Fluorohydride salt
Fluorohydride salts are ionic compounds containing a mixture of fluoride and hydride anions, generally with strongly electropositive metal cations. Unlike other types of mixed hydrides such as oxyhydrides, fluorohydride salts are typically solid solutions because of the similar sizes and identical charges of fluoride and hydride ions.
Examples
Fluorohydride salts typically contain one or more alkali or alkaline earth metals whose parent fluorides and hydrides are predominantly ionic. Examples with a single metal counterion include Li, Na, Mg2, and Ca2. More complex fluorohydride salts include the perovskite-structured NaMg3 and MCa3, and the high-pressure pyrochlore compound NaCaMg27.
Applications
Fluorohydride salts have drawn interest as thermoelectric storage materials because their continuous solid-solution range enables balancing of hydrogen-storage capacity with thermal stability. Replacing hydride with fluoride ions in solid solution reduces the hydrogen-storage capacity of the compound but also reduces the hydrogen-dissociation pressure, enabling higher-temperature operation. With sodium fluorohydride, the maximum rate of hydrogen release is found to be 443 °C for the solid solution with 1:1 H:F ratio, versus 408 °C for the pure sodium hydride. A cost comparison reveals that a hydrofluoride salt with the composition NaMgH2F can be operated at lower cost than the parent hydride NaMgH3 and other magnesium-hydride based materials, despite its lower hydrogen-storage capacity, because of the improved stability of the fluorohydride salt at high temperature.