Cation-dependent anomalous compression of gallosilicate zeolites with CGS topology: A high-pressure synchrotron powder diffraction study

The high-pressure compression behaviour of 3 different cation forms of gallosilicate zeolite with CGS topology has been investigated using in situ synchrotron X-ray powder diffraction and a diamond-anvil cell technique. Under hydrostatic conditions mediated by a nominally penetrating pressure-transmitting medium, unit-cell lengths and volume compression is modulated by different degrees of pressure-induced hydration and accompanying channel distortion. In a Na-exchanged CGS (Na₁₀Ga₁₀Si₂₂O₆₄·16H₂O), the unit-cell volume expands by ca. 0.6% upon applying hydrostatic pressure to 0.2 GPa, whereas, in an as-synthesized K-form (K₁₀Ga₁₀Si₂₂O₆₄·5H₂O), this initial volume expansion is suppressed to ca. 0.1% at 0.16 GPa. In the early stage of hydrostatic compression below ∼1 GPa, relative decrease in the ellipticity of the non-planar 10-rings is observed, which is then reverted to a gradual increase in the ellipticity at higher pressures above ∼1 GPa, implying a change in the compression mechanism. In a Sr-exchanged sample (Sr₅Ga₁₀Si₂₂O₆₄·19H₂O), on the other hand, no initial volume expansion is observed. Instead, a change in the slope of volume contraction is observed near 1.5 GPa, which leads to a 2-fold increase in the compressibility. This is interpreted as pressure-induced rearrangement of water molecules to facilitate further volume contraction at higher pressures.