First Principles Study On The Physical Properties Of Functional Materials A₂B₂O₇ （A=Cd,Nb;B=Nb, Ta）

The A2B2O7-type functional materials mainly have pyrochlore structures and layered perovskite structures. They have wide applications in new materials design and the progress of high-technology. Since the dielectric constant can be changed by the external electric field, cadmium pyroniobate (Cd2Nb2O7) is one of the most studied compounds for tunable microwave devices. Cd2Ta2O7 and Cd2Nb2O7 are isostructural. Until now, the phase transition of Cd2Ta2O7, the atomic structure and properties of its ground states are not clear. Among the layered perovskite compounds, Sr2Nb2O7 is one of the few materials that show ferroelectricity at high temperature. While it’s dielectric constant is not high. So far, there is no report on the theoretical analysis of the electronic structure. Sr2Nb207 and Sr2Ta2O7 are also a pair of isostructure. As a new type of photocatalyst, the photocatalytic effect of Sr2B2O7(B=Nb, Ta) in ultraviolet irradiation is significant. High absorb efficiency under visible light is important for practical applications. In this thesis, the structures, dielectric, piezoelectric and optical properties of pyrochlore type Cd2B2O7 (B=Nb, Ta) and layered perovskite type Sr2B2O7 (B=Nb, Ta) are studied using first principles calculations within density functional and density functional perturbation theory.Our work is mainly foucs on the following aspects. Firstly, the phonons of Cd2Nb2O7 with the Fd3m structure at the T point are calculated to study the structure instability and ferroelectric phase transition. The phonons of cubic Cd2Ta2O7 in the whole Brillouin zone are studied to investigate the anti-ferroelectric phase transition. Secondly, detailed analyses of the response properties of the metastable states and ground state for Cd2Nb2O7 and Cd2Ta2O7 are carried out. Thirdly, the dielectric, piezoelectric and polarization properties of orthorhombic Sr2Nb2O7 and Sr2Ta2O7 are investigated. Fourthly, the electronic structure and the imaginary part of dielectric constants are calculated to analyze the relation between the structure and optical properties for orthorhombic Sr2Nb2O7 and Sr2Ta2O7. The influences of N-doping and oxygen vacancies on optical properties of Sr2Nb2O7 and Sr2Ta2O7 are studied. The main results are as follows:(1) The tetragonal I41md, orthogonal Ima2 and monoclinic Cc phase are obtained by freezing the unstable IR phonon of Cd2Nb2O7 with the Fd3m structure through different ways. The dielectric, piezoelectric and polarization properties of these phases are studied. The response properties are investigated from the view of contributions from ions and phonons. The structure instability and phase transition are caused by the Nb-O octahedral distortion and hybridization between Nb-4d and O-2p orbital.(2) Cd2Ta2O7 does not contain any imaginary frequency at the T point, which is different with Cd2Nb2O7. It has only one soft mode X+ at the X-point with an imaginary frequency of 17.5 cm-1. We obtain the orthorhombic Pnma (No.63) Cd2Ta2O7 through freezing in the unstable X+ mode from the cubic Fd3m phase. Detailed analyses indicate that the Cd2Ta2O7 with Pnma structure is an anti-ferroelectric. The phase transition is related to the Nb-O octahedral distortion. The response properties of the Pnma phase Cd2Ta2O7 are also studied.(3) The band gaps of pure Sr2Nb2O7 and Sr2Ta2O7 computed by the HSE06 functional are very consistent with the experimental values. Nb-4d (Ta-5d) orbitals which have some hybridization with O-2p orbital can be divided into t2g and eg manifolds as expected from their octahedral coordination. The calculated optical properties show that pure Sr2Nb2O7 and Sr2Ta2O7 have good performance in the ultraviolet region. The N-doping and oxygen vacancies can reduce the effective gap and thus shift the absorption edge into the visible light region. The Sr2Ta2O5N2 has the best absorption effect in the visible region among the Sr2B2O7(B=Nb, Ta) modified systems. The N-doping systems display ferromagnetism. The ferromagnetism is weakened with the increasing of the doping ratio.