New Cubic Phase ofLi3N: Stability of theN3−Ion to 200 GPa

Abstract

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