[en] The ambient structure and pressure-induced structural changes of a synthetic sodium aluminogermanate with a natrolite (NAT) framework topology (Na-AlGe-NAT) were characterized by using Rietveld refinements of high-resolution synchrotron X-ray powder diffraction data at ambient and high pressures. Unlike a previously established model for Na₈Al₈Ge₁₂O₄₀ · 8H₂O based on a single-crystal study, the ambient structure of the Na-AlGe-NAT is found to adopt a monoclinic space group Cc (or Fd) with a ca. 6% expanded unit cell. The refined ambient structure of Na₈Al₈Ge₁₂O₄₀ · 12H₂O indicates an increased water content of 50%, compared to the single-crystal structure. The unit-cell volume and water-content relationships observed between the two Na-AlGe-NAT structures at ambient conditions with 8 and 12 H₂O respectively seem to mirror the ones found under hydrostatic pressure between the Na₈Al₈Ge₁₂O₄₀ · 8H₂O and the parantrolite phase Na₈Al₈Ge₁₂O₄₀ · 12H₂O. Under hydrostatic pressures mediated by a pore-penetrating alcohol and water mixture, the monoclinic Na-AlGe-NAT exhibits a gradual decrease of the unit-cell volume up to ca. 2.0 GPa, where the unit-cell volume then contracts abruptly by ca. 4.6%. This is in marked contrast to what is observed in the Na-AlSi-NAT and Na-GaSi-NAT systems, where one observes a pressure-induced hydration and volume expansion due to the auxetic nature of the frameworks. Above 2 GPa, the monoclinic phase of Na-AlGe-NAT transforms into a tetragonal structure with the unit-cell composition of Na₈Al₈Ge₁₂O₄₀ · 16H₂O, revealing pressure-induced hydration and a unit cell volume contraction. Unlike in the Na-Al,Si-paranatrolite phase, however, the sodium cations in the Na-AlGe-NAT maintain a 6-fold coordination in the monoclinic structure and only become 7-fold coordinated at higher pressures in the tetragonal structure. When comparing the pressure-induced hydration in the observed natrolite-type zeolites, Na-AlGe-NAT appears to have a nonauxetic framework and reveals the highest onset pressure for complete superhydration.

[en] The high-pressure, room temperature behavior of otavite (CdCO₃) was investigated by angle-dispersive synchrotron radiation powder diffraction up to 40 GPa, Raman spectroscopy up to 23 GPa and quantum mechanical calculations based on density functional theory. The calcite-type structure of CdCO₃ is stable up to at least ∼19 GPa as shown by Raman spectroscopy. The compression mechanism was obtained from structure refinements against the diffraction data. The quantum mechanical calculations propose a calcite-aragonite phase transition to occur at about 30 GPa. The existence of a pressure-induced phase transition is supported by the Raman and diffraction experiments. Evidence for the transformation is given by broadening of X-ray reflections and external Raman bands starting from about 19 GPa in both experiments.