Crystal Structure and Characterization of CsH_5(AsO_4)_2: A New Cesium Pentahydrogen Arsenate, and Comparison with CsH_5(PO_4)_2 and RbH_5(AsO_4)_2

摘要

Chemical preparation and X-ray single-crystal, vibrational, and calorimetric studies of CsH_5(AsO_4)_2 are described. The new compound crystallizes in themonoclinic ststem P2_1/c with cell parameters a = 10.983(1) A, b = 7.943(1) A, c = 9.844(1) A, #beta# = 96.15(1)~deg, V = 853.82(6) A~3, Z = 4, and p_cal = 3.235 g cm`(-3). The refinement of data leads to R_1 = 0/0396 and WR_2 = 0.0809 for 2377 observed reflections (I > 2#sigma# (I)) refined with 122 parameters. The structure is different from RbH_5(AsO_4)_2 (orthor-hombic) and similar to CsH_5(PO_4)_2 (monoclinic). The atomic araangement can be described as layered organization of isolated arsenate tetrahedra; interleaved cesium cations ensure the cohesion of the structure. As in all atomic arrangements, we can observe the formation of an infinite network of anions connected by strong H bonds (2.436(4)-2.597(5) A). The hydrogen atoms were located from difference maps and the short contact (2.436(4) A), which is not astride any symmetry element, is of the type O-H …O asymmetrical. The Raman and infrared spectra of CsH_5(AsO_4)_2 recorded at room temperature in the frequency ranges 10-1000 cm~(-1) and 250-4000 cm~(-1), respectively,confirm the presence of two independent AsO_4~(3-) groups in the crystal. An assignment of all the bands is given. Differential scanning calorimetry shows that the title compound does not exhibit any phase transition in the range 123-473 K.