Paraelectric resonance of noncentral ions

The review reports the main results, obtained by the paraelectric resonance method, of theoretical and experimental investigations of the physical properties of crystals with noncentral ions. The nature of the noncentrality of a number of impurities in cubic crystals is explained. It is shown that the cause of the noncentrality is a Jahn–Teller pseudoeffect. The form of the multiple-well potential in which the impurity moves is obtained. The spectrum of the paraelectric resonance of the noncentral ions and the effect of axial pressures on the spectrum are calculated in the tunnel approximation. Comparison of theory and experiment is used to determine the effective dipole moment, the tunnel splittings, and the coefficients of ion-lattice coupling. The line shape and the ion-lattice relaxation in paraelectric resonance are investigated. It is shown that at low temperatures and low impurity concentrations the principal broadening mechanisms are the internal electric fields and the stresses caused by the defects. Agreement is obtained between the observed and calculated lines shapes and relaxation times. The features of the magnetic resonances in crystal? with noncentral ions in external fields, or in the presence of axial pressures, are considered. It turns out that the characteristic field, temperature, and angular dependences of the frequencies can be used as a precise method of investigating the properties of noncentral ions. The features of the spin-lattice relaxation in crystals with noncentral ions are indicated. New methods for the study of crystals with noncentral ions, namely paraelectric-acoustic resonance and paraelastic resonance, are proposed.