#Solid helium code#

Helium was taken up around 17 GPa, enlarging the unit cell, and given off again when pressure dropped to 11 GPa. This has a much higher bulk modulus, and resists amorphization. When compressed with helium, a new clathrate forms. The mineral melanophlogite is a natural silica clathrate ( clathrasil) that normally would contain carbon dioxide, methane or nitrogen. Helium was first observed to enter into a silicate in 2007. Disodium helide is predicted to be an insulator and transparent. However the electron pairs can be considered as an eight-centre two-electron bond. The helium atoms do not participate in any bonding. The presence of these isolated electrons makes this an electride. Double electrons (2e −) are positioned on each edge and the centre of the unit cell. 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. At 300 GPa the edge of a unit cell of the crystal has a = 3.95 Å. Na 2He has a cubic crystal structure, resembling fluorite. It was predicted to be thermodynamically stable over 160 GPa and dynamically stable over 100 GPa.

#Solid helium code#

Disodium helide was first predicted using USPEX code and was first synthesised in 2016.

The 2s excited electron state resembles that of the lithium atom.ĭisodium helide (Na 2He) is a compound of helium and sodium that is stable at high pressures above 113 gigapascals (1,130,000 bar).

This requires 1,900 kilojoules (450 kcal) per gram of helium, which can be supplied by electron impact, or electric discharge. The excited helium atom has a 1s electron promoted to 2s. However, in some locations such as helium white dwarfs, conditions may be suitable to rapidly form excited helium atoms. 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. He + has an electron configuration like hydrogen, so as well as being ionic it can form covalent bonds. Ionised helium (He +), also known as He II, is a very high energy material able to extract an electron from any other atom. 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. Clathrates are also possible with helium under pressure in ice, and other small molecules such as nitrogen. Suitable pressures to force helium into solid combinations could be found inside planets. Helium has been shown to form a crystalline compound with sodium under pressure. Repulsive forces between helium and other atoms may be overcome by high pressures.

Helium has the lowest boiling point (4.2 K) of any known substance. This force may exceed repulsive forces, so at extremely low temperatures helium may form van der Waals molecules. It has the lowest polarizability of any kind of atom, however, very weak van der Waals forces exist between helium and other atoms. Helium is a very hard atom with a Pearson hardness of 12.3 eV. The helium atom is small with the radius of the outer electron shell at 0.29 Å. The electron affinity is 0.080 eV, which is very close to zero. 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. Helium's first ionization energy of 24.57 eV is the highest of any element. Helium is the smallest and the lightest noble gas and one of the most unreactive elements, so it was commonly considered that helium compounds cannot exist at all, or at least under normal conditions. Class of improbable chemical compounds only formed at very high pressure or low temperature