Crystal structure and characterization of CsH5(AsO4)(2): A new cesium pentahydrogen arsenate, and comparison with CsH5(PO4)(2) and RbH5(AsO4)(2)

Chemical preparation and X-ray single-crystal, vibrational, and calorimetri c studies of CsH5(AsO4)(2) are described. The new compound crystallizes in the monoclinic system P2(1)/c with cell parameters a = 10.983(1) Angstrom, b = 7.943(1) Angstrom, c = 9.844(l) Angstrom beta = 96.15(1)degrees, V= 853 .82(6) Angstrom (3), Z = 4, and P (cal) = 3.235 g cm(-3). The refinement of data leads to R-1 = 0.0396 and WR2 = 0.0809 for 2377 observed reflections (I > 2 sigma (I)) refined with 122 parameters. The structure is different f rom RbH5(AsO4)(2) (orthorhombic) and similar to CsH5(PO4)(2) (monoclinic). The atomic arrangement 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 ) Angstrom. The hydrogen atoms were located from difference maps and the sh ort contact (2.436(4) Angstrom), which is not astride any symmetry element, is of the type O-H . . .O asymmetrical. The Raman and infrared spectra of CsH5(AsO4)(2) recorded at room temperature in the frequency ranges 10-1000 cm(-1) and 250-4000 cm(-1), respectively, confirm the presence of two indep endent AsO43- groups in the crystal. An assignment of all the bands is give n. Differential scanning calorimetry shows that the title compound does not exhibit any phase transition in the range 123-473 K. (C) 2001 Academic Pre ss.