Crystalline solids metallic hydrides

The alkali metal halides are all high-melting, colourless crystalline solids which can be conveniently prepared by reaction of the appropriate hydroxide (MOH) or carbonate (M2CO3) with aqueous hydrohalic acid (HX), followed by recryslallization. Vast quantities of NaCl and KCl are available in nature and can be purihed if necessary by simple crystallization. The hydrides have already been discussed (p. 65). 

The compounds formed by the reaction of hydrogen with the alkali and alkaline earth metals contain H- ions for example, sodium hydride consists of Na+ and H- ions. These white crystalline solids are often referred to as saline hydrides because of their physical resemblance to NaCL Chemically, they behave quite differently from sodium chloride for example, they react with water to produce hydrogen gas. Typical reactions are... 

With hydrogen, the alkali metals form the mono-hydrides MeH, having salt-like properties and a partially ionic, Me H, NaCl-type structure. They are colourless crystalline solids having a fairly negative AH of formation. The mono-hydrides react with water. They may be prepared from hydrogen and the metal (heated at 700-800°C for Li, 350-400°C for the others) or through the reaction of hydrogen with the alkali mono-oxide, nitride, etc. 

V I ionic hydrides are crystalline solids with high melting points. They are formed by direct reaction with only the most active metals. 

The present paper is concerned with hydrogen storage in different crystalline solids. Such solids can be metal hydrides, carbon-based materials, and microporous materials. 

The saline hydrides are crystalline solids, white when pure but usually gray owing to traces of metal. They can be dissolved in molten alkali halides and on electrolysis of such a solution, for example, CaH2 in LiCl + KC1 at 360°C, hydrogen is released at the anode. They react instantly and completely with even the weakest acids, such as water, according to the reaction... 

Elements with very low electronegativeties of approximately 1.0 see Electronegativity), form compounds in which the hydrogen appears as an anion, H. These compounds have many of the properties associated with ionic substances and are sometimes called ionic or saline hydrides. Ionic hydrides, formed with alkali (M+H ) and alkaline earth (M +H2 ) metals, are colorless crystalline solids that either melt or decompose at temperatures above 600 °C. All of the hydrides can be formed by direct combination of the elements at elevated temperatures ... 

The surface of hydrogen-absorbing materials provides the contact point for interactions between gas-solid and liquid-solid phases. A typical example of the role and function of the surface is the transition of gaseous hydrogen at the metal hydride surface where it dissociates to monatomic hydrogen which is then located in the interstitial sites of the crystalline lattice (see Chapter 4.4). 

Protium is stored in the interstitial sites in the crystalline lattice of the metal hydride and the amount of protium varies in accordance with the levels of charge and discharge. In these electrochemical reactions, the hydrogen states between proton and protium change at the surface of the metal hydride. This is the typical interfacial transition of hydrogen between liquid and solid phases. 

Ionic (Saltlike) Hydrides With very reactive metals, such as those in Group 1A(1) and the larger members of Group 2A(2) (Ca, Sr, and Ba), hydrogen forms saltlike hydrides—white, crystalline solids composed of the metal cation and the hydride ion ... 

This is the second volume in the NATO ASI series dealing with the topic of hydrogen in solids. The first (V. B76, Metal Hydrides) appeared five years ago and focussed primarily on crystalline phases of hydrided metallic systems. In the intervening period, the amorphous solid state has become an area of intense research activity, encompassing both metallic and non-metallic, e.g. semiconducting, systems. At the same time the problem of storage of hydrogen, which motivated the first ASI, continues to be important. 

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