Fortes, AD and Wood, IG and Vocadlo, L and Chapon, L and Knight, KS and Smith, RI (2008) Neutron powder diffraction studies of sulfuric acid hydrates. II. The structure, thermal expansion, incompressibility, and polymorphism of sulfuric acid tetrahydrate (D2SO4 center dot 4D(2)O). Journal of Chemical Physics , 128 (5) , Article 054506. 10.1063/1.2827474.
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We report results of the first neutron powder diffraction study of sulfuric acid tetrahydrate (SAT); D2SO4 center dot 4D(2)O is tetragonal, space group P (4) over bar2(1)c, with two formula units per unit cell. At 1.7 K the unit-cell dimensions are a = b = 7.475 12(6) angstrom, c = 6.324 66(5) angstrom and V = 353.405(5) angstrom(3). At 225 K the unit-cell dimensions are a = b = 7.4833(1) angstrom, c = 6.4103(1) angstrom, and V = 358.98(1) angstrom(3). The deuteron positions refined from the neutron data are in excellent agreement with the single crystal x-ray analysis of Kjallman and Olovsson [Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. B28, 1692 (1972)]; the structure consists of SO42- and D5O2+ ions hydrogen bonded to form a three dimensional network. Although no structural change is observed between 2 K and the melting point at similar to 232 K, the thermal expansion and incompressibility of the crystal are highly anisotropic. The bulk modulus of SAT at 200 K is 9.2(2) GPa, (partial derivative K/partial derivative P)(T) = 7.9(8), and -(partial derivative K/partial derivative T)(P) = 10.6(5) MPa K-1, values which are very similar to D2O ice Ih. A new polymorph of SAT has been discovered above 235 K at 5.5 kbars. The structure of this phase could not be determined, but we have indexed the diffraction pattern with a monoclinic unit cell of likely space-group P2(1)/a (Z = 2). SAT-II has a lower density than SAT-I under the same P/T conditions; the refined unit-cell parameters at 235 K, 5.435 kbars are a = 6.1902(3) angstrom, b = 11.1234(5) angstrom, c = 5.6446(3) angstrom, beta = 110.287(4)degrees, and V = 364.56(2) angstrom(3). This phase has been quenched to low pressures and temperatures, and we have obtained estimates of the thermal expansivity and incompressibility which reveal SAT-II to be significantly stiffer and more isotropic than SAT-I. (c) 2008 American Institute of Physics.
|Title:||Neutron powder diffraction studies of sulfuric acid hydrates. II. The structure, thermal expansion, incompressibility, and polymorphism of sulfuric acid tetrahydrate (D2SO4 center dot 4D(2)O)|
|Keywords:||CRYSTAL-STRUCTURES, PROTON H&(H2O)N, SALT MINERALS, HYDROGEN-BOND, D2O ICE, 4 K, DIHYDRATE, SURFACE, EUROPA, OCEAN|
|UCL classification:||UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Earth Sciences|
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