First-principles Studies On The Electronic Structure And Properties Of Transition Metal Oxides

Transition metal oxides belong to the strongly correlated electronic systems. In these sytems, the lattice, spin, charge and orbital degrees of freedom are still active and strongly coupled each other, which make the transition metal oxides exhibit abundant peculiar physical and chemical properties, such as, the high transition temperature superconductivity in the cuprate, the colossal magnetoresi stance in the manganite, the multiferroics BiFeO₃, YMnO₃, TbMnO₃, TbMn₂O₅, LuFe₂O₄ etc.Due to the strongly coupling among the degrees of freedom, the magnetization can be controlled by the electric field or stress, and in turn the polarization could also be controlled by the magnetic field or stress. So, these materials have valuable potential applications.At present, there are many quantum simulation software packages, based on the density functional theory, such as CASTEP, VASP, PWscf, CPMD, GAUSSIAN etc. These packages consist of very important tools of the first-principles methods, which have been largely applied in the condensed matter physics, materials science, semiconductor, chemistry and made a great sucess. The dissertation mainly discusses the electronic structure and properties of transition metal oxides via first-principles calculation with CASTEP code. The dissertation selects the pressure induced phase transition:taking the 4d4 low spin state systems SrRuO₃ and BaRu3 for example to study the structure phase transition and the effects on the magnetic properties; temperature induced phase transition, taking the spinel AIV₂O₄ for example to study the charge disproportionation and the magnetic properties of the mixed-valence transition metal oxides at low temperature; finally select lithium ions induced the changes of electronic structure (electrochemical properties of cathode materials): taking the new cathode materials fluorosulfate LiMSO₄F (M=Fe, Co, Ni) for example to study their electrochemical properties and the effects of lithium ion extraction on the crystal structure and electronic structure of cathode materials.Pressure induced phase transition:SrRuO₃, BaRuO₃ are chemical composition and crystal structure related to CaRuO₃. For the ionic radii of Ca²⁺and Sr2÷are small (the tolerance factor t<1), CaRuO₃ and SrRuO₃ usually display orthomibic perovskite phase. In 2007, Akaogi M. research group discovered that the orthomibic perovskite structure of CaRuO₃ becomes into post-perovskite structure under high pressure (Pressure:21-25GPa, Temperature:1173-1473K). As for SrRuO₃, Hamlin J J et al. did not found the post-perovskite phase transition up to 34 GPa experimentally. Due to the large ionic radius of Ba+(the tolerance factor t>1), BaRuO₃ crystallizes as hexagonal polytypes. Sintering under high pressure,9R,4H, 6H and 3R polytypes can be gained in turn. The dissertation first studied the phase transition of ARuO₃(A=Sr, Ba) under high pressure by first-principles calculations. The calculations results mainly show as follows:(1) The structural distortion of orthorhombic SrRuO₃ perovskite is enhanced with increasing pressure. And it undergoes phase transition to post-perovskite structure at 40GPa. The SrRuO₃ post-perovskite phase transition companies with the discontinuous volume contraction and collapse of the magnetism. (2) In BaRuO₃, it was confirmed that BaRuO₃ undergoes phase transition from paramagnetic hexagonal phase to ferromagnetic cubic phase. With increasing pressure, BaRuO₃ undergoes 9R,4H,6H hexagonal phase and 3R cubic phase in sequence. The temperature plays an important role during these phase transitions. It was showned that the negative Clapeyron slope appears boundary between 6H-BaRuO₃ and 3C-BaRuO₃, which explains paramagnetic behavior of 6H-BaRuO₃, and while ferromagnetic behavior in 3C-BaRuO₃.Temperature induced phase transition:The AIV₂O₄ shows a phase transition at about 700K with anomalies of transport and magnetic properties, which was considered as a charge ordering transition. However, two research groups proposed different models (Matsuno et al.:three-one type charge odering and Horibe et al.: spin singlet of V heptamer). But it is still hard to understand the electronic structure of the V heptamer. So, in the second part of the dissertation, I studied the charge disproportionation of V ions in the rhombohedral spinel AIV₂O₄ by analysizing the electron population of V ions via first-principles calcution. The V heptamer structure was also gained by optimizing the experimental crystal structure of AIV₂O₄. The calculated results indicate that charge disproportionations take place in these V ions with valence states of+(2.5-δ1),+(2.5+δ2) and+(2.5+(δ1-δ2)/6) (δ1>δ2>0), respectively, and form a charge ordering along the c-axis direction layer by layer with a sequence as V1-V3-V2-V3-V1. However, the nature of the insulating state of AIV₂O₄ is still mysterious and needs further study.Lithium ions induced the changes of electronic structure (Electrochemical properties of cathode materials):LiFePO₄ was once considered as the promising cathode material for lithium-ion batteries for vehicles, due to the low price, high safety and non-toxic properties. However, the drawbacks of LiFePO₄ (low conductivity, complexity of the material synthesis, bad low-temperature performance) are also very hard to resovle. Recently, Tarascon J-M group synthesized a new fluorosulfate cathode material LiFeSO₄ F by introducing a fluorine atom and replacing the [PO₄]3+ group of LiFePO₄ with [SO₄]2-. The material shows a voltage plateau at 3.6 V vs. Li/Li+, with a reversible specific capacity of 130 mAhg-1. It was found that the ionic conductivity of LiFeSO₄F is about 103 times higher than that of LiFePO₄. The discovery of LiFeSO₄F not only provides a strong competitor of LiFePO₄, but also suggests a new class of fluoro-oxyanion cathode material for lithium ion batteries. Subsequently, Tarascon J-M group synthesized Li(Fe1-xMx)SO₄F (M= Co, Ni, Mn), they found that LiMSO₄F (M=Co, Ni, Mn) system did not show any electrochemical activity, cycling between 2.5 V and 4.2 V. At microscopic scale the electrochemical and physical properties of electrode materials are strongly correlated to their electronic structures. Therefore, we purpose to calculate the crystal and electronic structures of LiMSO₄F (M= Fe, Co, and Ni) as well as their delithiated forms using first-principles calculations, in order to present a deep understanding on the electrochemical and physical properties of the LiMSO₄F/MSO₄F systems. The main results are as follows (1) The crystallographic data of LiNiSO₄F was given. (2) The theoretical intercalation voltages of LiMSO₄F are 3.54 V (Fe),4.73 V (Co) and 5.16 V (Ni), respectively, which are close to corresponding LiMPO₄ phosphates, which explains none electrochemical activity of LiMSO₄F (M=Co, Ni), cycling between 2.5 V and 4.2 V. (3) The values of electron-charge transfer taking place on the oxygen anions are 56 % and 57 % for LiCoSO₄F/CoSO₄F and LiNiSO₄F/NiSO₄F respectively, which are much larger than that of LiFeSO₄/FeSO₄F(28%). The removal of such a lot of electrons from the O-2p band will result in a large amount of O- anions. This process is accompanied with the following peroxide formation, leading to ultimate loss of oxygen from the lattice surface, which may cause lattice collapse and safety problems. (4) LiFeSO₄F transforms from Mott-Hubbard insulator to charge-transfer insulator with Li+extraction. However, this transformation does not happen in L1CoSO₄F and LiNiSO₄F systems. The physical mechanism of this transformation is still unclear, which is worth of study in future.