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Abstract
Polynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches. Now, a one‐step synthesis of metal–inorganic frameworks Hf4N20⋅N2, WN8⋅N2, and Os5N28⋅3 N2 via direct reactions between elements in a diamond anvil cell at pressures exceeding 100 GPa is reported. The porous frameworks (Hf4N20, WN8, and Os5N28) are built from transition‐metal atoms linked either by polymeric polydiazenediyl (polyacetylene‐like) nitrogen chains or through dinitrogen units. Triply bound dinitrogen molecules occupy channels of these frameworks. Owing to conjugated polydiazenediyl chains, these compounds exhibit metallic properties. The high‐pressure reaction between Hf and N2 also leads to a non‐centrosymmetric polynitride Hf2N11 that features double‐helix catena‐poly[tetraz‐1‐ene‐1,4‐diyl] nitrogen chains [−N−N−N=N−]∞.
Highlights
Polynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches
ReN8·N2,[15] which enables us to suggest that the covalent character of the transition metal–nitrogen bonds is one of the important factors for the framework architecture: the imposed geometry constraints lead to the formation of framework structures, rather than to the densest packing of atoms even at ultrahigh pressures
We would like to note that small magnitude of the imaginary frequencies seen in Hf2N11 and WN8·N2 at ambient pressure is expected to be removed at finite temperature by anharmonic effects.[35]
Summary
Polynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches. The crystal-structure solution and refinement revealed the chemical formulas of new compounds as Hf4N20·N2, WN8·N2 and Os5N28·3 N2 (Figure 1).
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