Off-CenterCu+Ions in Potassium Halides Studied with Ionic Thermocurrents

From previous optical and theoretical work it had been concluded that substitutional ${\mathrm{Cu}}^{+}$ ions may occupy off-center positions in certain alkali halides. The method of ionic thermocurrents (ITC) was applied to verify directly the electric dipole properties of ${\mathrm{Cu}}^{+}$ ions in three potassium halides, and to study the temperature dependence of their relaxation behavior. ITC bands were observed for ${\mathrm{Cu}}^{+}$ in KCl, KBr, and KI, with maxima at 62, 70, and 83 °K, respectively. From the integrated ITC currents of crystals with calibrated ${\mathrm{Cu}}^{+}$ concentrations, the electric dipole moment was determined, yielding values of 1.5, 1.9, and 2.6 $e\mathrm{\AA }$ for ${\mathrm{Cu}}^{+}$ in KCl, KBr, and KI, respectively. The shape of the ITC bands was found in quantitative agreement with the calculated behavior for dipole reorientation by classical rate theory with a single relaxation time. The activation energy for reorientation of the ${\mathrm{Cu}}^{+}$ dipole was found to be 177, 196, and 232 meV for KCl, KBr, and KI, respectively. Using additional material on ${\mathrm{Cu}}^{+}$ and ${\mathrm{Ag}}^{+}$ ions, some systematic trends in the appearance of the off-center effect, and in the static and dynamic off-center dipole properties of heavy-metal ions, are demonstrated and discussed.