High-pressure Na3(N2)4, Ca3(N2)4, Sr3(N2)4, and Ba(N2)3 featuring nitrogen dimers with noninteger charges and anion-driven metallicity

Charged molecular species, such as ${{[{\mathrm{N}}₂]}^x}^{\text{--}}$, ${{[{\mathrm{O}}₂]}^x}^{\text{--}}$, ${{[{\mathrm{C}}₂]}^x}^{\text{--}}$, and ${{[{\mathrm{S}}₂]}^x}^{\text{--}}$, follow the paradigm of carrying integer values of electrons. Here, the ${\mathrm{Na}}₃{({\mathrm{N}}₂)}₄$, ${\mathrm{Ca}}₃{({\mathrm{N}}₂)}₄$, ${\mathrm{Sr}}₃{({\mathrm{N}}₂)}₄$, and $\mathrm{Ba}{({\mathrm{N}}₂)}₃$ compounds were produced and characterized 70 GPa and evidenced to be composed of paradigm-breaking ${{[{\mathrm{N}}₂]}^x}^{\text{--}}$ dimers with noninteger charges of \ensuremath{-}0.75, \ensuremath{-}1.5, \ensuremath{-}1.5, and \ensuremath{-}0.67, respectively. The anion-driven metallicity of the compounds is proposed as the physical mechanism enabling the noninteger electron count of the ${{[{\mathrm{N}}₂]}^x}^{\text{--}}$ dimers. The properties of these dimers and the compounds bearing them are demonstrated to depend on their noninteger charge, paving the way to materials with electron-tunable features.