Helium compound may form under pressure

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.

The scientists say they have created a novel insulating ionic crystal in which He atoms take residence inside cube-shaped voids present in the lattice of Na atoms; in doing so, the He atoms force Na electrons out into neighboring voids. Though the He atoms do not form bonds, they facilitate a new stable arrangement in which each non-He-occupied cube shares a pair of electrons.

Andreas Hermann, a materials scientist who was not involved in the research, is impressed by the theoretical analysis but says that “follow-up experiments seem necessary” to confirm the Na₂He interpretation. He notes that the x-ray diffraction pattern includes the peaks predicted for Na₂He but also some unexplained extra ones. And Hermann would like to see more details supporting the researchers’ claim that the compound Na₂HeO should also prove stable.

Assuming Na₂He has formed between the diamond tips at Carnegie, scientists will want to explore the possibility that helium is crushed into compounds inside the cores of gas giant planets. (X. Dong et al., Nat. Chem., in press.)