicate that Ca₈In₂SiN₄ is a void metal with calcium and indium states at the Fermi level and Ca₃SiN₃H is a semiconductor with a band gap of 3.1 eV.