Theoretical Study On The Spin Hamiltonian Parameters And Local Structure For D^5,7 Ions In Crystals

The performance of many funcational matierals denpend sensitively upon electronicenergy levels and local structure properties of doped transition-metal impurities in thehosts. And introduction of suitable transition-metal dopants can effectively enhance theoptical and magnetic properties of the materials. Since transition-metal ions haveunpaired d electrons, the electron paramagnetic resonance (EPR) technique can beconveniently adopted to investigate the properties of materials, and its experimentalresults are usually descrbied as the spin Hamiltonian parameters (e.g., zero-fieldsplitting, g factors, hyperfine structure constants and superhyperfine parameters). Byanalyzing these spectroscopic parameters, one can obtain important microscopicinformation about electronic spin energy levels and local structures of the dopedimpurities.The systems of d~5and d~7ions belong to very important topics among transition-metalgroups, which exhibit unqiue optical, magnetic and electrochemical properties whendoped into suitable crystals. In order to investigate the above ions doped into variouscrystals, the EPR experiments were carried out for AgX(X=Cl,Br):Fe3+, BeO:Cr+,[Fe(CN)4Cl2]5–group in NaCl:Fe+and PbTiO3:Pt3+, and the spin Hamiltonianparameters have been measured for these systems. However, the theoreticalexplanations for the above EPR experimental results are largely unsatisfactory in theprevious studies. For example, the conventional crystal-field model was adopted byneglecting the contributions from the ligand orbital and spin-orbit coupling interactions.Especially, the contribution from the charge transfer mechanism was not taken intoaccount for the systems with very strong covalency (i.e., lower charge transfer energylevels). As a result, the theoretical spin Hamiltonian parameters showed significantdescrepancies from the observed values. Meanwhile, the previous authors normallyobtained the adjusted unpaired spin densities by fitting the two experimentalsuperhyperfine parameters, failing to establish the quantitative relationships between theunpaired spin densities and covalency of the systems. In addition, the preivoustheoretical treatements did not correlate with the local structures of the impurity centers, and the defect structure information (e.g., impurity axial displacements) was notobtained for these systems. In this work, the perturbation formulas of the spinHamiltonian paramerters are established from the cluster approach for cubic andtrigonally distorted tetrahedral3d~5clusters, by including not only the contribution fromthe ligand orbital and spin-orbit coupling interactions in the conventional crystal-fieldmechanism but also that from the charge transfer mechanism. These formulas areadopted for the EPR studies of the cubic Fe3+centers in AgX (X=Cl, Br) and thetrigonal Cr+center in BeO. Morevoer, the perturbation formulas are also constructed for3d~7(strong crystal-field case) and5d~7(very strong crystal-field case) ions undertetragonally elongated octahedra. They are applied to the [Fe(CN)4Cl2]5–group inNaCl:Fe+and PbTiO3:Pt3+, respectively.1) As for AgX:Fe3+, the contribution to g-shift g from the charge transfer mechanismis opposite (positive) in sign and much larger in magnitude as compared with that fromthe crystal-field one. Moreover, importance of the charge transfer contribution increasesrapidly with increasing covalency and ligand spin-orbit coupling coefficient, i.e., Cl–<Br–. The unpaired spin densities are quantitatively obtained from the relevant molecularorbital coefficients using the cluster approach. And the shortcoming of treating theunpaired spin densities as adjustable parameters by fitting the experimentalsuperhyperfine parameters in the previous studies is thus overcome.2) As reagrds BeO:Cr+, the impurity Cr+is found not to occupy exactly the host Be2+site but to experience a small outward shift0.01away from the ligand triangle alongthe C3axis due to size mismuatch. The above impurity axial shift reduces considerablythe local trigonal distortion as compared with that of the host Be2+site in BeO. Theg-shift gCTfrom the charge transfer contribution is one order in magnitude larger than gCFfrom the crystal-field one, while the hyperfine structure constant ACTis about25%larger than ACFfrom the crystal-field one.3) For the [Fe(CN)4Cl2]5–cluster in NaCl:Fe+, the theoretical results of the spinHamiltonian parameters including the ligand orbital and spin-orbit couplingcontributions show good agreement with the experimental data in view of covalency ofthe system. The unpaired spin densities fs0.4%and f5.8%obtained from thecluster approach in this work are not far from those (0.6%and3.2%) obtained fromdirectly fitting the experimental superhyperfine parameters in the previous studies. 4) As for PbTiO3:Pt3+, the distance between the impurity Pt3+and the center of theoxygen octahedron is found to be about0.236, slightly shorter than that (0.3)between the host Ti4+site and the center of the ocathedron. This reveals the slightinward displacement (0.06) of the impurity towards the center of the octahedrondue to charge and size mismatching substitution of Ti4+by Pt3+.