Diamond-anvil cell experiments augmented by first-principles calculations have found a remarkable stability of the ${\mathrm{N}}^{3-}$ ion in ${\mathrm{Li}}_3\mathrm{N}$ to a sixfold volume reduction. A new ($\gamma$) phase is discovered above $40(\pm 5)\mathrm{GPa}$, with an 8% volume collapse and a band gap quadrupling at the transition determined by synchrotron x-ray diffraction and inelastic x-ray scattering. $\gamma \mathrm{\text{−}}{\mathrm{Li}}_3\mathrm{N}$ ($Fm3m$, ${\mathrm{Li}}_3\mathrm{Bi}$-like structure) remains stable up to 200 GPa, and calculations do not predict metallization until $\sim 8\mathrm{TPa}$. The high structural stability, wide band gap, and simple electronic structure make this ${\mathrm{N}}^{3-}$ based system analogous to lower valency compounds (MgO, NaCl, Ne), meriting its use as an internal pressure standard.

• Received 29 April 2005

DOI:https://doi.org/10.1103/PhysRevLett.95.165503