First-principles Calculations Of Mn⁴⁺-doped A₃XF₇ Fluoride Phosphors

With the deep investigation and fast development of light conversion materials and technologies,white LEDs using trichromatic phosphors have become very popular in the field of general and special lighting.Comparable with conventional incandescent light bulbs and fluorescent tubes,white LEDs are of much smaller volume and longer lifespan,and more energy-efficient,much lower power-consumed,and more friendly to our environment.However,there are still some disadvantages,such as high color temperature and low color rendering index,resulting in the cold sense and low softness to light sources for human eyes.In order to overcome such disadvantages,novel red emitting phosphors have been recently developed to enhance the red light component in the spectrum of white LEDs for the solid state lighting application.In particular,Mn⁴⁺-doped fluorides as red emitting phosphors for white LEDs have been at present paid more attention by scientists working in the field of lighting and display materials.Therefore,based on the hot research motivation of Mn⁴⁺-doped fluorides,the thesis will be mainly focused on A₃XF₇（A=Li,Na,K,Rb,Cs;X=Si,Ge,Sn,Pb,Ti,Zr,Hf）fluorides doped with Mn⁴⁺ions,and the implementation of a high-throughput first-principles screening to smartly search for novel red emitting phosphors with higher chemical and thermal stabilities among them.The main research contents are given below as follows:In Chapter 1,the research background of Mn⁴⁺-doped fluorides as red emitting phosphors for white LEDs and the comparison between the optical performances of two kinds of classical Mn⁴⁺-doped fluorides and their applications were reported and analyzed,and then the research significance of Mn⁴⁺-doped A₃XF₇ fluorides was highlighted due to their synthesis feasibility and short decay time with respect to Mn⁴⁺-doped A₂XF₆fluorides.In addition,it is also revealed by our reference retrieval on the theoretical investigations of Mn⁴⁺-doped red emitting phosphors that the high-throughput first-principles screening technique is the necessary choice to the discovery of new materials in the present work.In Chapter 2,the physics foundation of the first-principles model was simply introduced and described,including the basic concepts of density functional theory,various choices of exchange-correlation functions and the first-principles calculation software CRYSTAL14.In Chapter 3,the optimization scheme of Gaussian type basis sets was proposed and then its test on the model system was carried out.The validity of the calculation scheme with the optimization step of basis sets was confirmed by the direct application to Tl₂MF₆（M=Ti,Sn）fluorides.The thorough analysis of the calculated results shows the importance of the basis set optimization for the improvement of the calculation accuracy of the physical properties of materials.Such a theoretical training is very helpful for understanding the basic scientific knowledge and enriching the usage experience of the calculation software,and also establishes a solid basis to the next study of various physical properties of Mn⁴⁺-doped A₃XF₇fluorides.In Chapter 4,the chemical stabilities and structural,elastic and electronic properties of A₃XF₇fluorides with the P4/mbm phase structure providing doped Mn⁴⁺ions a6-coordinated octahedral site environment were theoretically investigated.Among 35kinds of A₃XF₇（A=Li,Na,K,Rb,Cs;X=Si,Ge,Sn,Pb,Ti,Zr,Hf）fluorides,the lithium-and sodium-based materials are chemically unstable and thus cannot be synthesized,whereas the other materials can be regarded as the good host candidates for Mn⁴⁺-doping.Moreover,the dependences of various physical properties of 21 chemically stable A₃XF₇fluorides on the host components A and X were demonstrated.The calculated results show the following interesting facts:1)the lattice constant increases linearly with X atomic number increasing;2)the bulk modulus decreases linearly with X element trending to be much heavier;3)the band gap is determined by the outer valence shell of X element,as heavier s²p²-type and lighter s²d²-type elements have a smaller band gap value.In Chapter 5,the thermal stabilities of 21 chemically stable A₃XF₇fluorides doped with Mn⁴⁺ions were theoretically analyzed to screen out the most thermally stable Mn⁴⁺-doped K₃SiF₇,Rb₃SiF₇ and Cs₃SiF₇,and then the structural,electronic and optical properties of the selective candidates with the best chemical and thermal stabilities were calculated in more details.The thermal stability of the luminescence intensity of doped Mn⁴⁺ions was found to be correlated to the average X-F chemical bond length,as the shorter the X-F bond length,the bigger the energy gap between ⁴A_2g and almost fixed ²E_genergy levels,suppressing the thermal loss of the luminescent population of ²E_g.The calculated crystal-field splittings between the t_2g and e_g energy states of Mn⁴⁺ions doped in K₃SiF₇,Rb₃SiF₇ and Cs₃SiF₇ show a decreasing trend following the K-Rb-Cs series,whereas there is an opposite case for the calculated emission energies from ²E_g to ⁴A_2g.These calculated facts are completely consistent with the increasing trend of the Si-F bond length on the K-Rb-Cs series.In addition,the limitation of density functional theory used for the calculation of the emission energy from ²E_g to ⁴A_2g of Mn⁴⁺ions was discussed,and an amplification factor of～1.5 was suggested to introduce to the HSE06 calculated results for the sake of correction to the comparable level with the experimental results.The final chapter summarizes all the obtained results and presents a perspective for the future study.All the results presented in the thesis are expected to be useful and a good reference for exploring novel Mn⁴⁺-doped red emitting phosphors with the best chemical and thermal stabilities.