X-ray and Neutron Study of Hydrogen Clathrate

Wendy Mao^1,2, Ho-kwang Mao², Alexander Goncharov², Viktor Struzhkin², Quanzhong Guo², Jingzhu Hu², Jinfu Shu², Russell J. Hemley², Maddury Somayazulu³, Konstantin Lokshin⁴, & Yusheng Zhao⁴

¹U of Chicago,

²Geophysical Lab, CIW,

³HPCAT, APS,

⁴LANSCE, LANL

wmao@uchicago.edu

NSLS-X17B (DAC)

Clathrates have attracted attention because of their fundamental physical and chemical significance and their applications to energy, environmental, and astrophysical issues. High-pressure Raman, infrared, x-ray, and neutron studies show that H₂ and H₂O mixtures crystallize into the sII clathrate structure with an extremely high hydrogen content (H₂/H₂O ~ 1:2). The clathrate cages are multiply occupied, i.e., a cluster of two H₂ molecules in the small cage and four in the large cage. Substantial softening of hydrogen vibrons indicates increased intermolecular interactions. The quenched clathrate is stable up to 145 K at ambient pressure. Retention of hydrogen at such high temperatures could greatly help its condensation in planetary nebulae and play a key role in the evolution of icy bodies.