Crystal structure phase transitions and a vibrational study of disordered RbH(SO4)0.81 (SeO4)0.19 solid solution

RbH(SO4)(0.81)(SeO4)(0.19) Mm = 189.58 single crystals were grown which appear to be different from RbHSO4 and RbHSeO4. The space group is monoclinic Cm, the lattice constants are a = 14.916(3) Angstrom, b = 24.860(3) Angstrom, c = 4.6419(8) Angstrom b = 90.01(2)degrees, V = 1721.3(5) Angstrom(3), Z = 16, Dx = 2.92 g cm(-3), lambda(MoKalpha) = 0.710 69 Angstrom, mu = 6.53 cm(-1), T = 293 K (room-temperature phase), R = 0.076 and wR2 = 0.174 for 3450 observed reflections. The structure consists of five crystallographically independent molecules in the asymmetric unit. Each of the independent sulphate or selenate tetrahedron is linked to a translationally equivalent sulphate or selenate by acentrically ordered hydrogen bonds. The separated chains of asymmetric hydrogen bonds run parallel to the c-axis. The distance O-H...O varies between 2.48 Angstrom and 2.60 Angstrom.