| Abstract
| - The influence of pressure, temperature, and composition on the crystal structure of Li-exchanged zeolite RHO with an aluminogermanate framework was investigated usingsynchrotron X-ray powder diffraction. At ambient conditions, aluminogermanate RHO adoptseither a rhombohedral (R3) or cubic (I23) symmetry depending on the Li-exchange level.Under hydrostatic conditions mediated by an alcohol and water mixture in a diamond-anvilcell, the rhombohedral 68% Li−AlGe−RHO transforms to a cubic structure near 3 GPa,whereas the cubic 57% Li−AlGe−RHO first transforms to a rhombohedral phase near 0.4GPa and then back to a cubic structure near 3 GPa. The rhombohedral distortion angle, anorder parameter of the low-symmetry structure, increases continuously with pressure whileapproaching the transition to the cubic form, where it decreases abruptly to the equivalentcubic angle. All of these materials show progressive volume contraction under increasingpressure and the calculated bulk moduli suggests that the rhombohedral phase atintermediate pressures is more compressible than its cubic form at higher pressures.Dehydrated samples of both compositions adopt cubic symmetry at ambient conditions andexhibit normal expansion upon heating. During in situ dehydration of hydrated 68% Li−AlGe−RHO, the rhombohedral angle decreases steadily toward the cubic equivalent valueuntil there is an abrupt volume contraction that starts at 300 K and an accompanyingtransition to a cubic form near 400 K. The hydrated 57% Li−AlGe−RHO maintains its cubicsymmetry during in situ dehydration but displays a similar abrupt volume contraction above350 K. The inverse pressure−temperature relationship is established as shown by theevolution of the rhombohedral angle and the corresponding cell length under pressure andtemperature. The rhombohedral-to-cubic phase transitions including the re-entrant behaviorobserved in the 57% Li−AlGe−RHO are predicted from a distance least-squares approachto framework minimization.
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