Electrocaloric properties of OH- molecules in KCl☆☆☆

Abstract The electrocaloric effect in the KCl: OH − system is used to cool the system down to 0·36°K starting from 1·3°K by a method of adiabatic depolarization. The limit to the temperature attainable by this means is set by the zero field splitting of the OH − energy levels in the lattice which is 0·3°K. The specific heat of KCl: OH − , measured as a function of applied E field at constant temperature, has a maximum at a value of E which depends on the direction of the applied field with respect to the crystal axes. The data is fitted by a non-interacting dipole theory that assumes the dipoles are oriented along [100] directions in the lattice. The position of the maximum gives a value of the uncorrected dipole moment of the OH − , p u , and the value of the maximum gives a measure of the concentration of OH − , N. The value of the dipole moment varies with concentration and for the low concentration of 10 17 /cm 3 is 4·4 ± 0·l deb. The reorientation time of the dipoles between [100] directions, t 1 , is estimated to be less than 5·10 −8 sec. The specific heat of KCl: OH − , measured as a function of temperature, shows a deviation from the Debye theory at low temperature. Below 1°K this is consistent with a contribution from the zero field splitting. In the region 1°K T − measured optically yields a higher value than the concentration measured from the specific heat maximum except at the lowest concentration where the two are equal. A qualitative explanation assumes regions of local order where the dipoles are frozen in and do not contribute to the caloric properties.