Abstract

The evolution of the unit-cell parameters of stottite [FeGe(OH)6], a compound with a tetragonal octahedral framework related to the perovskite structure, has been determined to a maximum pressure of 7.8 GPa by single-crystal X-ray diffraction at room temperature. Stottite does not exhibit any phase transitions in this pressure range. A fit of a third-order Birch-Murnaghan equation of state to the pressure-volume data yields values of V0 = 425.67(2) Å3, KT0 = 78.4(3) GPa and K0 = 6.18(10). Analysis of the unit-cell parameter data shows that c is approximately 10% more compressible a. Compressional moduli for the axes are Ka0 = 81.3(3) GPa and Ka0 = 6.4(1), Kc0 = 73.3(6) GPa and Kc0 = 5.7(2). We relate these axial compressibilities to the structure of stottite, which, unlike related cubic protonated octahedral frameworks such as burtite [CaSn(OH)6], is expected to have a highly anisotropic hydrogen-bonding topology: a high degree of hydrogen-bonded connectivity parallel to (001) and very little parallel to [001]. Enhanced hydrogen bonding within the (001) plane may stiffen the structure along <100>. We also make some provisional comparisons with structural and elasticity data for perovskites and show that the absence of a central, non-framework cation in the stottite structure allows octahedral tilts in excess of 40°. The stottite structure is much softer than any known oxide perovskite. The relative importance of an empty cavity site vs. the role of hydrogen bonding is likely to be a major issue in understanding the compressional behavior of protonated octahedral frameworks.

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