Lithium hydride

Lithium hydride (LiH) is an inorganic salt made from lithium and hydrogen. Though pure samples are colorless, the commercial material usually appears grey. As a typical ionic hydride, it melts at a high temperature, does not dissolve in protic organic solvents, and instead reacts with them. However, it does dissolve without reacting in certain molten salts, including lithium fluoride, lithium borohydride, and sodium hydride. With a molar mass of just 7.95 g/mol, it holds the title of the lightest ionic compound.

Physically, LiH is diamagnetic and conducts ions. Its electrical conductivity rises steadily from 443 °C to 754 °C, reaching 0.18 Ω⁻¹cm⁻¹, with no jump at the melting point. Its dielectric constant drops from 13.0 at low frequencies to 3.6 at visible-light frequencies. The material is soft, with a Mohs hardness of 3.5. When heated, it deforms easily: compressive creep reaches 100% per 100 hours at 475 °C, meaning it cannot bear mechanical loads at high temperatures. Thermal conductivity falls as temperature rises and depends on form—at 50 °C, crystals conduct 0.125 W/(cm·K) while compacts conduct 0.0695 W/(cm·K); at 500 °C, those values drop to 0.036 and 0.0432 W/(cm·K), respectively. Its linear thermal expansion coefficient is 4.2/°C at room temperature.

The main way to make LiH is by reacting lithium metal with hydrogen gas. This reaction is fastest above 600 °C, but it can start 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%. The reaction rate depends heavily on the surface condition of the lithium hydride. Less common routes include heating lithium aluminium hydride to 200 °C, lithium borohydride to 300 °C, n-butyllithium to 150 °C, or ethyllithium to 120 °C, plus several reactions with unstable lithium compounds that contain hydrogen.

The product is a lumped powder that can be pressed into pellets without a binder. More complex shapes come from casting the melt. Large single crystals—about 80 mm long and 16 mm in diameter—can be grown from molten LiH powder in hydrogen using the Bridgman–Stockbarger method. These crystals often have a bluish tint from colloidal lithium, which can be removed by annealing at around 550 °C under low thermal gradients. Common 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 LiH parts machine easily to micrometer precision with standard tools, but cast LiH is brittle and tends to crack during machining.

LiH powder reacts quickly with dry air to form LiOH, and in moist air it ignites spontaneously, producing a mix of products including some nitrogen compounds. Solid lumps react with humid air to form a viscous surface coating that slows further reaction, though a tarnish film is obvious. Little or no nitride forms in humid air. A lump in a metal dish can be heated in air to just below 200 °C without catching fire, but it ignites readily if touched by a flame. The ignition temperature is affected by surface condition and oxides on the dish. Dry oxygen does not react with crystalline LiH unless heated strongly, at which point it burns almost explosively.

LiH reacts violently with water and other protic reagents. It is less reactive with water than lithium metal itself, making it a weaker reducing agent for water, alcohols, and similar media. Pellets of LiH slowly expand in moist air as they form LiOH, but the expansion stays under 10% in 24 hours at a water vapor pressure of 2 Torr. If carbon dioxide is present, lithium carbonate forms instead. LiH reacts slowly with ammonia at room temperature, but the reaction speeds up above 300 °C. It reacts slowly with higher alcohols and phenols, but vigorously with lower ones. With sulfur dioxide, it gives dithionite; above 50 °C, the product is lithium sulfide. Acetylene reacts with LiH to form lithium carbide and hydrogen. Anhydrous organic acids, phenols, and acid anhydrides react slowly, releasing hydrogen and the lithium salt. With water-containing acids, the reaction is faster than with water alone. Many oxygen-containing reactions produce LiOH, which then irreversibly reacts with LiH above 300 °C. At moderate temperatures, LiH is fairly unreactive with certain gases, making it useful for synthesizing other hydrides.

Because it contains three times more hydrogen by mass than sodium hydride, LiH has the highest hydrogen content of any hydride. It has been considered for hydrogen storage, but its stability makes hydrogen release difficult—temperatures above 700 °C are needed, which are costly to maintain. The compound was once tested as a fuel component in a model rocket.