SINGLE-CRYSTAL NEUTRON-DIFFRACTION STUDY OF DIAMMONIUM HEXAAQUACOPPERDISULFATE AT 15-K AND 293-K - INVESTIGATION OF ANHARMONIC THERMAL MOTION

Crystal data: (ND4)(2)Cu(SO4)(2).6D(2)O, M(r)=419.96, monoclinic, P2(1 )/a. 15 K: a=9.386(3), b=12.637(7), c=6.055(2) Angstrom, beta=107.17(3 )degrees, V=686.8(5) Angstrom(3), Z=2, D-n=2.031(2) g cm(-3) neutron l ambda=1.013 Angstrom, mu=0.39 cm(-1), F(000)=44.270x10(-14) m (87.8% d euterated), T=15.0(5) K, R(F)=0.018, R(F)(2)=0.018 for 1457 unique ref lections at 0 sigma level. 293 K: a=9.302(4), b=12.519(9), c=6.188(3) Angstrom, beta=106.46(4)degrees, V=691.07 Angstrom(3), D-n =2.018 g cm (-3), R(F)=0.029, R(F)(2)=0.043 for 1585 unique reflections at 3 sigma level. Single-crystal neutron diffraction studies of copper Tutton sa lt at 15 and 293 K are reported. The Cu2+ ion of the structure has a s trongly Jahn-Teller distorted octahedral surrounding of water molecule s. It is shown that at temperatures near absolute zero the atomic pote ntials become close to harmonic, a hereas at room temperature the ammo nium ion, especially, exhibits considerable anharnlonic motion. Anharm onicity is also found at two of the sulfate oxygen positions. This anh armonicity is related to the switching of the Jahn-Teller minima when going from the deuterated to the hydrated structure. Earlier studies s how that within some approximation the CuO6 chromophore can be thought of as a fluxional system increasingly populating higher-energy minima with increasing temperature, leading to an apparent strong temperatur e dependence for the Cu-O bond lengths. Assuming the fluxional model, a value of Delta E=224+/-16 cm(-1) between the two lowest minima is ca lculated from the diffraction measurements at 15 and 293 K.