Helium compounds

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 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. 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. 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. 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 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. E. B. Gordon et al. suggested that this material may exist in 1974. 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. [Kr]/[He] contains 40–60 helium atoms per krypton atom [Xe]/[He] contains 40–60 helium atoms per xenon atom. Using H2 results in the product floating and stopping further production, but with deuterium, or a half-half mixture, it can sink and accumulate. Lowering temperatures into the millikelvin range can prolong the lifetime of atomic hydrogen condensates. Condensates containing heavy water or deuterium are under investigation for the production of ultracold neutrons. The water-helium condensate [H2O]/[He] contains water clusters of several nanometers in diameter, and pores from 8 to 800 nm. Oxygen O2 impurity helium contains solid oxygen clusters from 1 to 100 nm. ==Impurity solid helium==

Introducing impurities into solid helium yields a blue solid that melts at a higher temperature than pure He. For cesium the absorption has a peak at 750 nm, and for rubidium, maximal absorption is at 640 nm. These are due to metal clusters with diameters of 10 nm or so. However the low concentration of clusters in this substance should not be sufficient to solidify helium as the amount of metal in the solid is less than billionth that of the impurity helium condensate solids, and liquid helium does not "wet" cesium metal. The solid is possibly due to helium snowballs attached to Cs+ (or Rb+) ions. Free electrons in liquid helium are enclosed in a bubble 17 Å in diameter. Under 25 atmosphere pressure an electron bubble reduces to 11 Å. ==Solid solution==

Helium can dissolve to a limited extent in hot metal, with concentration proportional to pressure. At atmospheric pressure, 500 °C bismuth can absorb 1 part in a billion; at 649 °C lithium can take 5 parts per billion; and at 482 °C potassium can take 2.9 parts per million (all atom fractions). In nickel there can be 1 in 1010 atoms, and in gold 1 in 107. The supposition is that the higher the melting point the less helium can be dissolved. However, when a liquid metal is quenched, higher concentrations of helium can be left dissolved. So cooled liquid steel can have one part per million of helium. In order to get a helium atom into a metal lattice, a hole has to be formed. The energy to make that hole in the metal is basically the heat of solution. ==Nanowires==

Gold, copper, rubidium, caesium, or barium atoms evaporated into liquid helium form spiderweb-like structures. Rhenium produces nano flakes. Molybdenum, tungsten, and niobium produce thin nanowires with diameters of 20, 25 and 40 Å. When platinum, molybdenum or tungsten is evaporated into liquid helium, nanoclusters are first formed, accompanied by high temperature thermal emission pulse, above the melting point of the metals. In superfluid helium, these clusters migrate to the vortices and weld together to yield nanowires once the clusters are mostly solid. In higher temperature liquid helium, larger clusters of metal are formed instead of wires. The metal vapours can only penetrate about 0.5 mm into liquid helium. Indium, tin, lead and nickel produce nanowires about 80 Å in diameter. These same four metals also produce smooth spheres about 2 μm across that explode when examined with an electron microscope. Copper, permalloy, and bismuth also make nanowires. ==Two-dimensional ionic crystal==

Helium II ions (He+) in liquid helium when attracted by an electric field can form a two-dimensional crystal at temperatures below 100 mK. There are about half a trillion ions per square meter just below the surface of the helium. Free electrons float above the helium surface. ==Known van der Waals molecules==

• LiHe • Dihelium • Trihelium • Ag3He • HeCO is weakly bound by van der Waals forces. It is potentially important in cold interstellar media as both CO and He are common. • CF4He and CCl4He both exist. • HeI2 can be formed by supersonic expansion of high pressure helium with a trace of iodine into a vacuum. It was the first known triatomic helium van der Waals molecule. It can be detected by fluorescence. HeI2 has a similar optical spectrum to I2, except that the bands and lines are shifted to form two extra series. One series is blueshifted by between 2.4 and 4.0 cm−1, and the other between 9.4 and 9.9 cm−1. The two series may be due to different amounts of vibration in the He–I bond. The lines are narrow indicating that the molecules in their excited vibrational state have a long lifetime. • Na2He molecules can form on the surface of helium nanodroplets. • NOHe ==Known ions==

Helium has the highest ionisation energy, so a He+ ion will strip electrons off any other neutral atom or molecule. However it can also then bind to the ion produced. The He+ ion can be studied in gas, or in liquid helium. Its chemistry is not completely trivial. For example, He+ can react with SF6 to yield SF or SF and atomic fluorine. Ionised clusters He was predicted to exist by Linus Pauling in 1933. It was discovered when doing mass spectroscopy on ionised helium. The dihelium cation is formed by an ionised helium atom combining with a helium atom: He+ + He → He. The trihelium cation He Helium hydride The helium hydride ion HeH+ has been known since 1925. Noble gas Noble gas cluster ions exist for different noble gases. Singly charged cluster ions containing xenon exist with the formula HenXe, where n and m ≥ 1. The Ar+ argon ion can form many different sized clusters with helium ranging from HeAr+ to He50Ar+, but the most common clusters are He12Ar+ and smaller. These clusters are made by capturing an argon atom in a liquid helium nanodroplet, and then ionising with high speed electrons. He+ is formed, which can transfer charge to argon and then form a cluster ion when the rest of the droplet evaporates. NeHe can be made by ultraviolet photoionisation. Clusters only contain one neon atom. The number of helium atoms can vary from 1 to 23, but NeHe and NeHe are more likely to be observed. Cesium and rubidium cations also form clusters with helium. formed by high electric field off platinum surface in helium, VHe2+, Ta2+He, Mo2+He, W2+He, Re2+He, Ir2+He, Pt2+He2, W3+He2, W3+He3, and W3+He4. The energy needed to break up the molecule is 140 cm−1 which is quite a bit stronger than the van der Waals neutral molecules. HeN is tough enough to have several vibrational, bending and rotational states. He(OH)+ has been detected, although it is not produced when HTO (tritiated water) decays. When tritium substituted methane (CH3T) decays, CH3He+ is produced in a very small amount. The helium formyl cation, HeHCO+ is a linear molecule. It has a vibrational frequency red shifted 12.4 cm−1 compared to HCO+. It can be considered as a deenergized protonation reaction intermediate for the HeH+ + CO → HCO+ + He. HeHCO+ can be produced by a supersonic expansion of a gas mixture of He, CO, and H2, which is hit by a cross beam of electrons. CO and H2 are only supplied at 1% of the helium. The binding energy is 378 cm−1 in the 000 vibrational state, and 431 cm−1 in the 100 vibrational state. He2HN is also known. One helium atom is linked to a hydrogen, and the other is less tightly bound. ==Excimers==

The He excimer is responsible for the Hopfield continuum. Helium also forms an excimer with barium, Ba+He*. ==Predicted compounds==

Predicted solids MgF2He. Helium in white, magnesium in orange and fluorine in blue is predicted to form a solid with orthorhombic structure Ibam. Iron helide (FeHe) was early on claimed to have been found, perhaps in dense planetary cores, or, as suggested by Freeman Dyson, in neutron star crust material. Recent density functional theory calculations predict the formation of FeHe compounds at pressures above about 4 TPa, suggesting indeed that these compounds could be found inside giant planets, white dwarf stars, or neutron stars. Na2HeO is predicted to have a similar structure to Na2He, but with oxygen atoms in the same position as the electron pair, so that it becomes O2−. It would be stable from 13 to 106 GPa. La2/3-xLi3xTiO3He is a porous lithium ion conduction perovskite that can contain helium like a clathrate. Helium is predicted to form an inclusion compound with silicon, Si2He. This has a hexagonal lattice of silicon atoms with helium atoms lined up in the channels. It should be formed when liquid silicon is injected with helium at over 1GPa and cooled. Helium is predicted to form a fluoride He3F2 under extreme pressures well over 1 Tpa. Predicted van der Waals molecules The beryllium oxide helium adduct, HeBeO is believed to be bonded much more strongly than a normal van der Waals molecule with about 5 kcal/mol of binding energy. The bond is enhanced by a dipole induced positive charge on beryllium, and a vacancy in the σ orbital on beryllium where it faces the helium. Variations on the beryllium oxide adduct include HeBe2O2, and . Hydridohelium fluoride HHeF is predicted to have a . This molecule's metastability is slated due to electrostatic attraction between HHe+ and F− which increases the barrier to an exothermic breakup. Calculations for coinage metal fluorides include HeCuF as stable, HeAgF is unstable, Ag4He binding energy 1.85 cm−1, Au3He binding energy 4.91 cm−1, Bonds are predicted to form to nickel with helium as a weak ligand in HeNiCO and HeNiN2. 1-Tris(pyrazolyl)borate beryllium and 1-tris(pyrazolyl)borate magnesium are predicted to bind helium at low temperatures. There is also a prediction of a He-O bond in a molecule with caesium fluoride or tetramethyl ammonium fluoride. LiHe2 is predicted to be in an Efimov state when excited. Predicted ions Many ions have been investigated theoretically to see if they could exist. Just about every diatomic cation with helium has been studied. For the diatomic dications, for stability the second ionisation level of the partner atom has to be below the first ionisation level of helium, 24.6 eV. For Li, F, and Ne the ground state is repulsive, so molecules will not form. For N and O the molecule would break up to release He+. However HeBe2+, HeB2+ and HeC2+ are predicted to be stable. Also second row elements from Na to Cl are predicted to have a stable HeX2+ ion. Other possibly thermochemically stable ions include HeZr3+, HeHf3+, HeLa3+, HeNd3+, HeCe3+, HePr3+, HePm3+, HeSm3+, HeGa3+, HeTb3+, HeDy3+, HeHo3+, HeEr3+, HeTm3+, and HeLu3+ where the third ionisation point is below that of helium. The Fluoroheliate FHeO− ion should be stable but salts like LiFHeO are not stable. • FHeS− is predicted to be stable. • FHeBN− • HHeN2+ is unlikely to exist. • (HHe+)(OH2) is probably unstable. The lithium hydrohelide cation HLiHe+ is linear in theory. This molecular ion could exist with big bang nucleosynthesis elements. Other hydrohelide cations that exist in theory are HNaHe+ sodium hydrohelide cation, HKHe+ potassium hydrohelide cation, HBeHe2+ beryllium hydrohelide cation, HMgHe2+ magnesium hydrohelide cation, and HCaHe2+ calcium hydrohelide cation. Similarly the ethynylhelium cation [HeC≡CH]+ is predicted to be bound (with respect to ethynyl cation and helium) by 22 kcal/mol. However, vinylhelium ([HeCH=CH2]+, 0.03 kcal/mol) and methylhelium ([HeCH3]+, 1.5 kcal/mol) cations are predicted to be bound much more weakly. • HCHe+ ==Discredited or unlikely observations==

Numerous researchers attempted to create chemical compounds of helium in the early part of the twentieth century. In 1895 L. Troost and L. Ouvrard believed they had witnessed a reaction between magnesium vapour and helium (and also argon) due to the spectrum of helium disappearing from the tube they were passing it through. In 1906, W. Ternant Cooke claimed to have noticed a reaction of helium with cadmium or mercury vapour by observing an increase in the density of the vapour. Zinc vapour did not react with helium. J. J. Manley claimed to have found gaseous mercury helide HeHg in 1925 HgHe10; publishing the results in Nature, but then had trouble finding a stable composition, and eventually gave up. Between 1925 and 1940 in Buenos Aires, Horacio Damianovich studied various metal–helium combinations including beryllium (BeHe), iron (FeHe), palladium (PdHe), platinum (Pt3He), bismuth, and uranium. To make these substances, electrical discharges impacted helium into the surface of the metal. Later these were demoted from the status of compounds, to that of alloys. Palladium helide, PdHe is formed from tritium decay in palladium tritide, the helium (3He) is retained in the solid as a solution. Boomer claimed the discovery of tungsten helide WHe2 as a black solid. It is formed by way of an electric discharge in helium with a heated tungsten filament. When dissolved in nitric acid or potassium hydroxide, tungstic acid forms and helium escapes in bubbles. The electric discharge had a current of 5 mA and 1,000 V at a pressure between 0.05 and 0.5 mmHg for the helium. The process works slowly at 200 V. and 0.02 mmHg of mercury vapour accelerates tungsten evaporation by five times. The search for this was suggested by Ernest Rutherford. It was discredited by J. G. Waller in 1960. Sulfur and phosphorus helium combinations decomposed around −120 °C H. Krefft and R. Rompe claimed reactions between helium and sodium, potassium, zinc, rubidium, indium, and thallium. ==References==