First-principles Design Of Lanthanide Halides:Hosts And Impurities As Two Components Of Materials

In the present thesis,the structural,elastic,electronic and vibrational properties of lanthanide halides were theoretically designed and optimized by employing the first-principles method and all the calculations were implemented by considering the division of the studied optical materials into two components:host and dopant.Following the design scheme proposed above,our whole research work can be arranged into the following two aspects:1)Optimization of host compositions:forming the fundamental understanding on the physics properties referred above by evaluating the dependence of material properties on its chemical compositions in order to construct the tuning scheme of material properties for experimentalists based on the obtained correlation between the material properties and its chemical compositions;2)Introduction of dopants:considering the interaction between dopants and host lattices for the structural and spectroscopic calculations and then finding the reasonable design strategy for lanthanide-and/or transition-metal-doped optical materials.The research contents of the thesis can be mainly summarized into five chapters as follows:In Chapter 1,the real applications of lanthanide halides were firstly introduced,and then the difficulty for the trial-and-error experimental approach,such as the costly reagent consumption,was discussed based on our literature retrieval,which straightforwardly triggers the motivation of our present research work.In Chapter 2,we introduced the fundamental knowledge and related physics picture of two kinds of the calculation methodologies,i.e.,CF and hybrid DFT calculation models,and finally the usage details of the first-principles calculation software CRYSTAL09,such as the management of the input decks,were described and given.In Chapter 3,from the point of view of tuning host′s properties,we calculated the structural,elastic,electronic and vibrational properties of four kinds of lanthanide-based materials,i.e.,Cs₂NaLnX₆,LnF₃,LnOCl（Ln=La-Lu;X=F,Cl,Br,I）and LnX₂（Ln=La-Yb;X=F,Cl,Br,I）,by using the hybrid DFT model,and drew two interesting conclusions as follows:1)the calculated lattice constants linearly decrease across the lanthanide series,which can be explained by the lanthanide contraction effect;2)the calculated elastic constants and phonon energies increase together with the lanthanide atomic number increasing,which suggests the lanthanide-based materials can become more and more hard when the lanthanide composition varies from La to Lu.In Chapter 4,we focused on controlling the introduction of dopants and thus demonstrated the idea about our optimization strategy by two following examples:1)the most stable structural form of the solid solution Ln_1-xLn_x^′F₃（Ln=La-Pm;Ln^′=Sm-Lu）for any x mixture concentration was determined by the hybrid DFT calculations from the two kinds of the crystallographic phases Pnma and3?(8（81 of LnF₃ compounds;2）The multiplet energy levels of Mn⁴⁺ions doped in Rb₂HfF₆ and Cs₂HfF₆ and their positions with respect to the top of the valence bands of the host materials were evaluated in terms of the exchange charge model（ECM）in the framework of the CF theory.In Chapter 5,the theoretical scheme discussed above for tuning various physical properties of lanthanide halides was summarized and expected to be valid for experimental researches of lanthanide halides in future.