Ordered AsO4 and SO4 tetrahedra in diammonium trihydrogenarsenate sulfate

(NH4)2H3AsO4SO4, Msub(r)=274.08, monoclinic, P21/c, a=11.95(1), b=7.660(5), c=9.658(6) A, β=92.40(1)0, V=883.1 A3, Z=4, Dsub(chi)=2.0621 Mg m-3, MoKα, lambda=0.7107 A, μ=4.306 mm-1, F(000)=552, T=293 K, final R=0.043 for 1324 independent reflections. (NH4)2-H3AsO4SO4 has a structure closely related to those of the mixed salts M2H3PO4SO4 (M=K, NH4) but in the title compound order is observed between the two kinds of XO4 groups. [H3AsO4SO4]2- groups linked by two ammonium ions run along the [101] direction building up chains interconnected by hydrogen bonds. Mean bond lengths and angles in the tetrahedral oxo ions are: =1.686(6) A; =109.4(2)0; =1.473(5) A; =109.5(3)0. (orig.)