VANISHING EXCHANGE COUPLING BETWEEN DISSIMILAR COPPER IONS IN CUK2(SO4)(2)CENTER-DOT-6H2O AS EVIDENCED BY X-BAND ELECTRON-PARAMAGNETIC-RESONANCE

We have studied the electron paramagnetic resonance (EPR) of a single crystal of the CuK2(SO4)(2).6H(2)O Tutton salt when rotated around a and c* axes, and around beta(=b) and gamma axes at X-band and room temperature. When the static magnetic field lies in the (a, c) plane, both Cu2+ ions are magnetically equivalent and EPR spectrum reduces to a singlet whose shape varies considerably, from a quasi-lorentzian lineshape following the c axis, to an intermediate (between Gaussian and Lorentzian) lineshape following the a axis. For other orientations of the static magnetic field, the two Cu2+ ions are magnetically unequivalent and the EPR spectrum is a moreorless resolved doublet. When the static magnetic field is parallel to a tetragonal axis, the EPR spectrum is a perfectly resolved doublet, revealing a very weak exchange interaction (J') between dissimilar ions. For the compound under study, the computer simulation of the EPR spectrum was carried out by adopting a convolution for the shape of the elementary line. This reveals that the exchange interaction between dissimilar ions is vanishingly small at room temperature at an accuracy of 0.0002 cm(-1).