Helium is the smallest and the lightest noble gas and one of the most unreactive elements, so it was commonly assumed that helium compounds could not exist at all, or at least not under normal conditions. Helium's first ionization energy of 24.57 eV is the highest of any element. Helium has a complete shell of electrons, and in this form the atom does not readily accept any extra electrons nor join with anything to make covalent compounds. The electron affinity is 0.080 eV, which is very close to zero. The helium atom is small, with the radius of the outer electron shell at 0.29 Å. Helium is a very hard atom with a Pearson hardness of 12.3 eV. It has the lowest polarizability of any kind of atom; however, very weak van der Waals forces exist between helium and other atoms. This force may exceed repulsive forces, so at extremely low temperatures helium may form van der Waals molecules. Helium has the lowest boiling point (4.2 K) of any known substance.
Repulsive forces between helium and other atoms may be overcome by high pressures. Helium has been shown to form a crystalline compound with sodium under pressure. Suitable pressures to force helium into solid combinations could be found inside planets. Clathrates are also possible with helium under pressure in ice, and other small molecules such as nitrogen.
Other ways to make helium reactive are: to convert it into an ion, or to excite an electron to a higher level, allowing it to form excimers. Ionized helium (He+), also known as He II, is a very high energy material able to extract an electron from any other atom. He+ has an electron configuration like hydrogen, so as well as being ionic, it can form covalent bonds. Excimers do not last for long, as the molecule containing the higher energy level helium atom can rapidly decay back to a repulsive ground state, where the two atoms making up the bond repel. However, in some locations such as helium white dwarfs, conditions may be suitable to rapidly form excited helium atoms. The excited helium atom has a 1s electron promoted to 2s. This requires per gram of helium, which can be supplied by electron impact, or electric discharge. The 2s excited electron state resembles that of the lithium atom.
Known solid phases Most solid combinations of helium with other substances require high pressure. Helium does not bond with the other atoms, but the substances can have a well defined crystal structure.
Disodium helide
Disodium helide (Na2He) is a compound of helium and sodium that is stable at high pressures above . Disodium helide was first predicted using USPEX code and was first synthesised in 2016. It was predicted to be thermodynamically stable over 160 GPa and dynamically stable over 100 GPa. Na2He has a cubic crystal structure, resembling fluorite. At 300 GPa the edge of a unit cell of the crystal has . Each unit cell contains four helium atoms on the centre of the cube faces and corners, and eight sodium atoms at coordinates a quarter cell in from each face. Double electrons (2e−) are positioned on each edge and the centre of the unit cell.Each face is shared by two cells, each edge is shared by four cells, and each corner is shared by eight cells. Each pair of electrons is spin paired. The presence of these isolated electrons makes this an electride. The helium atoms do not participate in any bonding. However the electron pairs can be considered as an eight-centre two-electron bond. Disodium helide is predicted to be an insulator and transparent.
Silicates Helium was first observed to enter into a silicate in 2007. The mineral melanophlogite is a natural silica clathrate (clathrasil) that normally would contain carbon dioxide, methane or nitrogen. When compressed with helium, a new clathrate forms. This has a much higher bulk modulus, and resists amorphization. Helium was taken up around 17 GPa, enlarging the unit cell, and given off again when pressure dropped to 11 GPa.
Cristobalite He II (SiO2He) is stable between 1.7 and 6.4 GPa. It has a rhombohedral space group R-3c with unit cell dimensions and at 4 GPa.
Cristobalite He I (SiO2He) can be formed under higher helium pressures over 6.4 GPa. It has a monoclinic space group P21/C with unit cell dimensions and at 10 GPa.
Helium penetrates into fused silica at high pressure, reducing its compressibility.
Chibaite, another natural silica clathrate has its structure penetrated by helium under pressures higher than 2.5 GPa. The presence of guest hydrocarbons does not prevent this happening. Neon requires a higher pressure, 4.5 GPa to penetrate, and unlike helium shows hysteresis. Linde-type A zeolites are also rendered less compressible when penetrated by helium between 2 and 7 GPa.
Arsenolite helium inclusion compound
Arsenolite helium inclusion compound is stable from pressures over 3 GPa and up to at least 30 GPa. Arsenolite is one of the softest and most compressible minerals. Helium prevents amorphization that would otherwise occur in arsenolite under pressure. The solid containing helium is stronger and harder, with a higher sound velocity than plain arsenolite. The helium that is included into the crystal causes a more uniform stress on the As4O6 molecules. No actual bond is formed from arsenic to helium despite the lone pairs of electrons available. The diffusion of helium into arsenolite is a slow process taking days at a pressure around 3 GPa. However, if the pressure on the crystal is too high (13 GPa) helium penetration does not take place, as the gaps between arsenolite molecules become too small. Neon does not diffuse into arsenolite.
Perovskites Helium can be inserted into the A sites of negative thermal expansion perovskites that otherwise have defects at the A site. At room temperature and 350 MPa helium is included into CaZrF6 to expand its unit cell yielding HeCaZrF6. About half of the A sites are filled by helium atoms. This substance loses helium over several minutes on depressurisation at ambient temperature, but below 130 K it retains helium when depressurised. At 1 GPa all the A sites are filled by helium, yielding He2CaZrF6.
Formates Under pressure helium penetrates dimethylammonium iron formate (CH3)2NH2Fe(HCOO)3. It affects this by causing a change to a monoclinic ordered state at a lower pressure (around 4 GPa) than if helium were absent.
Small molecule is a van der Waals compound with hexagonal crystals. At 10 GPa the unit cell of 22 nitrogen atoms has a unit cell volume of 558 Å3, and about 512 Å3 at 15 GPa. These sizes are around 10 Å3 smaller than the equivalent amount of solid δ-N2 nitrogen at these pressures. The substance is made by compressing nitrogen and helium in a diamond anvil cell.
NeHe2 has a crystal structure of hexagonal MgZn2 type at 13.7 GPa. The unit cell has dimensions and at 21.8 GPa, There are four atoms in each unit cell. It melts at 12.8 GPa and 296 K, stable to over 90 GPa.
Clathrates Helium clathrates only form under pressure. With ice II at pressures between 280 and 480 MPa a solid helium hydrate with He:H2O ratio of 1:6 exists. Another clathrate with a water to helium ratio of 2.833 has been made in the SII clathrate structure. It has two different cages in the ice, the small one can contain one helium atom, and the large can contain four atoms. It was produced from neon clathrate that lost its neon, and then replaced by helium at 141 K and 150 MPa Other helium hydrates with the ice-Ih, ice-Ic 1:1, and ice-Ic 2:1 He to H2O ratio have been predicted. These could exist in planets like Neptune or Uranus. Helium clathrate hydrates should be similar to hydrogen clathrate due to the similar size of the hydrogen molecule.
Helium may enter into crystals of other molecular solids under pressure to alter their structure and properties; for example, with chlorpropamide over 0.3 GPa in helium changes to a monoclinic structure, and yet another structural form at 1.0 GPa.
Fullerites Helium can form intercalation compounds with the fullerites, including buckminsterfullerene C60 and C70. In solid C60 there are spaces between the C60 balls, either tetrahedral or octahedral in shape. Helium can diffuse into the solid fullerite even at one atmosphere pressure. Helium enters the lattice in two stages. The first rapid stage takes a couple of days, and expands the lattice by 0.16% (that is 2.2 pm) filling the larger octahedral sites. The second stage takes thousands of hours to absorb more helium and expands the lattice twice as much again (0.32%) filling the tetrahedral sites. However the solid C60•3He is not stable and loses helium on a timescale of 340 hours when not under a helium atmosphere. When the helium intercalated fullerite is cooled, it has an orientational phase transition that is 10 K higher than for pure solid C60. The actual discontinuous change in volume at that point is smaller, but there are more rapid changes near the transition temperature, perhaps due to varying occupancy of the voids by helium.
Endohedral Helium atoms can be trapped inside molecular cages such as the fullerenes He@C60, He@C70, He2@C60 and He2@C70 have all been made using compressed helium and fullerenes. When using only pressure and heat, the yield is quite low, under 1%. However, by breaking and reforming the carbon ball, much higher concentrations of He@C60 or He@C70 can be made. High-performance liquid chromatography can concentrate the helium containing material. HeN@C60 and HeN@C70 have also been made. These have a lower symmetry due to the two atoms being trapped together in the same cavity. This also causes ESR line broadening.
Dodecahedrane can trap helium from a helium ion beam to yield He@C20H20.
Other cage like inorganic or organic molecules may also trap helium, for example C8He with He inside a cube, or He@Mo6Cl8F6.
Impurity helium condensates Impurity helium condensates (IHCs) (or impurity helium gels) are deposited as a snow-like gel in liquid helium when various atoms or molecules are absorbed on the surface of superfluid helium. Atoms can include H, N, Na, Ne, Ar, Kr, Xe, alkalis or alkaline earths. The impurities form nanoparticle clusters coated with localised helium held by van der Waals force. Helium atoms are unable to move towards or away from the impurity, but perhaps can move perpendicularly around the impurity. The snow like solid is structured like an aerogel. When free atoms are included in the condensate a high energy density can be achieved, up to 860 J cm−1 or 5 kJ g−1. These condensates were first investigated as a possible rocket fuel. The mixtures are given a notation involving square brackets so that [N]/[He] represents a nitrogen atom impurity in helium.
[N]/[He] atomic nitrogen impurity helium is produced when a radio frequency discharge in a nitrogen helium mixture is absorbed into superfluid helium, it can have up to 4% nitrogen atoms included. The substance resembles crumbly snow and condenses and settles from the liquid helium. It also contains variable proportions of N2 molecules. This substance is a high energy solid, with as much power as conventional explosives. When it is heated above 2.19 K (the lambda point of helium), the solid decomposes and explodes. This substance is not a true compound, but more like a solid solution. E. B. Gordon et al. suggested that this material may exist in 1974. The localised helium shells around an individual atom are termed van der Waals spheres. However the idea that the nitrogen atoms are dispersed in the helium has been replaced by the concept of nitrogen atoms attached to the surface of clusters of nitrogen molecules. The energy density of the solid can be increased by pressing it.
Other inert gas impurity helium condensates can also be made from a gas beam into superfluid helium. [Ne]/[He] decomposes at 8.5 K with release of heat and formation of solid neon. Its composition approximates NeHe16.
[Ar]/[He] contains 40–60 helium atoms per argon atom.