High-pressure synthesis of lithium hydride

By compressing elemental lithium and hydrogen in a diamond anvil cell, we have synthesized lithium hydride (LiH) at pressures as low as 50 MPa at room temperature. Combined Raman spectroscopy and synchrotron x-ray diffraction measurements reveal that, once synthesized, LiH remains stable at 300 K up to 160 GPa in the presence of molecular hydrogen. The mixture of lithium hydride and molecular hydrogen and application of pressure alone cannot form a higher H${}_2$ content hydride (LiH${}_x$, $x>1$) as was suggested from the theoretical ab initio calculations and therefore, cannot be considered as a route to low-pressure hydrogen rich material metallization.

Figure 1

Representative XRD profile collected from Li+H${}_2$ phase assembly at 0.05 GPa (top) and 0.32 GPa (bottom) at room temperature (crosses) and its LeBail fit (solid line); the background baseline has been subtracted. Positions of diffraction peaks of $bcc$-Li and $\mathit{fcc}$-LiH are shown by tick marks below; XRD reflections from rhenium gasket are also marked.

Figure 2

(a) Representative second-order Raman spectra of synthesised LiH+H${}_2$. Arrows indicate peaks corresponding to rotational modes from pure hydrogen and ruby fluorescence used as a pressure gauge. Inset: photomicrograph of sample at 0.05 GPa. (b) Raman spectra of recovered synthesized LiH at various pressures. Inset: photomicrograph of recovered transparent LiH sample from (a). (c) High temperature Raman measurements of the recovered synthesized LiH.

Figure 3

$d$ spacings of observed phases as a function of pressure (open and solid symbols) plotted against previously reported compression data of Li (Refs. 20 and 21), LiH (Refs. 14 and 18), and H${}_2$ (Refs. 22 and 23).

Figure 4

Pressure evolution of the frequency of the H${}_2$ vibron with pressure. Symbols and lines represent measurements on the LiH part and hydrogen part of the sample, respectively. Triangles and line (blue) show measurements at 120 K while diamonds and line (red) represent measurements at room temperature. The inset shows the synthesized LiH sample in hydrogen media at a pressure of 155 GPa.