Theoretical Study On The Spin Hamiltonian Parameters And Local Structure For Transition Metal Ions In Glass Systems

Properties and applications of glasses depend largely upon the compositions and structures of the systems.Transition-metal dopants play a core role in modifying optical properties of glasses due to their abundant energy levels.The 3d orbtals of transition-metal ions which are sensitive to the local structures ensure these dopants to act as the probes to reveal delicate environmental variations nearby by means of optical and electron paramagnetic resonance?EPR?techniques.Extensive studies have indicated that optical and EPR analysis can help to bridge performances and local structures of glasses which can also be conveniently modified by varying the concentrations of impurities and modifiers.The optical absorption and EPR experiments were carried out for CdO–SrO–B₂O₃–SiO₂?CSBS?doped with Cr₂O₃,Li₂O–PbO–B₂O₃–P₂O₅?LPBP?doped with V₂O₅ and PbO–Y₂O₃–P₂O₅?PYP?glasses doped with MoO3,and the d-d transition bands and g factors were measured for various concentrations of dopants.However,the above experimental results have not been theoretically explained so far.In this work,the theoretical analysis is performed for the d-d transition optical spectra,g factors and local structures as well as their concentration dependences of the impurities Cr³⁺?V⁴⁺ and Mo⁵⁺ in these glass systems.The new concept,local optical basicity ?loc,is proposed to describe the coordination and cloud admixtures in the vicinity of the impurities.1)The concentration dependences of g factor and d-d transition band are theoretically studied for Cr³⁺ in CSBS glasses.To determine the local structure around dopant Cr³⁺,the non-bridge oxygen?NBO?occupation ratio o is calculated for different dopant concentrations x.In order to evaluate explicitly the influences of other cations?e.g.,Sr2+?and to refine the conventional bulk optical basicity of whole glass systems,a new quantity local optical basicity ?loc around dopant Cr³⁺ is introduced to describe the influences of local environments on the studied [CrO6] clusters,by involving the NBO occupation ratio o related to NBO concentration f?NBO?.The concentration dependences of cubic field parameter Dq and isotropic g factor are suitably reproduced by three piecewise linear functions of Dq and orbital reduction factor k with x.The microscopic mechanisms of the relationships of crystal-field strength and covalency of [CrO6] clusters with x are analyzed in view of the variations of ?loc and high ionicity of Sr2+ nearby at different stages.2)The concentration dependences of g factors,d-d transition band and local distortion are theoretically studied for V⁴⁺ in LPBP glasses from the quasi-continuously shifting of the coordination number in global level and the variation of the covalency of V⁴⁺–O₂? bond,crystal-field strength and compression distortion of the octahedral [VO6] clusters in local level.The tetrahedral [VO4] clusters in the relatively stable glass network are suitably treated as invariable with concentration.The varying distributions between octahedral [VO6] and tetrahedral [VO4] clusters are characterized by the diffusivity ? of the exponential relationship with x.The increases of covalency factor N?and also the orbital reduction factors k and k'?and the decreases of the orbital admixture coefficients ?e and ?t are suitably supported by the present calculated decreasing basicity ?O for the octahedral groups.This is ascribed to the decreases of the effective valence electrons of central ion V⁴⁺ with increasing V₂O₅ dopants arising from the overlap with the empty 3d states of the neighboring V⁵⁺ ions.The exponentially decreasing DqO is due to the decreasing effective valence electrons of V⁴⁺ and the difference of repulsive energy between ? and ? bonds,characterized by ?loc and ?t – ?e,respectively.The decreases of the relative tetragonal compression ratio ? can be attributed to the weakening V⁴⁺–O₂? bonding due to the weakening crystal-field strength and hence the decreasing Jahn-Teller compression distortions.3)The similar analysis is performed for Mo⁵⁺ in PYP glasses.The tetrahedral [MoO4] clusters in glass network are treated as invariable with concentration.Local optical basicity ?loc are calculated to definitely characterize the ability of adjacent ligands O₂– around the central ions Mo⁵⁺ to donate electrons.The increases of N are suitably supported by the decreasing ?loc,attributable to the decreasing effective valence electrons of central Mo⁵⁺ with increasing MoO3 dopants due to the overlap with the empty 4d states of the neighboring Mo6+ ions.