Crystal structure of lithium beryllium hydride

Analysis of powder diffraction data, published by Bell and Coates, reveals that ${\mathrm{LiBeH}}_3$ and ${\mathrm{Li}}_2$ ${\mathrm{BeH}}_4$ both have an fcc translation group with lattice constants 5.09 and 5.14 Å, respectively. The cubic cell of each contains eight formula units; so the total atomic concentrations, 3.03×${10}^{23}$ and 4.12×${10}^{23}$ ${\mathrm{cm}}^{\mathrm{-}3}$, exceed that of diamond by factors of 1.7 and 2.3. The computed densities are 1.91 and 2.63 g/${\mathrm{cm}}^3$. Both crystals have a modified perovskite structure involving cubes with edges half the size of the lattice constant. The (otherwise) sc lattice is broken by a chemical superstructure in ${\mathrm{LiBeH}}_3$ and by an orientational superstructure in ${\mathrm{Li}}_2$ ${\mathrm{BeH}}_4$. The ‘‘conduction’’-electron density of the latter is 4.71×${10}^{23}$ ${\mathrm{cm}}^3$; so the equivalent-sphere-radius parameter is $r_s$=1.51 Bohr, a value in the range typically assumed for metallic hydrogen. Infrared absorption at 1600 ${\mathrm{cm}}^{\mathrm{-}1}$ (≊2300 K) was reported. Consequently, these compounds, if metallic, may manifest the high-temperature superconductivity often envisioned for metallic hydrogen.