STRUCTURE OF (NH4)3H(SEO4)2 IN HIGH-TEMPERATURE PHASE-I AND PHASE-II

Triammonium hydrogenbis(tetraoxo-selenate), (NH4)3H(SeO4)2, M(r) = 341.04, Z = 3, lambda(Mo KalphaBAR) = 0.71073 angstrom, F(000) = 498. Phase I: trigonal, R3mBAR, a = 6.090 (1), c = 22.759 (5) angstrom, V = 731.0 (2) angstrom3, D(x) = 2.32 g cm-3, mu = 68.7 cm-1, T = 355 K, R = 0.0336 for 241 unique reflections with I > 4sigma(I). Phase II: trigonal, R3BAR, a = 6.064 (1), c = 22.904 (5) angstrom, V = 729.4 (2) angstrom3, D(x) = 2.33 g cm-3, mu = 68.8 cm-1, T = 310 K, R = 0.03 74 for 397 unique reflections with I > 4sigma(I). In both phases, SeO4 tetrahedra are linked by a dynamic system of symmetrically disordered hydrogen bonds in planes perpendicular to the trigonal axis. In phase II, SeO4 tetrahedra deviate from the (210) plane owing to a small rotation of about 4-degrees about the trigonal axis. In phase I, owing to symmetry enhancement, both orientations of SeO4 tetrahedra are equally probable on both sides of (210), which is therefore a mirror plane.