First-Principles Study Of The Ordered And A-Site Partial Substituted KTN Solid Solutions

Potassium tantalite niobate (KTN) are applied in holographic data, electroop -tic switches and modulators with waveguides, etc., will be significantly used in making electro-optic modulators. The Li-doped KTN (KLTN) are widely employ -ed in voltage controlled photorefractive, electroholographic wavelength selective switching, and single mode fiber integration with spatial solitons. The KNbO₃ -based crystals (of course including KTN) have good dielectric and piezoelectric performance among lead-free materials. In 2004, Y. Saito et al have found out that most of the piezoelectric properties of KNbO₃-based materials are comparable to those of PZT piezoelectric ceramics, it is expected to be one of the most promising lead-free materials. Therefore, the series of KTN crystals have been the subject of extensive investigation in the scopes of functional materials. Experimentally, it is found that many disordered solid solutions are ordered on the nanoscale, and the ordered microregions have an obvious influence on the properties of solid solutions.The first principles method is a reliable tool to study the microscopic structures of ferroelectric or piezoelectric materials. In this paper, we carried out a systematic investigation on the physical properties of paraelectric ordered KTa_1/2Nb_1/2O₃ using the density functional theory. The physical properties were compared for three ordered KTN supercells. The BO₆ octahedron and K-O bond lengths have plenty of differences, although the optimized structure parameters of three supercells are almost consistent. The Ta ions in KTN have obvious unusual behaves. The effect of B site cations ordering evidently influences the energy gaps, total density of states and partial density of states, leading to the differences of optical characters.The calculations of ferroelectric properties for disorder KTa_1/2Nb_1/2O₃ solid solutions were performed by generalized gradient approximation. Stable ferro -electric ground states are found, and the electronic structures and optical propert -ies were calculated. The ferroelectric states are caused by the structural instabilities involving B site cations off-centering. Different chemical order of B site cations result in different phases in ordered KTN crystals. In other words, the ferroelectric states are particularly sensitive to the chemical ordering of B site cations. We also present the evidence to demonstrate the existence of antiferro -electric phases in KTN solid solutions, which is in agreement with experimental data. The calculated results show that optical properties are different for three supercells.We present a detailed investigation on the physical properties of Cu:KNbO₃ with first principles method. The calculated lattice constants is in good accord with the experimental results. The theoretical energy gaps are better than previous calculated values. The considered peak of the absorption spectra is not just due to the single transition, and the Cu 3d states generate a peak at Fermi level. It is predicted that KNbO₃ crystal will be metallic if the Cu-doping concentration is increased enough.Li/Na-doping can obviously change the physical performances of the KTN crystals, which is suitable for the optical applications. The electronic structures and optical properties of KLTN and Na-doped KTN (KNTN) have been investigated theoretically, in order to explain the above fact. It is found that Li ions have more space to move, and its covalency is very strong. While the Na ions are almost completely ionized. Their optical spectra are significantly different from those of KTN crystals, which shows that the substituted A atoms have evident effect on the physical properties of KTN solid solutions. The reflectivity spectra of KLTN and KNTN varies widely as a function of energy, which make them suitable for a variety of applications in fabricating optical devices.In conclusion, the effect of the local ordering and A-site partial substitution influences the structures and performances of KTN crystals importantly. At the present time there are no related experimental data, however, our calculated results that could serve as the theoretical predications for future work. The theoretical data are also the guides for the experimental development, which has the important practical significance in the field of exploring highperformance and low -cost lead-free piezoelectric materials.