Neutron powder diffraction studies of sulfuric acid hydrates. I. The structure of sulfuric acid hemitriskaidekahydrate D2SO4.6(1/2)D2O

We report the first neutron diffraction data from D2SO4.6(1/2)D2O. The crystal is monoclinic, space group Cm, with four formula units per unit cell. At 4.2 K the unit cell dimensions are a = 6.253 26(4) A, b = 26.813 62(10) A, c = 5.908 45(2) A, and beta = 112.1939(3) degrees [V = 917.286(6) A3 and rho(deuterated) = 1664.14(2) kg m(-3)]. The deuteron positions refined from the neutron data are in agreement with those established by single crystal x-ray analysis [D. Mootz and A. Merschenz-Quack, Z. Naturforsch. B 42, 1231 (1987)], but not with those found from the ab initio simulation of Hirsch and Ojamae [Acta Crystallogr, Sect. B: Struct. Sci. 60, 179 (2004)]. The crystal consists of SO4(2-), D3O+ ions, and D2O molecules hydrogen bonded to form a layered structure in which sheets of "icelike" D3O+ and D2O are separated by layers of opposing SO4(2-) tetrahedra.