Monoclinic–Orthorhombic Phase Transition with a Positive Volume Change in the Siliceous Zeolite Mobil-Five Induced by the High-Pressure Insertion of Dense Fluid Helium

Synchrotron X-ray diffraction was used to investigate the insertion of helium in the archetypal siliceous zeolite Mobil-Five (MFI). Due to the very high degree of helium penetration into the pores and the framework, the monoclinic form of MFI is stabilized over a large pressure interval, and the compressibility is much lower than when the pores are empty or filled with any other guest molecules or atoms except hydrogen. The structure of MFI transformed from monoclinic P2$_1$/n to orthorhombic Pnma with a discontinuous volume increase of 3% at pressures close to 10 GPa due to the filling process. The material is highly strained at pressures above 42 GPa but remains crystalline. The strain is released on decompression and a highly crystalline material is recovered