Polyhydride

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,, and. By contrast, the more well known lithium hydride only has one hydrogen atom.
Polyhydrides are only known to be stable under high pressure.
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 units, and can be a superconductor at and a pressure of 1.5 million atmospheres.

Structures

The polyhydrides of alkaline earth and alkali metals contain cage structures. Also hydrogen may be clustered into,, or 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. The substructure is linear.
[Trihydrogen cation|] would form triangular structures in the hypothetical.

Compounds

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

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
	lithium dihydride	27	130
	Lithium hexahydride
	Lithium heptahydride
	sodium trihydride			orthorhombic	Cmcm	3.332 Å	6.354 Å	4.142 Å	90	87.69	4
	sodium heptahydride			monoclinic	Cc	6.99	3.597	5.541	69.465	130.5
		500	22	double hexagon
		600	121
RbH_9−x			10		Cccm
RbH_9−x					Cm
	strontium hexahydride			pseudo cubic	Pm'm								semiconductor metallize > 220 GPa
	tristrontium tridecahydride				C2/m
	strontium docosahydride		138	triclinic	P1
	Barium dodecahydride		75	pseudo cubic		5.43	5.41	5.37		39.48		20K
	iron pentahydride	1200	66	tetragonal	I4/mmm
	Sulfur trihydride	25	150	cubic	Im'm							203K
	Selenium trihydride		10
	yttrium tetrahydride	700	160		I4/mmm
	yttrium hexahydride	700	160		Im-3m							224
	yttrium nonahydride	400	237		P6₃/mmc							243
	caesium heptahydride			tetragonal	P4/nmm
CsH_15+x				triclinic	P1
	Lanthanum decahydride	1000	170	cubic	Fmm	5.09	5.09	5.09		132	4	250K
	Lanthanum decahydride	25	121	Hexagonal	Rm	3.67	3.67	8.83			1
	Lanthanum undecahydride	2150	130-160	Tetragonal	P4/nmm							168
	Lanthanum dodecahydride			Cubic								insulating
	Lanthanum heptahydride	25	109	monoclinic	C2/m	6.44	3.8	3.69	135	63.9	2
	Cerium nonahydride		93	hexagonal	P6₃/mmc	3.711		5.543		33.053		100K
	Cerium decahydride				Fm'm							115K
	Praseodymium nonahydride		90-140		P6₃/mmc	3.60		5.47		61.5		55K 9K
	Praseodymium nonahydride		120		F43m	4.98				124		69K
	Neodymium tetrahydride		85-135	tetragonal	I4/mmm	2.8234		5,7808
	Neodymium heptahydride		85-135	monoclinic	C2/c	3.3177	6.252	5.707	89.354
	Neodymium nonahydride		110-130	hexagonal	P6₃/mmc	3.458		5.935
	europium tetrahydride		50-130		I4/mmm
	octaeuropium hexatetracontahydride	1600	130	cubic	Pm'n	5.865
	Europium nonahydride		86-130	cubic	F3''m
	Europium nonahydride		>130	hexagonal	P''6₃/mmc
	Thorium tetrahydride		86		I4/mmm	2.903		4.421		57.23	2
	Thorium tetrahydride		88	trigonal	P321	5.500		3.29		86.18
	Thorium tetrahydride			orthorhombic	Fmmm
	Thorium hexahydride		86-104		Cmc2₁					32.36
	Thorium nonahydride	2100	152	hexagonal	P6₃/mmc	3.713		5.541		66.20
	Thorium decahydride	1800	85-185	cubic	Fm''m''	5.29				148.0		161
	Thorium decahydride		<85		Immm	5.304	3.287	3.647		74.03
	Uranium heptahydride	2000	63	fcc	P6₃/mmc
	Uranium octahydride	300	1-55	fcc	Fmm
	Uranium nonahydride		40-55	fcc	P6₃/mmc

Predicted

Using computational chemistry many other polyhydrides are predicted, including,
,,,,,,,,,,,,,,,,,, and,,,,,,,,,,,, and,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,.
Among the transition elements, in a C2/m structure around 200 GPa is predicted to have a superconducting transition temperature of 71.4 K. in a P6₃/mmm space group has a lower transition temperature.

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. 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.
For example, lithium hexahydride is predicted to lose all electrical resistance below 38 K at a pressure of 150 GPa. The hypothetical has a predicted superconducting transition temperature at 31 K at 200 GPa. is predicted to have a T_c of 400 K around 300 GPa. could have a T_c of 260 K at 120 GPa. doped is also predicted to have a transition temperature above the 203 K measured for . Rare earth and actinide polyhydrides may also have highish transition temperatures, for example, with T_c = 241 K., which can be decompressed to room temperature without decomposition, is predicted to have a transition temperature of 193 K., if it could be ever made, is predicted to superconduct at temperatures over 204 K, and would be similarly conducting under lower pressures.
actually is a van der Waals solid with formula with a measured T_c of 105 K under a pressure of 135 GPa.

Ternary superhydrides

Ternary superhydrides open up the possibility of many more formulas. For example, may also be superconducting at high temperatures. A compound of lanthanum, boron and hydrogen is speculated to be a "hot" superconductor. Elements may substitute for others and so modify the properties eg and can be made to have a slightly higher critical temperature than or.