Oxonickelates

For the most common nickel oxide, see Nickel(II) oxide.

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.^[1]

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.^[2] For two decades after that these nickelates received very little attention.^[3] 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.^[4][5][6] 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.^[7]

List of oxides

formula	name	other names	structure	Remarks	references
LiNiO2	lithium nickelate		rhombohedral a = 2.88 Å, c = 14.2 Å, density = 4.78 / 4.81		[8]
Li2NiO3			monoclinic C2/m a = 4.898 Å, b = 8.449 Å, c = 4.9692 Å, β = 109.02°, V = 194.60 Å3	Nickel in +4 state	[1]
NaNiO2	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.	[8][9]
KNiO2	potassium nickelate				[8][10]
SrTiNiO3[dubious – discuss]	strontium titanate nickelate	STN			[11]
YNiO3	yttrium nickelate		monoclinic P21/n; orthorhombic a = 5.516 Å, b = 7.419 Å, c = 5.178 Å, V = 211.9 Å3, Z = 4, density = 6.13	insulator changes to metal under pressure	[12][13]
Y2BaNiO5	chain nickelate		Orthorhombic Immm, a = 3.7589, b = 5.7604, c = 11.3311		[14][15]
2H-AgNiO2			hexagonal P63/mmc, a = 2.93653 Å, b = 2.93653 Å, c = 12.2369 Å, V = 91.384 Å3, Z = 2, density = 7.216 g/cm3	Ni in +3 state	[16]
3R-AgNiO2			trigonal R32/m, a = 2.9390 Å, c = 18.3700 Å	Ni in +3 state	[16][17]
Ag2NiO2	silveroxonickelate		trigonal R32/m, a = 2.926 Å, c = 24.0888 Å	lustrous black solid, stable in air; Ni3+ and subvalent Ag2+	[17]
Ag3Ni2O4			hexagonal P63/mmc, a = 2.9331 Å, b = 2.9331 Å, c = 28.31 Å, V = 210.9 Å3, Z = 2, density = 7.951 g/cm3	electric conductor	[18]
BaNiO2			orthorhombic a = 5.73 Å, b = 9.2 Å, c = 4.73 Å, V = 249 Å3, Z = 4	black	[19]
BaNiO3			hexagonal a = 5.580 Å, c = 4.832 Å, V = 130.4 Å3, Z = 2	black powder dec 730 °C N-type semiconductor; decompose in acid	[19][20]
Ba2Ni2O5			hexagonal a = 5.72, c = 4.30, density = 6.4	black needles melt 1200 °C	[19][20]
LaNiO2	lanthanum nickelite		a = 3.959, c = 3.375	Ni in +1 state	[21]
LaNiO3	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	[22]
La2NiO4		LN	tetragonal a = 3.86 Å, b = 3.86 Å, c = 12.67 Å, V = 188.8 Å3, Z = 2, density = 7.05		[23][24]
La3Ni2O6			tetragonal a = 3.968 Å, c = 19.32 Å		[23]
La3Ni2O7			a = 5.3961 Å, b = 5.4498 Å, c = 20.522 Å, V = 603.5, Z = 4, density = 7.1	superconductor under pressure Tc=80K	[23][25][26]
La4Ni3O8				antiferromagnetic below 105 K, mixed valence I and II	[23][27]
La4Ni3O10					[27]
La2−xSrxNiO4		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	[28]
CeNiO3	cerium nickelate			decomposes 1984 °C	[29]
PrNiO2					[23]
PrNiO3			perovskite	metallic insulator transition=130K	[30]
Pr4Ni3O8					[23]
Pr2BaNiO5	chain nickelate		Orthorhombic		[14]
La2PrNi2O7			orthorhombic		[31]
La2PrNi2O7			tetragonal	Superconductor under pressure Tc = 82.5°C	[31]
NdNiO3	neodymium nickelate		perovskite orthorhombic Pbnm, a = 5.38712 Å, b = 5.38267 Å, c = 7.60940 Å	metallic insulator transition=200K	[13][30]
NdNiO2			orthorhombic a = 5.402 Å, b = 7.608 Å, c = 5.377 Å, V = 221.0 Å3, density = 7.54		[23][32][33]
Nd4Ni3O8			orthorhombic a = 3.9171 Å, b = 3.9171 Å, c = 25.307 Å, V = 388.3 Å3, Z = 2, density = 7.54		[23][34]
Nd2NiO4			Cmca a = 5.383 Å, b = 12.342 Å, c = 5.445 Å, V = 361.7 Å3, density = 7.55		[35]
Nd2BaNiO5	chain nickelate		Orthorhombic Immm, a = 2.8268 Å, b = 5.9272 Å, c = 11.651 Å		[14][15]
SmNiO3	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	[30][36]
Sm1.5Sr0.5NiO4		SSNO	orthorhombic Bmab	giant dielectric constant 100,000	[37]
EuNiO3	europium nickelate		perovskite orthorhombic a = 5.466 Å, b = 7.542 Å, c = 5.293 Å, V = 218.2 Å3, Z = 4, density = 7.87	metallic insulator transition=460K	[30]
GdNiO3	gadolinium nickelate		perovskite orthorhombic a = 0.5492 Å, b = 0.7506 Å, c = 0.5258 Å, V = 216.8 Å3, Z = 4, density = 8.09	metallic insulator transition=510.9K	[38]
Gd2NiO4	digadolinium nickelate		Orthorhombic a = 3.851 Å, b = 3.851 Å, c = 6.8817 Å, V = 187.5 Å3, Z = 2, density = 7.75		[39]
BaGd2NiO5	barium digadolinium nickelate	chain nickellate	?orthorhombic	low thermal conductance	[40]
Tb2BaNiO5	chain nickelate		Orthorhombic		[14]
DyNiO3	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	[30][38][41]
Dy2BaNiO5	chain nickelate		Orthorhombic		[14]
HoNiO3	holmium nickelate		perovskite orthorhombic a = 3.96 Å, b = 3.96 Å, c = 5.04 Å, V = 212 Å3 Z = 4, density=8.51	metallic insulator transition=560K	[38]
Ho2BaNiO5	chain nickelate		Orthorhombic Immm, a = 3.764 Å, b = 5.761 Å, c=11.336 Å		[14][42]
ErNiO3	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	[38][43]
Er2BaNiO5	chain nickelate		Orthorhombic Immm a = 3.7541 Å, b = 5.7442 Å c=11.3019 Å V=243.71 Å3 Z=2		[14][15][44]
TmNiO3	thulium nickelate		orthorhombic a = 5.495 Å, b = 7.375 Å, c = 5.149 Å V = 208.7 Z = 4 density = 8.77		[45]
Tm2BaNiO5	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	[14][46]
YbNiO3	ytterbium nickelate		Orthorhombic a = 5.496 Å, b = 7.353 Å, c = 5.131 Å Z=4 V=207.4 Å3 density=8.96		[47]
Yb2BaNiO5	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	[46]
LuNiO3	lutetium nickelate		perovskite a = 5.499 Å, b = 7.356 Å, c = 5.117 Å, V = 207 Å3, Z = 4, density = 9.04	metallic insulator transition=600K	[38][48]
Lu2BaNiO5			Orthorhombic Pnma		[15]
TlNiO3	thallium nickelate(III)		perovskite a = 5.2549 Å, b = 5.3677 Å, c = 7.5620 Å, V = 213.3 Å3		[49]
PbNiO3
BiNiO3	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 Å3	Ni in +2 state, Bi in +3 and +5; stable 5–420K, antiferromagnetic	[50][51]

See also

• Nickel manganese oxides for what are considered nickel manganates