Disodium helide
Disodium helide is a compound of helium and sodium that is stable at high pressures above. It was first predicted using the USPEX crystal structure prediction algorithm and then synthesised in 2016.
Synthesis
was predicted to be thermodynamically stable over and dynamically stable over. This means it should be possible to form at the higher pressure and then decompress to, but below that it would decompose. Compared with other binary compounds of other elements and helium, it was predicted to be stable at the lowest pressure of any such combination. This also means, for example, that a helium-potassium compound is predicted to require much higher pressures of the order of terapascals.The material was synthesized by putting tiny plates of sodium in a diamond anvil cell along with helium at and then compressing to and heating to with a laser. Disodium helide is predicted to be an insulator and transparent. At the sodium atoms have a Bader charge of +0.599, the helium charge is −0.174, and the two-electron spots are each near −0.511. This phase could be called disodium helium electride. Disodium helide melts at a high temperature near, much higher than the melting point of sodium. When decompressed, it can keep its form as low as. As pressure increases, the sodium is predicted to gain more positive charge, the helium to lose negative charge and the free electron density to increase. Energy is compensated by the relative shrinking of the helium atoms and the space for electrons.