Theoretical Studies on the Local Structures of D~7 Ions in NaCl-type Crystals

Author:Wu Li Na

Supervisor:zuo zuo zuo

Database:Doctor

Degree Year:2019

Download:15

Pages:141

Size:11098K

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The local structures,electronic stuctures,EPR parameters and optical properties of NaCl-type crystals doped with d7 ions are theoretically studied by density functional theory(DFT)calculations.The results are discussed and compared with those of the previous calculations by using the perturbation formulas of the EPR parameters based on the improved ligand field theory.The local distortions of the systems with d7 dopants under the Jahn-Teller effect are obtained from the energy curve(energy vs.local bond lengths),and then compared with the results of the structural optimization.The localization of unpaired electrons,the source of the paramagnetism of the systems,is analyzed from the electronic structures.By comparing the UV-Vis absorption spectra before and after d7 doping,the utilization of visible light is effectively improved with respect to the undoped systems.The contributions from the outer atoms and more valence orbitals are considered in the DFT calculations of the EPR parameters.(1)Based on the DFT calculations,geometry optimization(GEO-OPT)and single-point energy curve(ENE-CUR)are performed for the local structures of Rh2+doped AgCl and AgBr crystals,and the influences of the Jahn-Teller effect,impurity-host cation size mismatch and cation vacancy on the local structures are also analyzed.Secondly,the electronic structures of pure and impurity doped systems(i.e.,various Rh2+centers)are calculated,and charge density,ELF,molecular orbital and DOS are compared for these systems.The covalency of Rh-X bonds in all centers is found to be stronger than pure crystals based on the analysis of the overlap population.The simulated UV-Vis and electron paramagnetic resonance(EPR)spectra with the embedded cluster model are consistent with the experimental data.Rh dopants result in significant red-shifts of absorption bands in the UV-Vis region and some new d-d transition peaks in the visible and near-infrared regions.And the EPR parameters(g tensor and HFC constants)are compared with the related results of previous calculations based on the perturbation formulas of these parameters.(2)Based on the DFT calculations,the local structures of two Rh2+centers in LiD:Rh2+are studied by GEO-OPT and ENE-CUR.Then,the covalency of LiD:Rh2+is discussed from Mulliken population analysis,band gap and DOS.The UV-Vis absorption spectra are analysized with TD-DFT for both centers,which also exhibit red-shifts of the absorption bands.Finally,the calculated EPR parameters of both Rh2+centers are in agreement with the experimental results and those based on the previous perturbation calculations.(3)Based on the DFT calculations,the structures(lattice parameters and local structures)of LiH crystal doped with metals M(=Al,Fe and Ru)are studied by GEO-OPT and energy-volume(E-V)curve.The electronic structures are discussed with the anaylsis on band structures,DOS,spin states of d7 ions and Mulliken charge.A MIGB for the metal M is formed in the forbidden band region.Then,the linear optical properties,such as dielectric tensor and optical absorption spectra,are also modifed.A new absorption peak occurs in visable region for Ru-doped system.In addition,M doping can effectively improve the hydrogen storage of LiH by decreasing the enthalpy of formation.(4)Based on the DFT calculations,the electronic structures and optical properties of silver halides AgX(X=F,Cl and Br)are systematically discussed.The anomalously lower band gap of AgF can be ascribed to the fact that the strongest peak of Ag 4d band is higher than the F 2p band,quite different from that of AgCl or AgBr which is lower than the halogen np band.The effective masses and migration ability of the photogenerated carriers are calculated to evaluate the optical activity and potential photocatalytic properties of AgX.Secondly,the effects of nd7 dopants on linear optical properties are studied and compared with pure AgX crystals.Possibly due to the stronger spin polarization,the optical absoption spectra of the systems with Co2+,Rh2+and Ir2+dopants show less significant changes in linear optical properties(especially dielectric constants at extremely low frequency)than those with the other d7 dopants of the same VIII group.(5)Based on the high order perturbation formulas of g tensor and HFC constants,the local structures and EPR parameters are studied for the distorted octahedral d1 and d9clusters with similar low spin(S=1/2)and the Jahn-Teller effect to the above d7 ions.The calculated EPR parameters of tetragonally compressd V4+centers in SZB glasses and rhombically elongated Cu2+centers in various bi-copper paddle wheel coordinated polymers agree well with the experimental results.