Experimental and theoretical study ofLi3Nat high pressure

Abstract

X rays from the Cornell High Energy Synchrotron Source were used to study the crystal structure of ${\mathrm{Li}}_{3}\mathrm{N}$ over a pressure range of 0 to 35 GPa at ambient temperature. Before loading, both the hexagonal ${D}_{6h}^{4}$ ${(P6}_{3}/mmc)$ and the hexagonal ${D}_{6h}^{1}$ $(P6/mmm)$ phases were present. At modest pressure, the ${D}_{6h}^{1}$ structure transforms completely into the ${D}_{6h}^{4}$ structure. A two-parameter Birch equation was fitted to the data to determine the equation of state (EOS) of the ${D}_{6h}^{4}$ structure, which is compared with a theoretical equation of state produced by our total-energy calculations in the framework of the local-density approximation. The experimental EOS was then used to determine the change in Gibb's free energy as a function of pressure for ${\mathrm{Li}}_{3}\mathrm{N}$ to 35 GPa at ambient temperature. The likelihood of a phase transition to a cubic structure at 35 GPa (as calculated by theory) based on observed optical changes is discussed.