Crystal structure of lithium beryllium hydride

Analysis of powder diffraction data, published by Bell and Coates, reveals that LiBeH₃ and Li₂{BeH}₄$ both have an fcc translation group with lattice constants 5.09 and 5.14 A{}, respectively. The cubic cell of each contains eight formula units; so the total atomic concentrations, 3.03×{}10²³ and 4.12×{}10²³ cm^-3, exceed that of diamond by factors of 1.7 and 2.3. The computed densities are 1.91 and 2.63 g/cm³. 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 LiBeH₃ and by an orientational superstructure in Li₂{BeH}₄. The ``conduction''-electron density of the latter is 4.71×{10}²³$ ${cm}³$; 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 ${cm}^{{{-}}1}$ ({}2300 K) was reported. Consequently, these compounds, if metallic, may manifest the high-temperature superconductivity often envisioned for metallic hydrogen.