MgSe has been studied using energy dispersive x-ray diffraction to 202 GPa and local density approximation and ultrasoft pseudopotentials to 500 GPa. MgSe undergoes a "continuous" phase transformation from the rocksalt to FeSi (B28) beginning at around 99 6 8 GPa and approaching sevenfold coordination at 202 GPa. Theoretical computation finds the B28 transition beginning at 58 GPa followed by a transition to an orthorhombic distortion of the B2 structure at 429 GPa.[S0031-9007(98)

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

Second-order Raman spectra of LiH and LiD have been measured as functions of pressure up to 15 GPa (⌬V/V 0 ϭϪ0.25) at ambient temperature. The assignments of the many features in these spectra have been thoroughly studied. The features have been compared with the two-phonon dispersion curves derived from neutron-scattering data and with isotope effect scaling. In addition, an internal self-consistency procedure is employed to validate the phonon assignments. Most of the features are assigned to combinations of zoneboundary phonons. The pressure and volume dependence of many zone-boundary phonons have been determined. These include all of the phonons at the X point as well as some at the L and the W points. Most of these phonons have constant mode Grüneisen parameters, while others exhibit volume-dependent mode Grüneisen parameters. There is significant variation in the magnitude of these parameters between the various modes. The results are shown to be in good agreement with our ab initio calculations. ͓S0163-1829͑97͒00322-6͔

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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.