Orientationally disordered H2 in the high-pressure van der Waals compound SiH4(H2)2

Recent high-pressure synthesis of H-2-rich van der Waals compounds have attracted great interests in probing novel H-2 physics, which in general remain elusive. We have solved the crystal structure of the synthesized SiH4(H-2)(2) by first-principles calculations and revealed that SiH4 molecules in the formation of SiH4(H-2)(2) remain nearly unaltered with Si atoms forming a peculiar tetragonal lattice, which can also be viewed as a distorted face-centered-cubic lattice. We have provided direct evidences on that H-2 molecules in SiH4(H-2)(2) occupy the interstitial sites, and more intriguingly are orientationally disordered. Our argument has been supported by the excellent mutual agreement between theoretical and experimental equation of states, Raman, and x-ray diffraction data. The current study has strong implications on other high-pressure van der Waals compounds, e. g., H2O-H-2, CH4-H-2, NH3BH3-H-2, Ar-H-2, and Xe-H-2.