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Compressibility of stottite, FeGe(OH)₆: An octahedral framework with protonated O atoms

¹ Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, U.S.A.
² Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, U.K.
³ Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, U.K.

Correspondence: ^* E-mail: nross{at}vt.edu

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) ų, K_T0 = 78.4(3) GPa and K'₀ = 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 K_a0 = 81.3(3) GPa and K'_a0 = 6.4(1), K_c0 = 73.3(6) GPa and K'_c0 = 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.

This article has been cited by other articles:

				M. D. Welch, W. A. Crichton, and N. L. Ross Compression of the perovskite-related mineral bernalite Fe(OH)3 to 9 GPa and a reappraisal of its structure Mineralogical Magazine, June 1, 2005; 69(3): 309 - 315. [Abstract] [Full Text] [PDF]