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 anions are electropositive enough, and the conditions are reducing enough, solid materials can be made that combine hydrogen and oxygen in the negative ion role.

ProductionEdit

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.[1] However heating transition metal oxides with hydrogen, usually results in reduction to the metal.[1]

Topochemical synthesis retains the basic structure of the parent compound, and only does the minimum rearrangements of atoms to convert to the final product.[1] Topotactic reactions retain the original crystal symmetry.[1] Reactions at lower temperatures do not distorub 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.[2] TiH2 or LiH can also be used as an agent to introduce hydride.[1] If calcium hydroxide or sodium hydroxide is formed, it might be able to be washed away.[1] However for some starting oxides, this kind of hydride reduction might just yield an oxygen deficient oxide.[1]

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.[3]

Oxyhydrides all contain an alkali, alkaline earth, or rare earth metal, which are needed in order to put electronic charge on hydrogen.[3]

PropertiesEdit

The hydrogen bonding in oxyhydrides can be covalent, metallic, and ionic bonding, dependent on the metals present in the compound.[3] Oxyhydrides lose their hydrogen less than the pure metal hydrides.[2] 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)[2] Acidic attack can replace the hydrogen, for example moderate heating in hydrogen fluoride yields compounds containing oxide, fluoride and hydride ions. (oxyfluorohydride[4]) The hydrogen is more thermolabile, and can be lost by heating yielding a reduced valence metal compound.[2]

Changing the ratio of hydrogen and oxygen can modify electrical or magnetic properties. Then band gap can be altered.[2] The hydride atom can be mobile in a compound undergoing electron coupled hydride transfer.[3] The hydride ion is highly polarisable, so it presence raised the dielectric constant and refractive index.[3]

Some oxyhydrides have photocatalytic capability. For example BaTiO2.5H0.5 can function as a catalyst for ammonia production from hydrogen and nitrogen.[2]

The hydride ion is quite variable in size, ranging from 130 to 153 pm.[3]

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−.[3] In oxyhydrides, the hydride ion is much more compressible than the other atoms in compounds.[3] 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.[3]

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.[1]

ListEdit

Formula Structure Space group Unit cell volume Comments Reference
Na3SO4H tetrahedral P4/nmm a=7.0034 c=4.8569 [5]
CaTiO3−xHx (x ≤ 0.6) Conducting; H in disordered position [2]
Mg2AlNiXHZOY [6]
Sr2LiH3O ionic conductor [7]
Sr3AlO4H tetragonal I4/mcm a =6.7560 c =11.1568 [8]
Sr2CaAlO4H tetragonal I4/mcm a= 6.6220 c= 10.9812 481.531 [8]
Sr21Si2O5H14 cubic [9]
Sr5(BO3)3H orthorhombic Pnma a=7.1982, b=14.1461, c=9.8215 1000.10 decomposed by water [10]
LiSr2SiO4H monoclinic P21/m a = 6.5863, b = 5.4236, c = 6.9501, β = 112.5637 air stable [11]
Sr5(PO4)3H hexagonal P63/m a = 9.7169, c = 7.2747 594.83 for deuteride [12]
SrTiO3−xHx (x ≤ 0.6) Conducting; H in disordered position [2]
SrVO2H [2]
Sr2VO3H [2]
Sr3V2O5H2 [2]
SrCrO2H cubic produced under 5GPa 1000 °C [2]
Sr3Co2O4.33H0.84 insulator [2]
YHO orthorhombic Pnma a = 7.5367, b = 3.7578, c = 5.3249 [13]
YOxHy photochromic; band gap 2.6 eV [14]
Zr3V3OD5 [1]
Zr5Al3OH5 [1]
Ba3AlO4H orthorhombic Pnma Z=4,a=10.4911,b=8.1518,c=7.2399 [15]
BaTiO3−xHx (x ≤ 0.6) Conducting; H in disordered position [2]
BaVO3−xHx (x = .3) 5 GPa hexagonal, 7GPa cubic [2]
BaCrO2H hexagonal P63/mmc a =5.6559 c =13.7707 [16]
Ba21Zn2O5H12 cubic Fd3m a = 20.417 [9]
Sr2BaAlO4H tetragonal I4/mcm a =6.9093 c =11.2107 [8]
Ba21Cd2O5H12 cubic Fd3m a=20.633 [9]
Ba21Hg2O5H12 cubic Fd3m a=20.507 [9]
Ba21In2O5H12 cubic Fd3m a=20.607 [9]
Ba21Tl2O5H12 cubic Fd3m a=20.68 [9]
Ba21Si2O5H14 cubic Fd3m a=20.336 [9]
Ba21Ge2O5H14 cubic Fd3m a=20.356 [9]
Ba21Sn2O5H14 cubic Fd3m a=20.532 [9]
Ba21Pb2O5H14 cubic Fd3m a=20.597 [9]
Ba21As2O5H16 cubic Fd3m a=20.230 [9]
Ba21Sb2O5H16 cubic Fd3m a=20.419 [9]
BaScO2H Cubic Pmm a=4.1518 [17]
Ba2ScHO3 H conductor [18]
Ba2YHO3 a=4.38035 c=13.8234 H conductor [19]
Ba3AlO4H [1]
Ba21Si2O5H24 Fd3m Zintl phase [1]
Ba21Ge2O5H24 Fd3m Zintl phase [1]
Ba21Ga2O5H24 Fd3m Zintl phase [1]
Ba21In2O5H24 Fd3m Zintl phase [1]
La2LiHO3 [3]
La0.6Sr1.4LiH1.6O2 H conductor [3]
LaSr3NiRuO4H4 [2]
LaSrMnO3.3H0.7 high pressure fabrication [2]
LaSrCoO3H0.7 insulator [2]
EuTiO3−xHx (x ≤ 0.6) Conducting; H in disordered position [2]
LiEu2HOCl2 orthorhombic Cmcm a = 14.923, b = 5.7012, c = 11.4371, Z = 8 density 5.444; yellow [20]
LaHO [21]
CeHO [21]
PrHO [21]
NdHO P4/nmm a=7.8480, c=5.5601 V=342.46 [21]
GdHO Fmm a = 5.38450 [22]
HoHO F4̅3m a = 5.2755 light-yellow under the sun; pink indoors [23]
DyHO cubic F4̅3m a=5.3095 [24]
ErHO cubic F4̅3m a=5.24615 [24]
LuHO cubic F4̅3m a=5.17159 [24]
LuHO orthorhombic Pnma a = 7.3493, b = 3.6747, c = 5.1985 [24]
CeNiHZOY Catalyse ethanol to H2 [25]
Ba21Tl2O5H24 cubic Fd3m a = 20.68 Zintl phase [1]
Ba21Bi2O5H16 cubic Fd3m a=20.459 [9]

Three or more anionsEdit

Formula Structure Space group Unit cell Comments Reference
LiEu2HOCl2 orthorhombic Cmcm a = 1492.30(11) pm, b = 570.12(4) pm, c = 1143.71(8) pm, Z = 8 yellow [26]

ReferencesEdit

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