SINGLE-CRYSTAL NEUTRON-DIFFRACTION STUDY OF DIAMMONIUM HEXAAQUACOPPER DISULFATE 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 lambda=1.013 Angstrom, mu=0.39 cm(-1), F(000)=44.270x10(-14) m (87.8% deuterated), T=15.0(5) K, R(F)=0.018, R(F)(2)=0.018 for 1457 unique reflections 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 salt at 15 and 293 K are reported. The Cu2+ ion of the structure has a strongly Jahn-Teller distorted octahedral surrounding of water molecules. It is shown that at temperatures near absolute zero the atomic potentials become close to harmonic, a hereas at room temperature the ammonium ion, especially, exhibits considerable anharnlonic motion. Anharmonicity is also found at two of the sulfate oxygen positions. This anharmonicity is related to the switching of the Jahn-Teller minima when going from the deuterated to the hydrated structure. Earlier studies show 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 temperature 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 calculated from the diffraction measurements at 15 and 293 K.