Journal of Applied Crystallography

Volume 46, Part 2 (April 2013)

research papers

J. Appl. Cryst. (2013). 46, 448-460    [ doi:10.1107/S0021889813001362 ]

P-V-T equation of state of synthetic mirabilite (Na2SO4·10D2O) determined by powder neutron diffraction

A. D. Fortes, H. E. A. Brand, L. Vocadlo, A. Lindsay-Scott, F. Fernandez-Alonso and I. G. Wood

Abstract: Neutron powder diffraction data have been collected from Na2SO4·10D2O (the deuterated analogue of mirabilite), a highly hydrated sulfate salt that is thought to be a candidate rock-forming mineral in some icy satellites of the outer solar system. These measurements, made using the OSIRIS instrument on the ISIS neutron spallation source, covered the range 0.1 < P < 545 MPa and 150 < T < 270 K. The refined unit-cell volumes as a function of pressure and temperature are parameterized in the form of a Birch-Murnaghan third-order equation of state, and the anisotropic linear incompressibilities are represented in terms of the elastic strain tensor. At 270 K, the bulk modulus K0,270 = 19.6 (1) GPa, its first pressure derivative [partial differential]K/[partial differential]P = 5.8 (5) and its temperature dependence [partial differential]K/[partial differential]T = -0.0175 (6) GPa K-1. The stiffest direction at 270 K, with a linear bulk modulus of ~82 GPa, is coincident with the twofold axis of this monoclinic crystal. Of the remaining two principal directions, the most compressible (K [asymptotically equal to] 44 GPa) is roughly aligned with the c axis, and the intermediate value (K [asymptotically equal to] 59 GPa) is therefore approximately collinear with a*. With the aid of additional published data, a number of other important thermodynamic quantities have been derived, including the Grüneisen and Anderson-Grüneisen parameters, and the volume and enthalpy of melting along the high-pressure melting curve. Additional data obtained during this work, concerning the elastic properties of deuterated ice IV, are also presented.

Keywords: mirabilite; equations of state; neutron diffraction; ice IV.

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