UCL Discovery

Abstract

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.