Scientists at the Carnegie Institution have found that high pressure can be used to make a unique hydrogen-storage material.
The researchers found that the normally unreactive, noble gas xenon combines with molecular hydrogen under pressure to form a previously unknown solid with unusual bonding chemistry. The experiments are the first time these elements have been combined to form a stable compound. The discovery debuts a new range of materials, which could boost new hydrogen technologies
Maddury Somayazulu, research scientist at Carnegie’s Geophysical Laboratory, said: ‘We subjected a series of gas mixtures of xenon in combination with hydrogen to high pressures in a diamond anvil cell. At about 41,000 times the pressure at sea level, the atoms became arranged in a lattice structure dominated by hydrogen, but interspersed with layers of loosely bonded xenon pairs. When we increased the pressure, the distances between the xenon pairs changed the distances contracted to those observed in dense metallic xenon.’
The researchers imaged the compound at varying pressures using X-ray diffraction, infrared and Raman spectroscopy. When they looked at the xenon part of the structure, they realised that the interaction of xenon with the surrounding hydrogen was responsible for the unusual stability and the continuous change in xenon-xenon distances as pressure was adjusted from 41,000 to 255,000 atmospheres.
‘We were taken off guard by both the structure and stability of this material,’ said Przemek Dera, the lead crystallographer who looked at the changes in electron density at different pressures using single-crystal diffraction. As electron density from the xenon atoms spreads towards the surrounding hydrogen molecules, it seems to stabilise the compound and the xenon pairs.
‘Xenon is too heavy and expensive to be practical for use in hydrogen-storage applications,’ said Somayazulu. ‘But by understanding how it works in this situation, researchers can come up with lighter substitutes.’
This schematic shows the structure of the new material, Xe(H2)7. Freely rotating hydrogen molecules (red dumbbells) surround xenon atoms (yellow)
Russell Hemley, director of the Geophysical Laboratory, added: ‘This hydrogen-rich solid represents a new pathway to forming novel hydrogen-storage compounds and the new pressure-induced chemistry opens the possibility of synthesising new energetic materials.’