An oxyhydride is a mixed anion compound containing both oxide O2− and hydride ions H−. These compounds may be unexpected as the hydrogen and oxygen could be expected to react to form water. But if the metals making up the cations are electropositive enough, and the conditions are reducing enough, solid materials can be made that combine hydrogen and oxygen in the negative ion role.

Production The first oxyhydride to be discovered was lanthanum oxyhydride, a 1982 discovery. It was made by heating lanthanum oxide in an atmosphere of hydrogen at 900 °C. However, heating transition metal oxides with hydrogen usually results in water and the reduced metal.

Topochemical synthesis retains the basic structure of the parent compound, and only does the minimum rearrangements of atoms to convert to the final product. Topotactic transitions retain the original crystal symmetry. Reactions at lower temperatures do not distort the existing structure. Oxyhydrides in a topochemical synthesis can be produced by heating oxides with sodium hydride NaH or calcium hydride CaH2 at temperatures from 200–600 °C. TiH2 or LiH can also be used as an agent to introduce hydride. If calcium hydroxide or sodium hydroxide is formed, it might be able to be washed away. However for some starting oxides, this kind of hydride reduction might just yield an oxygen-deficient oxide.

Reactions under hot high-pressure hydrogen can result from heating hydrides with oxides. A suitable seal for the lid on the container is required, and one such substance is sodium chloride.

Oxyhydrides all contain an alkali metal, alkaline earth metal, or rare-earth element, which are needed in order to put electronic charge on hydrogen.

Properties The hydrogen bonding in oxyhydrides can be covalent, metallic, and ionic bonding, depending on the metals present in the compound.

Oxyhydrides lose their hydrogen less than the pure metal hydrides.

The hydrogen in oxyhydrides is much more exchangeable. For example oxynitrides can be made at much lower temperatures by heating the oxyhydride in ammonia or nitrogen gas (say around 400 °C rather than 900 °C required for an oxide) Acidic attack can replace the hydrogen, for example moderate heating in hydrogen fluoride yields compounds containing oxide, fluoride, and hydride ions (oxyfluorohydride.) The hydrogen is more thermolabile, and can be lost by heating yielding a reduced valence metal compound.

Changing the ratio of hydrogen and oxygen can modify electrical or magnetic properties. Then band gap can be altered. The hydride atom can be mobile in a compound undergoing electron coupled hydride transfer. The hydride ion is highly polarisable, so it presence raised the dielectric constant and refractive index.

Some oxyhydrides have photocatalytic capability. For example BaTiO2.5H0.5 can function as a catalyst for ammonia production from hydrogen and nitrogen.

The hydride ion is quite variable in size, ranging from 130 to 153 pm.

The hydride ion actually does not only have a −1 charge, but will have a charge dependent on its environment, so it is often written as Hδ−. In oxyhydrides, the hydride ion is much more compressible than the other atoms in compounds. Hydride is the only anion with no π orbital, so if it is incorporated into a compound, it acts as a π-blocker, reducing dimensionality of the solid.

Oxyhydride structures with heavy metals cannot be properly studied with X-ray diffraction, as hydrogen hardly has any effect on X-rays. Neutron diffraction can be used to observe hydrogen, but not if there are heavy neutron absorbers like Eu, Sm, Gd, Dy in the material.

List FormulaStructureSpace groupUnit cellVolumeDensityCommentsReferenceNa3SO4HtetrahedralP4/nmma=7.0034 c=4.8569[η1-3,5-tBu2pz(η-Al)H)2O]2 pz=pyrazolatotriclinicPa=10.202 b=13.128 c=13.612 α=112.39 β=101.90 γ=96.936 Z=11608.71.162(MeLAlH)2(μ-O) MeL = HC[(CMe)N(2,4,6-Me3C6H2)]2–whiteCaTiO3−xHx (x ≤ 0.6)Conducting; H in disordered positionMg2AlNiXHZOYSr2LiH3Oionic conductorSr3AlO4HtetragonalI4/mcma =6.7560 c =11.1568Sr21Si2O5H14cubicSr5(BO3)3HorthorhombicPnmaa=7.1982, b=14.1461, c=9.82151000.10decomposed by waterLiSr2SiO4HmonoclinicP21/ma = 6.5863, b = 5.4236, c = 6.9501, β = 112.5637air stableSr21Si2O5H12+xcubicFdma = 19.1190Sr5(PO4)3HhexagonalP63/ma = 9.7169, c = 7.2747594.83for deuterideSr2CaAlO4HtetragonalI4/mcma= 6.6220 c= 10.9812481.531SrScO2HcubicPmma = 4.0483mechanochemical synthesisSrTiO3−xHx (x ≤ 0.6)Conducting; H in disordered positionSrVO2HSr2VO3HSr3V2O5H2SrCrO2Hcubicproduced under 5GPa 1000 °CSr3Co2O4.33H0.84insulatorYHOorthorhombicPnmaa = 7.5367, b = 3.7578, c = 5.3249YOxHyphotochromic; band gap 2.6 eVZr3V3OD5Zr5Al3OH5Ba3AlO4HorthorhombicPnmaZ=4,a=10.4911,b=8.1518,c=7.2399Ba21Si2O5H14cubicFdma=20.336Ba21Si2O5H24cubicFdma = 20.336Zintl phaseBaScO2HcubicPmma=4.1518Ba2ScHO3H− conductorBaTiO3−xHx (x ≤ 0.6)Conducting; H in disordered positionBa2NaTiO3H3cubicFmma=8.29714BaVO3−xHx (x = .3)5 GPa hexagonal, 7GPa cubicBa2NaVO2.4H3.6cubicFmma=8.22670BaCrO2HhexagonalP63/mmca =5.6559 c =13.7707Ba2NaCrO2.2H3.8cubicFmma=8.17470Ba21Zn2O5H12cubicFdma = 20.417Ba21Zn2O5H24cubicFdma = 20.417Ba21Ga2O5H24cubicFdmZintl phaseBa21Ge2O5H14cubicFdma=20.356Ba21Ge2O5H24cubicFdma = 20.356Zintl phaseBa21As2O5H16cubicFdma=20.230Ba21As2O5H24cubicFdma = 20.230Sr2BaAlO4HtetragonalI4/mcma =6.9093 c =11.2107BaYO2HcubicPmma = 4.3068mechanochemical synthesisBa2YHO3a=4.38035 c=13.8234H− conductorBa21Cd2O5H12cubicFdma=20.633Ba21Cd2O5H24cubicFdma = 20.633Ba21In2O5H12cubicFdma=20.607Ba21In2O5H24cubicFdma = 20.607Zintl phaseBa21Sn2O5H14cubicFdma=20.532Ba21Sn2O5H24cubicFdma = 20.532Ba21Sb2O5H16cubicFdma=20.419Ba21Sb2O5H24cubicFdma = 20.419Ba21Pb2O5H14cubicFdma=20.597LaHOLa2LiHO3orthorhombicImmma=3.57152 b=3.76353 c=12.9785La0.6Sr1.4LiH1.6O2H− conductorLaSr3NiRuO4H4LaSrMnO3.3H0.7high-pressure fabricationLaSrCoO3H0.7insulatorCeHOCeNiHZOYCatalyse ethanol to H2Nd0.8Sr0.2NiO2Hx (x = 0.2–0.5)superconductor for x between 0.22 and 0.28NdHOP4/nmm P2ma=7.8480, c=5.5601 V=342.46 a = 5.99429, c = 3.492523 GPaPrHOGdHOFmm P2ma = 5.38450 a = 5.84710, c = 3.389835 GPaEuTiO3−xHx (x ≤ 0.6)Conducting; H in disordered positionHoHOF3ma = 5.2755light-yellow under the sun; pink indoorsDyHOcubicF3ma=5.3095ErHOcubicF3m P2ma=5.24615 a = 5.70986, c = 3.299155 GPaLuHOcubicF3ma=5.17159LuHOorthorhombicPnmaa = 7.3493, b = 3.6747, c = 5.1985Ba21Hg2O5H12cubicFdma=20.507Ba21Tl2O5H12cubicFdma=20.68Ba21Tl2O5H24cubicFdma = 20.68Zintl phaseBa21Hg2O5H24cubicFdma = 20.507Ba21Pb2O5H24cubicFdma = 20.597Ba21Bi2O5H16cubicFdma=20.459PuHOFormed during corrosion of plutonium metal in waterJohn M. Haschke Thomas H. Allen: Plutonium Hydride, Sesquioxide and Monoxide Monohydride: Pyrophoricity and Catalysis of Plutonium Corrosion, Journal of Alloys and Compounds, 320, 1, 2001, 58–71, .

Three or more anions FormulaStructureSpace groupUnit cellCommentsReferenceEu2LiHOCl2orthorhombicCmcma = 14.923 b = 5.7012 c = 11.4371 Z = 8density 5.444; yellowSr2LiHOCl2orthorhombicCmcma = 15.0235 b = 5.69899 c = 11.4501synthesized at ambient pressure and 2 GPa; ordered H/OSr2LiHOCl2tetragonalI4/mmma = 4.04215 c = 15.04359synthesized at 5 GPa; disordered H/OSr2LiHOBr2tetragonalI4/mmma = 4.1097 c = 16.1864synthesized at 5 GPa; disordered H/OBa2LiHOCl2tetragonalI4/mmma = 4.26816 c = 15.6877synthesized at 5 GPa; disordered H/O

See also Hydrous oxide (oxide-hydroxide) Aldehyde

References