Lithium hydride

Lithium hydride (LiH) is a salt-like, ionic compound made from lithium and hydrogen. Though pure samples are colorless, the commercial material is typically grey. With a molar mass of just 7.95 g/mol, it holds the title of the lightest ionic compound. However, it does not have the highest hydrogen content of any hydride—beryllium hydride and methane, for example, contain more hydrogen by mass.

This soft solid (Mohs hardness of 3.5) is diamagnetic and acts as an ionic conductor. Its electrical conductivity rises steadily from 2×10⁻⁵ Ω⁻¹cm⁻¹ at 443 °C to 0.18 Ω⁻¹cm⁻¹ at 754 °C, with no jump at the melting point. Its dielectric constant drops from 13.0 at low frequencies to 3.6 at visible-light frequencies. Thermally, its conductivity varies with both temperature and form: at 50 °C, crystals conduct 0.125 W/(cm·K) while compacts conduct 0.0695 W/(cm·K); at 500 °C, these values fall to 0.036 and 0.0432 W/(cm·K), respectively. The linear thermal expansion coefficient is 4.2×10⁻⁵/°C at room temperature. Notably, its compressive creep accelerates sharply with heat—under 1% over 100 hours at 350 °C, but over 100% at 475 °C—meaning it cannot bear loads when hot.

LiH is typically made by reacting lithium metal with hydrogen gas: 2 Li + H₂ → 2 LiH. This reaction is fastest above 600 °C, but it can begin as low as 29 °C. Adding 0.001–0.003% carbon or raising temperature and pressure can boost yield to 98% in two hours. At 99 °C the yield is 60%, and at 125 °C it reaches 85%, with the rate heavily dependent on the lithium’s surface condition. Alternative syntheses include thermally decomposing lithium aluminium hydride (200 °C), lithium borohydride (300 °C), n-butyllithium (150 °C), or ethyllithium (120 °C). The product is a lumped powder that can be pressed into pellets without a binder or cast from the melt into complex shapes. Large single crystals (roughly 80 mm long, 16 mm wide) can be grown from molten LiH in hydrogen using the Bridgman–Stockbarger technique; these often appear bluish from colloidal lithium, a color removable by annealing at ~550 °C. Major crystal impurities include sodium (20–200 ppm), oxygen (10–100 ppm), magnesium (0.5–6 ppm), iron (0.5–2 ppm), and copper (0.5–2 ppm). Cold-pressed parts machine easily to micrometer precision, but cast LiH is brittle and prone to cracking.