The Effects Of Applied Electric Fields On The EPR Spectra Of ～6S-state (3d～5) Ions In Crystals With Inversion Symmetry

The studies of the physical properties of solids in the extreme conditions (e.g., strength electric field, strength magnetic field, high temperature, low pressure, high pressure and low temperature, etc.) have received much attention because they can help to understand the microscopic mechanisms of properties in solids, to discover the new functions of materials and to improve the applications of materials. In this paper, we study the effects of applied electric fields on the electron paramagnetic resonance (EPR) spectra for 3dn ions in crystals with inversion symmetry.The electric-field-induced changes of EPR spectra can often be found in the systems of 3dn ions without inversion symmetry (first-order electric-field effect). When a system possesses inversion symmetry, the induced EPR changes, in principle, cannot be observed because the second-order electric-field effect is very weak. However, for 3dn ions in some systems with inversion symmetry, the strong electric-field effects on the EPR spectra were observed. It is found that these strong electric-field effects occur in the cases of (i) the loose binding between transition metal ion and lattice and of (ii) the strong local electric field at transition metal ion site. So, some people suggested that these effects come from the off-center displacements of 3dn ions induced by the electric field. However, until now, few quantitative and theoretical studies have been made for them.We apply the microscopic mechanism theory and the empirical superposition model to study the electric-field-induced zero-field splittings for Mn2+ in cubic SrCl2, Fe3+ in cubic KTaO3 and tetragonal SrTiO3 crystals. In the microscopic theory, three important mechanisms (i.e., spin-orbit (SO) coupling mechanism, relativistic (RE) mechanism, covalency and overlap (CO) mechanism) are used and the parameters used in the calculations are obtained from the optical spectra and the structural data. The electric-field-induced displacements of 3d5 ions obtained from the above theory and model are comparable with those estimated from the force-balance equations. The results are as follows:1. For the cubic SrCl2: Mn2+, we found that zero-field splitting D can be attributed primarily to the electric-field-induced displacement of Mn2+ ion along the electric-field direction (i.e., C3 axis) and the displacements at various strengths of electric-field are estimated. They are close to those obtained from the force-balance equation.2. The displacements of Fe3+ along [001] direction in KTaO3 crystal are caused by the electric field. We obtain the displacements of Fe3+ in different electric field strengths and temperatures. It is suggested that the changes of electric-field-induced zero-field splitting can be attributed primarily to the above displacements and the temperature dependences of the electric-field-induced zero-field splitting are due mainly to the change in the dielectric constant of KTaO3 with temperature.3. From the studies of the electric-field-induced zero-field splittings for Fe3+ in tetragonal phase of SrTiO3, it is found that the induced zero-field splittings can be reasonably explained by considering suitable Fe3+ displacements along the electric field direction in the tetragonal (FeO6)9( groups. The strong temperature dependences of electric-field-induced Fe3+ displacement and hence of zero-field splitting are due to the strong temperature dependences of dielectric constant and local electric field (internal electric field) in SrTiO3.So, the above methods can calculate and explain the changes of electric-field-induced zero-field splittings for 6S-state(3d5)ions in crystals with inversion symmetry. The methods can also be applied in the investigations of other similar cases.