Fluorohydride salts are a unique class of inorganic compounds containing both fluoride (F⁻) and hydride (H⁻) anions coordinated to a metal cation. These 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.
They are sometimes referred to as mixed-anion salts because they incorporate two distinct types of anions with very different chemical properties—fluoride being highly electronegative and hydride being strongly reducing. These salts are typically formed by alkali or alkaline earth metals, such as lithium, sodium, calcium, or strontium, which can stabilize both anions simultaneously.
Structurally, fluorohydrides often crystallize in lattice frameworks where the metal cation balances the charges of the F⁻ and H⁻ ions. Their stability depends on the cation size and electronegativity, with smaller cations (like Li⁺) favoring stronger bonding. Some compounds, such as LiHF₂, also contain bridging species where hydrogen is shared between fluoride ions.
Chemically, fluorohydrides exhibit interesting reactivity. The hydride component can act as a reducing agent, while the fluoride contributes to thermal stability and ionic conductivity. This combination makes certain fluorohydrides promising for use in hydrogen storage, solid electrolytes, and advanced materials research. They are sensitive to moisture, often releasing HF or H₂ when exposed to water, so they require careful handling under inert atmospheres.
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(H,F), Na(H,F), Mg(H,F)2, and Ca(H,F)2. More complex fluorohydride salts include the perovskite-structured NaMg(H,F)3[5] and MCa(H,F)3 (M = Rb or Cs),[6] and the high-pressure pyrochlore compound NaCaMg2(H,F)7.
Properties
Fluorohydride salts are inorganic compounds containing both fluoride (F⁻) and hydride (H⁻) anions, typically stabilized by metal cations such as alkali or alkaline earth metals. They are often represented as M(FH)x or M(HF)x, where M is a metal. These salts can form mixed anionic lattices due to the high electronegativity of fluorine and the strong reducing nature of hydride. They generally exhibit high lattice energies, are ionic crystalline solids, and may show ionic conductivity at elevated temperatures. Fluorohydrides are often sensitive to moisture and may hydrolyze, releasing hydrogen or hydrogen fluoride (HF). Their stability depends strongly on the cation; for example, light alkali-metal fluorohydrides tend to be more reactive than heavier counterparts.
Occurrences
Fluorohydride salts are not abundant in nature due to their high reactivity with water and oxygen. They are mostly synthetic compounds, prepared under controlled laboratory or industrial conditions, often by reaction of metal fluorides with hydrides or hydrogen fluoride. They have been studied for their potential roles in hydrogen storage materials, solid electrolytes, and nuclear applications, particularly because of the unique bonding and ion-exchange properties of the F⁻ and H⁻ combination. Some rare mixed hydride–fluoride phases may be found in specialized chemical environments but are not naturally widespread.
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.
