Structural phase transitions of sodium nitride at high pressure

Abstract

The structural evolution of recently characterized sodium nitride ${\text{Na}}_{3}\text{N}$ as a function of pressure was investigated at room temperature by the angle-dispersive powder x-ray diffraction in a diamond-anvil cell up to 36 GPa. The rather open cubic anti-${\text{ReO}}_{3}$-type structure stable at ambient pressure is followed by a series of four high-pressure modifications. Along the route, the coordination number for the nitride anion increases from 6 in ${\text{Na}}_{3}\text{N-I}$ to 8 in hexagonal ${\text{Li}}_{3}\text{N}$-type ${\text{Na}}_{3}\text{N-II}$, 9 in orthorhombic anti-${\text{YF}}_{3}$-type ${\text{Na}}_{3}\text{N-III}$, 11 in hexagonal ${\text{Cu}}_{3}\text{P}$-type ${\text{Na}}_{3}\text{N-IV}$, and finally 14 in cubic ${\text{Li}}_{3}\text{Bi}$-type ${\text{Na}}_{3}\text{N-V}$ structures. The experimental data are compared to the results of total-energy calculations and are discussed with regard to the structural details of the five phases and their equations of state.