Structural stability of metal hydrides, alanates and borohydrides of alkali and alkali- earth elements: A review

Alkali and alkali– earth metal hydrides have high hydrogen storage capacity, but high operation temperature hinders their use. The alanates and borohydrides of alkali and alkali– earth metals are widely studied because of their light weight and high hydrogen content. Borohydrides are highly stable and decompose only at elevated temperatures while alanates decompose in two steps. A detailed study of the properties of these hydrides is required for further understanding of their stability. A lot of thermodynamic information can be derived from investigation of materials under pressure or temperature. Structural measurements on the hydride compounds at ambient and high P–T conditions result in a better understanding of the stability of the hydride structures and will assist us in the design of suitable storage materials with desired thermodynamic properties. The structural data are potential source of information regarding inter-atomic forces which determine pressure/temperature induced changes. During the past few years, structural stability of hydrides is widely investigated under pressure and temperature, both experimentally and theoretically. In this review we discuss structural phase transition and decomposition behavior of light metal hydrides, borohydrides and alanates of the elements that belong to first and second group in the periodic table.