X-BAND ELECTRON-PARAMAGNETIC-RESONANCE OF A SINGLE-CRYSTAL CU(NH4)2(SO4)2.6H2O - EXCHANGE INTERACTION BETWEEN DISSIMILAR COPPER IONS

We have studied the EPR of a single crystal of the Tutton salt Cu(NH4) 2(SO4)2.6H2O in rotations around the a and c axes, and around the mag netic beta (= b) and gamma axes at X-band and room temperature. When t he magnetic field lies in the (a, c) plane, the two Cu2+ ions are magn etically equivalent and the EPR spectrum reduces to a quasi-Lorentzian single line. Due to the two dissimilar copper ions, the EPR spectrum is a more or less resolved doublet, according to the orientation of th e magnetic field. When the magnetic field is parallel to one tetragona l axis, the EPR spectrum is a perfectly resolved doublet, characterist ic of a very weak exchange interaction J' between the dissimilar ions. The simulation of the EPR spectrum for the compound under study revea ls that the exchange coupling between dissimilar ions is practically e qual to zero at room temperature at the accuracy of 0.0002 cm-1.