High pressure behaviour of bikitaite: an integrated theoretical and experimental approach

The pressure-induced structural modifications in the zeolite bikitaite are studied by means of in-situ synchrotron X-ray powder diffraction and ab initio molecular dynamics. The experimental cell parameters were refined up to 9 GPa, pressure at which we found a reduction of 4.5, 4.5, 6.3 and 15% in a, b, c, and V, respectively. Minor variations were observed for the cell angles. The complete X-ray amorphization is not achieved in the investigated P range, moreover the P-induced effects on bikitaite structure are completely reversible. Since it was possible to extract only the cell parameters from the powder patterns, atomic coordinates at 5.7 and 9.0 GPa were obtained by means of Car-Parrinello simulations using the unit cells parameters experimentally determined at these pressures. The analysis of the computational results at the increasing pressures evidenced that: the volume contraction is essentially due to the rotations of the tetrahedra; the 8-ring channels become more circular; the pyroxene chain becomes more corrugated in the bc plane; the mean Li-O bond distances and the coordination polyhedral volumes decrease with increasing pressure without significant distortion of the internal angles. The peculiar aspect of bikitaite structure, i.e. the presence in the channels of a "floating" one-dimensional water chain, is only partially maintained at high pressure: the compression brings framework oxygens close enough to water hydrogens to allow the formation of host-guest hydrogen bonds, without, however, destroying the one-dimensional chain.