Transformation pathways and isothermal compressibility of a MTN-type clathrasil using penetrating and non-penetrating fluids

Abstract

The high-pressure behavior of the natural MTN-type clathrasil chibaite was investigated with in situ single-crystal X-ray diffraction and Raman spectroscopy under hydrostatic pressures up to 10.3 GPa. The experiments were conducted in diamond-anvil cells using 4:1 methanol-ethanol mixture (ME), helium (He) and neon (Ne) as pressure-transmitting media. The pressure dependent unit-cell volumes of the room-pressure polymorph yield an isothermal bulk modulus KT0 = 25.75 (19) GPa for the compression in the non-penetrating ME fluid. Due to the penetration of the Ne and He atoms into the cages of the framework, the crystal structure is significantly stiffened resulting in KT0 = 42.5 (1.2) GPa (Ne) and KT0 = 58 (2) GPa (He). Under the influence of pressure both the evolution of the Raman spectra and the change in cell metrics indicate a distortion of the lattice without leading to a complete pressure-induced amorphization, as observed for many comparable porous structures. Compressed in the non-penetrating ME, the cubic Fd3¯m framework of chibaite undergoes a first transformation step in the pressure range between 1.7 GPa and 2.2 GPa and a second one between 3.9 GPa and 4.3 GPa. The accompanied formation of crystal domains did not allow a reliable determination of the distorted crystal structures. The unit-cell parameters might suggest a monoclinic metric for pressures >1.7 GPa and a monoclinic or tetragonal metric for pressures >3.9 GPa. However, in some samples even the co-existence of crystal domains of a different degree of lattice distortions has been proved.