# Oxonickelates

Nickel forms a series of mixed oxide compounds which are commonly called nickelates. A nickelate is an anion containing nickel or a salt containing a nickelate anion, or a double compound containing nickel bound to oxygen and other elements. Nickel can be in different or even mixed oxidation states, ranging from +1, +2, +3 to +4. The anions can contain a single nickel ion, or multiple to form a cluster ion. The solid mixed oxide compounds are often ceramics, but can also be metallic. They have a variety of electrical and magnetic properties. Rare-earth elements form a range of perovskite nickelates, in which the properties vary systematically as the rare-earth element changes. Fine tuning of properties is achievable with mixtures of elements, applying stress or pressure, or varying the physical form.
Inorganic chemists call many compounds that contain nickel centred anions "nickelates". These include the chloronickelates, fluoronickelates, tetrabromonickelates, tetraiodonickelates, cyanonickelates, nitronickelates and other nickel-organic acid complexes such as oxalatonickelates.

## Alkali nickelates

The lithium nickelates are of interest to researchers as cathodes in lithium cells, as these substance can hold a variable amount of lithium, with the nickel varying in oxidation state.

## Rare-earth nickelates

Rare-earth nickelates were first made by Demazeau et al. in 1971, by heating a mixture of oxides under high pressure oxygen, or potassium perchlorate. For two decades after that these nickelates received very little attention. Many rare-earth nickelates have the Ruddlesden–Popper phase structure.
Rare-earth nickelates with nickel in a +1 oxidation state have an electronic configuration to same as for cuprates and so are of interest to high-temperature superconductor researchers. Other rare-earth nickelates can function as fuel cell catalysts. The ability to switch between an insulating and a conducting state in some of these materials is of interest in the development of new transistors, that have higher on to off current ratios.

## List of oxides

| formula | name | other names | structure | Remarks |
| LiNiO$_2$ | lithium nickelate | | rhombohedral a = 2.88 Å, c = 14.2 Å, density = 4.78 / 4.81 | |
| Li$_2$NiO$_3$ | | | monoclinic C2/m a = 4.898 Å, b = 8.449 Å, c = 4.9692 Å, β = 109.02°, V = 194.60 Å$^3$ | Nickel in +4 state |
| NaNiO$_2$ | sodium nickelate | | monoclinic a = 5.33 Å, b = 2.86 Å, c = 5.59 Å, β = 110°30′, Z = 2, density = 4.74; over 220 °C: rhombohedral a = 2.96 Å, b = 15.77 Å | Carbon dissolved in the molten salt can precipitate diamond. |
| KNiO$_2$ | potassium nickelate | | | |
| SrTiNiO$_3$ | strontium titanate nickelate | STN | | |
| YNiO$_3$ | yttrium nickelate | | monoclinic P2$_1$/n; orthorhombic a = 5.516 Å, b = 7.419 Å, c = 5.178 Å, V = 211.9 Å$^3$, Z = 4, density = 6.13 | insulator changes to metal under pressure |
| Y$_2$BaNiO$_5$ | chain nickelate | | Orthorhombic Immm, a = 3.7589, b = 5.7604, c = 11.3311 | |
| 2H-AgNiO$_2$ | | | hexagonal P6$_3$/mmc, a = 2.93653 Å, b = 2.93653 Å, c = 12.2369 Å, V = 91.384 Å$^3$, Z = 2, density = 7.216 g/cm$^3$ | Ni in +3 state |
| 3R-AgNiO$_2$ | | | trigonal R32/m, a = 2.9390 Å, c = 18.3700 Å | Ni in +3 state |
| Ag$_2$NiO$_2$ | silveroxonickelate | | trigonal R32/m, a = 2.926 Å, c = 24.0888 Å | lustrous black solid, stable in air; Ni$^{3+}$ and subvalent Ag$_2$$^+$ |
| Ag$_3$Ni$_2$O$_4$ | | | hexagonal P6$_3$/mmc, a = 2.9331 Å, b = 2.9331 Å, c = 28.31 Å, V = 210.9 Å$^3$, Z = 2, density = 7.951 g/cm$^3$ | electric conductor |
| BaNiO$_2$ | | | orthorhombic a = 5.73 Å, b = 9.2 Å, c = 4.73 Å, V = 249 Å$^3$, Z = 4 | black |
| BaNiO$_3$ | | | hexagonal a = 5.580 Å, c = 4.832 Å, V = 130.4 Å$^3$, Z = 2 | black powder dec 730 °C N-type semiconductor; decompose in acid |
| Ba$_2$Ni$_2$O$_5$ | | | hexagonal a = 5.72, c = 4.30, density = 6.4 | black needles melt 1200 °C |
| LaNiO$_2$ | lanthanum nickelite | | a = 3.959, c = 3.375 | Ni in +1 state |
| LaNiO$_3$ | lanthanum nickelate | | a = 5.4827 Å, b = 5.4827 Å, c = 3.2726 Å, γ = 120°, V = 345.5, Z = 6, density = 7.08 | metallic, no insulating transition polar metal |
| La$_2$NiO$_4$ | | LN | tetragonal a = 3.86 Å, b = 3.86 Å, c = 12.67 Å, V = 188.8 Å$^3$, Z = 2, density = 7.05 | |
| La$_3$Ni$_2$O$_6$ | | | tetragonal a = 3.968 Å, c = 19.32 Å | |
| La$_3$Ni$_2$O$_7$ | | | a = 5.3961 Å, b = 5.4498 Å, c = 20.522 Å, V = 603.5, Z = 4, density = 7.1 | superconductor under pressure T$_c$=80K |
| La$_4$Ni$_3$O$_8$ | | | | antiferromagnetic below 105 K, mixed valence I and II |
| La$_4$Ni$_3$O$_{10}$ | | | | |
| La$_{2−x}$Sr$_x$NiO$_4$ | | LSN | a varies from 3.86 to 3.81 as x changes from 0 to 0.5, then ≈ 3.81; c ≈ 12.7 for x ≤ 0.8, the it falls to 12.4 at x = 1.2 | polarization-specific metal |
| CeNiO$_3$ | cerium nickelate | | | decomposes 1984 °C |
| PrNiO$_2$ | | | | |
| PrNiO$_3$ | | | perovskite | metallic insulator transition=130K |
| Pr$_4$Ni$_3$O$_8$ | | | | |
| Pr$_2$BaNiO$_5$ | chain nickelate | | Orthorhombic | |
| La$_2$PrNi$_2$O$_7$ | | | orthorhombic | |
| La$_2$PrNi$_2$O$_7$ | | | tetragonal | Superconductor under pressure Tc = 82.5°C |
| NdNiO$_3$ | neodymium nickelate | | perovskite orthorhombic Pbnm, a = 5.38712 Å, b = 5.38267 Å, c = 7.60940 Å | metallic insulator transition=200K |
| NdNiO$_2$ | | | orthorhombic a = 5.402 Å, b = 7.608 Å, c = 5.377 Å, V = 221.0 Å$^3$, density = 7.54 | |
| Nd$_4$Ni$_3$O$_8$ | | | orthorhombic a = 3.9171 Å, b = 3.9171 Å, c = 25.307 Å, V = 388.3 Å$^3$, Z = 2, density = 7.54 | |
| Nd$_2$NiO$_4$ | | | Cmca a = 5.383 Å, b = 12.342 Å, c = 5.445 Å, V = 361.7 Å$^3$, density = 7.55 | |
| Nd$_2$BaNiO$_5$ | chain nickelate | | Orthorhombic Immm, a = 2.8268 Å, b = 5.9272 Å, c = 11.651 Å | |
| SmNiO$_3$ | samarium nickelate | SNO | perovskite Pnma, a = 5.431 Å, b = 7.568 Å, c = 5.336 Å, V = 219.3 Å, Z = 4, density = 7.79 | metallic insulator transition=400K |
| Sm$_{1.5}$Sr$_{0.5}$NiO$_4$ | | SSNO | orthorhombic Bmab | giant dielectric constant 100,000 |
| EuNiO$_3$ | europium nickelate | | perovskite orthorhombic a = 5.466 Å, b = 7.542 Å, c = 5.293 Å, V = 218.2 Å$^3$, Z = 4, density = 7.87 | metallic insulator transition=460K |
| GdNiO$_3$ | gadolinium nickelate | | perovskite orthorhombic a = 0.5492 Å, b = 0.7506 Å, c = 0.5258 Å, V = 216.8 Å$^3$, Z = 4, density = 8.09 | metallic insulator transition=510.9K |
| Gd$_2$NiO$_4$ | digadolinium nickelate | | Orthorhombic a = 3.851 Å, b = 3.851 Å, c = 6.8817 Å, V = 187.5 Å$^3$, Z = 2, density = 7.75 | |
| BaGd$_2$NiO$_5$ | barium digadolinium nickelate | chain nickellate | ?orthorhombic | low thermal conductance |
| Tb$_2$BaNiO$_5$ | chain nickelate | | Orthorhombic | |
| DyNiO$_3$ | dysprosium nickelate | | perovskite orthorhombic a = 0.55 Å, b = 0.7445 Å, c = 0.5212 Å V=213.4 Z=4 density=8.38 | metallic insulator transition=564.1K |
| Dy$_2$BaNiO$_5$ | chain nickelate | | Orthorhombic | |
| HoNiO$_3$ | holmium nickelate | | perovskite orthorhombic a = 3.96 Å, b = 3.96 Å, c = 5.04 Å, V = 212 Å$^3$ Z = 4, density=8.51 | metallic insulator transition=560K |
| Ho$_2$BaNiO$_5$ | chain nickelate | | Orthorhombic Immm, a = 3.764 Å, b = 5.761 Å, c=11.336 Å | |
| ErNiO$_3$ | erbium nickelate | | perovskite orthorhombic a = 5.514 Å, b =7.381 Å, c = 5.16 V=201 Z=4 density=8.67 | metallic insulator transition=580K |
| Er$_2$BaNiO$_5$ | chain nickelate | | Orthorhombic Immm a = 3.7541 Å, b = 5.7442 Å c=11.3019 Å V=243.71 Å$^3$ Z=2 | |
| TmNiO$_3$ | thulium nickelate | | orthorhombic a = 5.495 Å, b = 7.375 Å, c = 5.149 Å V = 208.7 Z = 4 density = 8.77 | |
| Tm$_2$BaNiO$_5$ | thulium barium nickelate | | Orthorhombic low temperature Pnma a = 12.2003 Å b = 5.65845 Å c = 6.9745 Å Z = 4; high T: Immm a = 3.75128 b = 5.7214 c = 11.2456 | Pnma form is brown Immm form is dark green |
| YbNiO$_3$ | ytterbium nickelate | | Orthorhombic a = 5.496 Å, b = 7.353 Å, c = 5.131 Å Z=4 V=207.4 Å$^3$ density=8.96 | |
| Yb$_2$BaNiO$_5$ | ytterbium barium nickelate | | Orthorhombic Pnma a = 5.6423 Å, b = 6.9545 Å, c = 12.1583 Å V=477.1 Z=4 density=8.66 | Pnma form is brown |
| LuNiO$_3$ | lutetium nickelate | | perovskite a = 5.499 Å, b = 7.356 Å, c = 5.117 Å, V = 207 Å$^3$, Z = 4, density = 9.04 | metallic insulator transition=600K |
| Lu$_2$BaNiO$_5$ | | | Orthorhombic Pnma | |
| TlNiO$_3$ | thallium nickelate(III) | | perovskite a = 5.2549 Å, b = 5.3677 Å, c = 7.5620 Å, V = 213.3 Å$^3$ | |
| PbNiO$_3$ | |
| BiNiO$_3$ | bismuth nickelate(III) | | perovskite triclinic a = 5.3852, b = 5.6498, c = 7.7078 Å, α = 91.9529°, β = 89.8097°, γ = 91.5411, V = 234.29 Å$^3$ | Ni in +2 state, Bi in +3 and +5; stable 5–420K, antiferromagnetic |