Grace Under Pressure...

The sample appears as a dark area near the center of this micrograph of the diamond-anvil cell.
Credit: X. Dong et al. Nat. Chem. 2017

Topics: Chemistry, Chemical Physics, Materials Science

Helium doesn’t play well with others. Beyond its noble gas designation on the periodic table, it has the lowest electron affinity—zero—among the elements, and the highest ionization energy. Scientists have managed to mechanically pack He atoms with other elements, but the He has little effect on those compounds’ characteristics.

Now an international team has presented evidence for a compound whose electronic structure and thus its physical properties are influenced by its He components. Researchers led by Artem Oganov ran a crystal structure prediction algorithm to play matchmaker for He and found that the compound Na₂He should form at high pressures. The researchers shared their prediction with Alexander Goncharov and colleagues, who loaded He gas and solid sodium into a diamond-anvil cell at the Carnegie Institution for Science. After increasing the pressure to 140 GPa and heating the sample, Goncharov’s team noticed a marked shift in material properties. New peaks appeared in x-ray diffraction patterns, and the sample’s melting point rose to more than 1500 K; pure Na melts at about 550 K.

Scientific American: Helium compound may form under pressure, Andrew Grant