Polymeric nitrogen frameworks are of interest for high-energy-density materials because their conversion to molecular ${\mathrm{N}}_2$ is associated with substantial energy release, while their experimental synthesis remains challenging. Here we combine particle-swarm structure prediction, first-principles calculations, and laser-heated diamond-anvil-cell experiments to investigate the phase stability and bonding of the In-N system under pressure. Two pressure-stabilized ${\mathrm{InN}}_{10}$ polymorphs were obtained experimentally, with in situ synchrotron x-ray diffraction and Raman spectroscopy supporting their assignment to tetragonal $I4$ at 55 GPa and orthorhombic Ama2 at 80 GPa. The $I4$ phase contains isolated pentazolate ${\mathrm{N}}_5$ rings, whereas the Ama2 structure contains an extended chain-ladder nitrogen framework involving ${\mathrm{N}}_{17}$-based motifs. Electronic-structure and bonding analyses indicate substantial charge transfer from indium to the nitrogen sublattice, with the dominant bonding contributions arising from covalent N-N interactions. The calculated energetic metrics show that Ama2 ${\mathrm{InN}}_{10}$ has a higher volumetric energy density than the other predicted In-N phases, associated with its higher packing density and extended nitrogen connectivity. The results establish two pressure-stabilized ${\mathrm{InN}}_{10}$ polymorphs with distinct ring-based and chain-ladder polynitrogen frameworks and clarify the role of indium in stabilizing different nitrogen connectivities under pressure.

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• High-pressure studies
• Crystal structures
• Nitrides
• Density functional theory
• First-principles calculations
• X-ray diffraction