First-principles Study On Several Ce³⁺ And Bi³⁺ Activated Luminescent Materials

First-principles calculation plays an important role in the study of photofunctional materials as an effective auxiliary method for material performance analysis and pre-diction.Ce3+ or Bi3+ions have a wide range of applications or prospects as activator of photofunctional materials.The atomic geometry structure and energy level structure of the activator in the hosts determine the properties of the isolated luminescence cen-ters.This dissertation is entitled to explore the first-principles calculation scheme of Ce3+activated photofunctional materials and Bi3+activated new photofunctional ma-terials,to realize the exploration of the phototransition mechanism and the prediction of the luminescent properties of the related materials.For Ce3+as the activation cen-ter,formers' calculation method has been improved in our work and used to calculate the stability,local structure and transition properties of multiple luminescence centers in CaAl2O4:Ce3+and Na3LuSi2O7:Ce3+.This method can be widely used to predict the luminescence properties of luminescent materials doped with rare earth ions.Ions containing(ns)2 lone-pair electrons,such as Bi3+,Pb2+,Sb3+,etc.,become important activators of new photofunctional materials,and can also act as sensitizers of lumines-cent materials activated by a large number of rare earth ions or transition metal ions.Existing methods of calculation and analysis for those activators are mainly empirical model or calculation of single-electron energy state density based on energy band struc-ture of doping center.The development of computational methods that fully take into account of the effects of electron correlation and relativistic effects,such as spin-orbit coupling,is of great importance for the study of such ion-activated photofunctional ma-terials.Using CaMO3(M=Ti,Sn,Zr)as the model system,we have explored a system-atic first-principles calculation scheme,and realized the calculation of the equilibrium geometry and energy level structure of various excited states involved in the photon transition.Based on the calculation results,the photon absorption,energy transfer,ex-cited state relaxation,photon emission and other luminescence dynamics are analyzed.This method has been systematically used to calculate the defect energy level structure and study the luminescence process of Bi3+ions in ABO3 perovskites.It not only ex-plains the experimental results,but also predicts the energy level positions of Bi3+as donor and recipient and the properties of the corresponding electronic states.It provides an important basis for exploring the related long afterglow materials and optical storage materials.The structure and main contents of this dissertation are summarized as follows:This dissertation is divided into five chapters.In the first chapter,we introduce the density-functional theory(DFT)involved in first-principles calculation of point defects and their developments.We presented the semi-empirical and first-principles research background for point defect doped systems,such as the luminescence materials with Ce3+and Bi3+ions as the activation centers.The second chapter introduced the first-principles calculations that were carried out for the electronic structures of Ce3+in calcium aluminate phosphors,CaAl2O4,and their effects on luminescence properties.Hybrid density functional approaches were used to overcome the well-known underestimation of band gaps of conventional density func-tional approaches and to calculate the energy levels of Ce3+ions more accurately.The obtained 4f-5d excitation and emission energies show good consistency with measured values.A detailed energy diagram of all three sites is obtained,which explains quali-tatively all of the luminescent phenomena.With the results of energy levels calculated by combining the hybrid functional and the constraint occupancy approach,we are able to construct a configurational coordinate diagram to analyze the processes of capture of a hole or an electron and luminescence.This approach can be applied for systematic high-throughput calculations in predicting Ce3+activated luminescent materials with a moderate computing requirement.In the third chapter,we introduced the first-principles study of Na3LuSi2O77:Ce3+,which exhibits strongly temperature-dependent luminescence and indicates potential applications in temperature sensing.The direct assignments of the measured excitation spectra to Ce3+ions at two different Lu sites are at odds with the crystal-field split-ting analysis.Thus,first-principles calculations were carried out in order to resolve site occupancies of Na3LuSi2O7:Ce3+by considering both the influence of the oxygen vacancy and the energy level structures of Ce3+ ions.Hybrid density functional ap-proaches were utilized to overcome the well-known underestimation of the host band gap so as to obtain energy levels of Ce3+ions in the band gap more accurately.Based on our calculated results,one set of the excitation spectrum is assigned to Ce3+ at Lu(2)site and the other to the composition of Ce3+at Lu(2)site with an adjacent O(3)vacancy.First-principles calculations presented in this work exhibit the significant effects of both the different occupied sites of lanthanide ions and the introduced intrinsic defects on the experimental excited spectra,which sometimes are confused.The forth chapter exhibits the first-ptinciples calculation of Bi3+ion,which is an excellent activator and sensitizer for luminescent materials and shows different prop-erties with Ce3+ ions.However,the complexity and variety of the Bi3+related tran-sitions bring a great challenge to the study of luminescence processes of Bi3+doped materials.Here,we presented first-principles calculations to determine the excitation,relaxation and emission processes of Bi3+activated materials by using CaMO3:Bi3+(M=Zr,Sn,Ti)as prototype systems,where Bi3+substitutes Ca2+in similar coor-dinate environments but presents tremendously different excitation and emission spec-tra.The-equilibrium geometric structures of excited states were calculated based on density-functional theory,with appropriately constraining the electron occupation and including the spin-orbit couplings.Then the hybrid DFT calculations were carried out to obtain the electronic structures and defect levels.Different metastable excited states and Stokes shift were obtained for M=Zr,Sn and Ti,which explain the remarkable differences in the measured emission spectra.The relative energies of three types of transitions are obtained from the calculations,including intra Bi3+ bands transition and charge transfer between Bi3+ions and the band edges.This leads to a clear and reliable interpretation of all the excitation spectra in the series.The method and its applications to CaMO3:Bi3+show the potential of first-principles calculations in analyzing and pre-dicting luminescent properties of Bi3+activated materials.In the fifth chapter,we investigated the Bi3+doped ABO3-type perovskites,in-cluding the luminescence behavior of Bi3+ ions as activators and Bi3+ related electron and hole trap levels.Herein,the electron constrained occupancy approach and hybrid density functional method were applied on the Bi3+ dopants in perovskites MA103(M=Gd,Y,La)and La(In/Ga)O3 to obtain the vacuum referred binding energy diagram of Bi3+-related defect levels relative to band edges,and to reveal the origin of the ex-citation and emission bands.The variation of the electron trap depth is shown mainly due to the shift of the conduction band,but the hole trap is strongly correlated with the BiM-O bond length,with some dependence on the chemical environment as well.The lowest excited level that produces the photoluminescence is the valence band to Bi3+charge transfer state in LaAlO3:Bi3+but is 3P0,1 in other hosts considered.Further-more,our calculations show that there is no tendency of forming Bi3+ pairs in any of the MA103:Bi3+(M=Gd,Y,La),while the Gd3+-Gd3+coupling in GdAlO3:Bi3+pro-vides a network of energy migration from an excited single Bi3+ ion to Bi3+pair,leading to the observation of 495 nm emission form Bi2+-Bi4+excited state.Our results on Bi3+ion activated ABO3-type perovskite provide the theoretical basis for manipulating the excitation and emission wavelengths and trap depth.