First-Principles Studies On Ultraviolet Fluorine Crystal And Complex Oxides

Deep-ultraviolet(DUV) laser below 200nm attact great interest in photoe-mission spectroscopy, laser spectroscopy, semiconductor photolithography, micro-machining and photochemical synthesis. DUV all-solid-state laser through direct frequency-doubled present great application merits and margets for its simplicity, stability, narrower spectral bandwidth, better beam quality and easier mainte-nance compared to to other DUV coherent light devices. There are several non-linear optical(NLO) crystals, such as BBO, CLBO and LBO can produce DUV laser by nonlinear frequency conversion. However, for DUV region below 200nm, they can only be used with sum-grequency mixing, which makes the laser sys-tem complex and efficiency low. So the ferroelectric fluoride named BaMgF4 and KBe2BO3F2 which show a wide transparency range and good nonlinear optical properties attract great attention. Although the BaMgF4 and KBBF are good candidates for the applications of DUV all-solid state laser, both of them has their own pro and cons. For KBBF, it is too fragile to grow because of its strong layer tendency along the z-axix, make it hard to grow thicker then even 2mm. Besides, the composition used to grow the crystal is toxic. For BaMgF4, the birefringence of this crystal is too small(0.02) to directly archive the birefringent phase matching in the DUV region. Since the difficulty of crystal growing and the limitations of the experiment, theoretical investigation could be a promising tool to assist the crystal engineering to find candidates possess more attractive properties.There are three important properties that determine whether a optical crys-tal possess good ultralviolet ability and nonlinear optical capacity:the first one is the electronic band gap; the second one is the optical band gap and the last one is the nonlinear optical susceptibility. the first-two properties are related with each other, they both determine whether the optical crystal is still transparent in the ultralviolet region. The third one determine the nonlinear optical capacity of the crystal. The third properties can be obtained through the density functional perturbation theory. However, because of the "band-gap" problem and "single-particle" image of the density functional theory, make it is hard to describe these two properties properly.In this thesis, we have studied the electronic and optical properties of ul-tralviolet nonlinear crystals BaMgF4 and KBe2BO3F2 through the many-body perturbation theory, which strongly improve the previous calculations and be in good agreement with experimental results. By comparing the results between hybrid functionals calculation and GW calculations, we find that the hybrid func-tionals also give a good description of the properties of the crystal with much less computational time. For the linear optical properties, by comparing the re-sults with and without the electron-hole interaction, we point out that in these systems, strong excitonic effects play an important role in the description of the linear optical properties. As a result, BSE equation is required for these systems.The quasi-particle band-structure and linear optical properties of GaSe have been report as well following the same procedure as above. By introducing the QP(quasi-particle) correction and BSE equation. The electronic and optical properties of GaSe have been described well.As the first-principles theory is a promising tool to study the solid systems. We used it to investigate the MFeO2(M=Sr,Ba) systems. The MFeO2 system have attracted considerable interest since its first synthesis because of the puz-zles acompanied with this compound.Among these, the absence of Jahn-Teller distortion, which is unexpected from the crystal-field theory, have raised many discussions. In this thesis, we found that the system undergo a Jahn-Teller-like distortion when external strain is applied on. By analyzing the DOS of these sys-tem, we pointed out that this phenomenon is not a normal Jahn-Teller distortion but a lattice unstability caused by the strain. Besides the structural and electron-ic properties, a transition of magnetic orderings from G-type anti-ferromagnetic phase to C-type anti-ferromagnetic phase is predicted in both compounds.