New Cubic Phase of Li₃N: Stability of the N^3- Ion to 200 GPa

Diamond-anvil cell experiments augmented by first-principles calculations have found a remarkable stability of the N^3- ion in Li₃N to a sixfold volume reduction. A new (γ) phase is discovered above 40(±5)  GPa, with an 8% volume collapse and a band gap quadrupling at the transition determined by synchrotron x-ray diffraction and inelastic x-ray scattering. γ-Li₃N (Fm3m, Li₃Bi-like structure) remains stable up to 200 GPa, and calculations do not predict metallization until ∼8  TPa. The high structural stability, wide band gap, and simple electronic structure make this N^3- based system analogous to lower valency compounds (MgO, NaCl, Ne), meriting its use as an internal pressure standard.