PRESSURE-DEPENDENCE OF THE CRYSTAL-STRUCTURES AND EPR-SPECTRA OF POTASSIUM HEXAAQUACOPPER(II) SULFATE AND DEUTERATED AMMONIUM HEXAAQUACOPPER(II) SULFATE

The crystal structures of (ND4)(2)[Cu(D2O)(6)](SO4)(2) at 295 K, deter mined by X-ray diffraction at pressures of 1 bar, similar to 1.5 kbar and similar to 3.0 kbar, are reported. Between 1 bar and 1.5 kbar, the crystal structure changes to one almost identical to that of the corr esponding hydrogenous compound at 1 bar. The structural change involve s a 90 degrees switch in the direction of the long bonds of the Cu(D2O )(6)(2+) ion, accompanied by a change in the hydrogen-bonding interact ions of the ammonium cations. Comparison of these two structures with those at similar to 15 K shows that at room temperature, for both the high (H) and low (L) pressure phases, the Cu complexes are in thermal equilibrium with the other structural isomer. Though one of these is e nergetically preferred in the H and the other in the L phase, the vari ation of the powder EPR spectrum with pressure indicates a continuous transition between both within similar to 1 kbar and suggests that at similar to 0.5 kbar the two orientations of the copper complex are sim ilar in energy. However, the underlying thermally induced equilibrium is little affected by pressure, and this is also the case for the Cu(H 2O)(6)(2+) ion in Cu2+-doped K-2[Zn(H2O)(6)](SO4)(2). The crystal stru cture of K-2[Cu(H2O)(6)](SO4)(2) at 15 K and 1.4 kbar, determined by t ime-of-flight neutron diffraction, is similar to that at 295 K and 1 b ar, except for indications that here also a thermal population of the higher energy form in which the directions of the long and intermediat e Cu-O bonds interchange occurs at 295 K. The powder EPR spectrum of t he potassium salt at 295 K shows little change between 1 bar and 11 kb ar.