Polyhydride

For the reducing agent known as Superhydride, see Lithium triethylborohydride.

A polyhydride or superhydride is a compound that contains an abnormally large amount of hydrogen. This can be described as high hydrogen stoichiometry. Examples include iron pentahydride FeH₅, LiH₆, and LiH₇. By contrast, the more well known lithium hydride only has one hydrogen atom.^[1]

Polyhydrides are only known to be stable under high pressure.^[1]

Polyhydrides are important because they can form substances with a very high density of hydrogen. They may resemble the elusive metallic hydrogen, but can be made under lower pressures. One possibility is that they could be superconductors. Hydrogen sulfide under high pressures forms SH₃ units, and can be a superconductor at 203 K (−70 °C) and a pressure of 1.5 million atmospheres.^[1]

Structures

Unit cell diagram showing the structure of NaH₇, which contains H−3 complexes. The coloured balls in the isosurface, plotted at the level of 0.07 electrons*Å⁻³. One of H₂ molecules is bonded to a hydrogen atom in the NaH unit with a bond length of 1.25 Å, forming a H−3 linear anion.

The polyhydrides of alkaline earth and alkali metals contain cage structures. Also hydrogen may be clustered into H⁻, H−3, or H₂ units. Polyhydrides of transition metals may have the hydrogen atoms arranged around the metal atom. Computations suggest that increasing hydrogen levels will reduce the dimensionality of the metal arrangement, so that layers form separated by hydrogen sheets.^[1] The H−3 substructure is linear.^[2]

H+3 would form triangular structures in the hypothetical H₅Cl.^[2]

Compounds

When sodium hydride is compressed with hydrogen, NaH₃ and NaH₇ form. These are formed at 30 GPa and 2,100 K.^[2]

Heating and compressing a metal with ammonia borane avoids using bulky hydrogen, and produces boron nitride as a decomposition product in addition to the polyhydride.^[3]

formula	name	temperature °C	pressure GPa	crystal structure	space group	a Å	b	c	β	cell volume	formulae per unit cell	Tc K	Comment	refs
LiH2	lithium dihydride	27	130											[4]
LiH6	Lithium hexahydride													[1]
LiH7	Lithium heptahydride													[1]
NaH3	sodium trihydride			orthorhombic	Cmcm	3.332 Å	6.354 Å	4.142 Å	90	87.69	4			[2]
NaH7	sodium heptahydride			monoclinic	Cc	6.99	3.597	5.541	69.465	130.5				[2]
CaHx		500	22	double hexagon										[5]
CaHx		600	121											[5]
RbH9-x			10		Cccm									[6]
RbH9-x					Cm
SrH6				pseudo cubic	Pm3m								semiconductor metallize > 220 GPa	[7]
Sr3H13					C2/m									[7]
SrH22			138	triclinic	P1									[7]
BaH12	Barium dodecahydride		75	pseudo cubic		5.43	5.41	5.37		39.48		20K		[8][9]
FeH5	iron pentahydride	1200	66	tetragonal	I4/mmm									[1]
H3S	Sulfur trihydride	25	150	cubic	Im3m							203K		[10]
H3Se	Selenium trihydride		10											[11]
YH4	yttrium tetrahydride	700	160		I4/mmm									[12]
YH6	yttrium hexahydride	700	160		Im-3m							224		[12][13][14]
YH9	yttrium nonahydride	400	237		P63/mmc							243		[12]
CsH7				tetragonal	P4/nmm									[6]
CsH15+x				triclinic	P1									[6]
LaH10	Lanthanum decahydride	1000	170	cubic	Fm3m	5.09	5.09	5.09		132	4	250K		[15][16]
LaH10	Lanthanum decahydride	25	121	Hexagonal	R3m	3.67	3.67	8.83			1			[15]
LaD11	Lanthanum undecahydride	2150	130-160	Tetragonal	P4/nmm							168		[16]
LaH12	Lanthanum dodecahydride			Cubic								insulating		[16]
LaH7	Lanthanum heptahydride	25	109	monoclinic	C2/m	6.44	3.8	3.69	135	63.9	2			[15]
CeH9	Cerium nonahydride		93	hexagonal	P63/mmc	3.711		5.543		33.053		100K		[17]
CeH10	Cerium decahydride				Fm3m							115K		[18]
PrH9	Praseodymium nonahydride		90-140		P63/mmc	3.60		5.47		61.5		55K 9K		[19][20]
PrH9	Praseodymium nonahydride		120		F43m	4.98				124		69K		[19]
NdH4	Neodymium tetrahydride		85-135	tetragonal	I4/mmm	2.8234		5,7808						[21]
NdH7	Neodymium heptahydride		85-135	monoclinic	C2/c	3.3177	6.252	5.707	89.354					[21]
NdH9	Neodymium nonahydride		110-130	hexagonal	P63/mmc	3.458		5.935						[21]
EuH4			50-130		I4/mmm									[22]
Eu8H46		1600	130	cubic	Pm3n	5.865								[22]
EuH9	Europium nonahydride		86-130	cubic	F43m									[22]
EuH9	Europium nonahydride		>130	hexagonal	P63/mmc									[22]
ThH4	Thorium tetrahydride		86		I4/mmm	2.903		4.421		57.23	2			[3]
ThH4	Thorium tetrahydride		88	trigonal	P321	5.500		3.29		86.18				[3]
ThH4	Thorium tetrahydride			orthorhombic	Fmmm									[3]
ThH6	Thorium hexahydride		86-104		Cmc21					32.36				[3]
ThH9	Thorium nonahydride	2100	152	hexagonal	P63/mmc	3.713		5.541		66.20				[3]
ThH10	Thorium decahydride	1800	85-185	cubic	Fm3m	5.29				148.0		161		[3]
ThH10	Thorium decahydride		<85		Immm	5.304	3.287	3.647		74.03				[3]
UH7	Uranium heptahydride	2000	63	fcc	P63/mmc									[23]
UH8	Uranium octahydride	300	1-55	fcc	Fm3m									[23]
UH9	Uranium nonahydride		40-55	fcc	P63/mmc									[23]

Predicted

Using computational chemistry many other polyhydrides are predicted, including LiH₈,^[24] LiH₉,^[25] LiH₁₀,^[25] CsH₃,^[26] KH₅, RbH₅,^[27] RbH₉,^[24] NaH₉, BaH₆,^[27] CaH₆,^[28] MgH₄, MgH₁₂, MgH₁₆,^[29] SrH₄,^[30] SrH₁₀, SrH₁₂,^[24] ScH₄, ScH₆, ScH₈,^[31] YH₄ and YH₆,^[32] YH₂₄, LaH₈, LaH₁₀,^[33] YH₉, LaH₁₁, CeH₈, CeH₉, CeH₁₀,^[34] PrH₈, PrH₉,^[35] ThH₆, ThH₇ and ThH₁₀,^[36] U₂H₁₃, UH₇, UH₈, UH₉,^[23] AlH₅,^[37] GaH₅, InH₅,^[24] SnH₈, SnH₁₂, SnH₁₄,^[38] PbH₈,^[39] SiH₈ (subsequently discovered),^[24] GeH₈,^[40] (although Ge₃H₁₁ may be stable instead)^[41] AsH₈, SbH₄,^[42] BiH₄, BiH₅, BiH₆,^[43] H₃Se,^[44] H₃S,^[45] Te₂H₅, TeH₄,^[46] PoH₄, PoH₆,^[24] H₂F, H₃F,^[24] H₂Cl, H₃Cl, H₅Cl, H₇Cl,^[47] H₂Br, H₃Br, H₄Br, H₅Br, H₅I,^[24] XeH₂, XeH₄.^[48]

Among the transition elements, VH₈ in a C2/m structure around 200 GPa is predicted to have a superconducting transition temperature of 71.4 K. VH₅ in a P6₃/mmm space group has a lower transition temperature.^[49]

Properties

Superconduction

Under suitably high pressures polyhydrides may become superconducting. Characteristics of substances that are predicted to have high superconducting temperatures are a high phonon frequency, which will happen for light elements, and strong bonds. Hydrogen is the lightest and so will have the highest frequency of vibration. Even changing the isotope to deuterium will lower the frequency and lower the transition temperature. Compounds with more hydrogen will resemble the predicted metallic hydrogen. However, superconductors also tend to be substances with high symmetry and also need the electrons not to be locked into molecular subunits, and require large numbers of electrons in states near the Fermi level. There should also be electron-phonon coupling which happens when the electric properties are tied to the mechanical position of the hydrogen atoms.^[35][50][51] The highest superconduction critical temperatures are predicted to be in groups 3 and 3 of the periodic table. Late transitions elements, heavy lanthanides or actinides have extra d- or f-electrons that interfere with superconductivity.^[52]

For example, lithium hexahydride is predicted to lose all electrical resistance below 38 K at a pressure of 150 GPa. The hypothetical LiH₈ has a predicted superconducting transition temperature at 31 K at 200 GPa.^[53] MgH₆ is predicted to have a T_c of 400 K around 300 GPa.^[54] CaH₆ could have a T_c of 260 K at 120 GPa. PH₃ doped H₃S is also predicted to have a transition temperature above the 203 K measured for H₃S (contaminated with solid sulfur).^[55] Rare earth and actinide polyhydrides may also have highish transition temperatures, for example, ThH₁₀ with T_c = 241 K.^[36] UH₈, which can be decompressed to room temperature without decomposition, is predicted to have a transition temperature of 193 K.^[36] AcH₁₀, if it could be ever made, is predicted to superconduct at temperatures over 204 K, and AcH₁₀ would be similarly conducting under lower pressures (150 GPa).^[56]

H₃Se actually is a van der Waals solid with formula 2H₂Se·H₂ with a measured T_c of 105 K under a pressure of 135 GPa.^[11]

Ternary superhydrides

Ternary superhydrides open up the possibility of many more formulas.^[57] For example, Li₂MgH₁₆ may also be superconducting at high temperatures (200 °C).^[58] A compound of lanthanum, boron and hydrogen is speculated to be a "hot" superconductor (550 K).^[59][60] Elements may substitute for others and so modify the properties eg (La,Y)H₆ and (La,Y)H₁₀ can be made to have a slightly higher critical temperature than YH₆ or LaH₁₀.^[61]

See also

• Potassium nonahydridorhenate, stable at ordinary pressures

References

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