Neutron powder diffraction studies of sulfuric acid hydrates. II. The structure, thermal expansion, incompressibility, and polymorphism of sulfuric acid tetrahydrate (D2SO4⋅4D2O)

Abstract

We report results of the first neutron powder diffraction study of sulfuric acid tetrahydrate (SAT); D(2)SO(4)4D(2)O is tetragonal, space group P42(1)c, with two formula units per unit cell. At 1.7 K the unit-cell dimensions are a=b=7.475 12(6) A, c=6.324 66(5) A and V=353.405(5) A(3). At 225 K the unit-cell dimensions are a=b=7.4833(1) A, c=6.4103(1) A, and V=358.98(1) A(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 SO(4) (2-) and D(5)O(2) (+) ions hydrogen bonded to form a three dimensional network. Although no structural change is observed between 2 K and the melting point at approximately 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 differentialK partial differentialP)(T)=7.9(8), and -( partial differentialK partial differentialT)(P)=10.6(5) MPa K(-1), values which are very similar to D(2)O 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 PT conditions; the refined unit-cell parameters at 235 K, 5.435 kbars are a=6.1902(3) A, b=11.1234(5) A, c=5.6446(3) A, beta=110.287(4) degrees , and V=364.56(2) A(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.