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 $D_{6h}^4$ ${(P6\mathrm{}}_3/mmc)$ and the hexagonal $D_{6h}^1$ $\mathrm{}(P6/mmm)\mathrm{}$ 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.

• Received 16 December 1997

DOI:https://doi.org/10.1103/PhysRevB.59.6083