X rays from the Cornell High Energy Synchrotron Source were used to study the crystal structure of Li 3 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 ͑EOS͒ 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 Li 3 N to 35 GPa at ambient temperature. The likelihood of a phase transition to a cubic structure at 35 GPa ͑as calculated by theory͒ based on observed optical changes is discussed. ͓S0163-1829͑99͒06609-6͔

The equation of state (EOS) of Li3N has been determined by energy-dispersive x-ray diffraction (EDXD) using synchrotron radiation up to 35 GPa at ambient temperature. Both the hexagonal D6h4(P63/mmc) and the hexagonal D6h1(P6/mmm) phases were present at ambient pressure. The D6h1 -structure completely transforms into the D6h4 -structure at modest pressure. The change in Gibb's free energy as a function of pressure for Li3N was calculated using the experimental EOS.