光折变晶体KTa0.5Nb0.5O3光学特性的第一性原理研究

用全电势线性缀加平面波法(FLAPW)计算了KTa0.5Nb0.5O3四方相和立方相 的光学特性，即介电函数虚部ε２(ω)、光学吸收系数I(ω)和反射率R(ω). 在四方相，介电虚部沿a，b轴，在3，7和23eV附近，分别有三个介电峰.沿c轴的三个介电峰分别位于4，8和23eV.其中4eV附近的介电峰非常尖锐而且高.从8至18eV，沿a，b ，c轴三个方向都有许多低的介电峰.通过对两相光学特性的对比分析发现铁电相KTa0.5Nb0.5O3具有更强的各向异性. 关键词： 平面波法计算 光学常量和参数 铁电体     

Optical properties of cubic and tetragonal KTa0.5Nb0.5O3 by density functional theory

Using density functional methods, optical properties of KTa_0.5Nb_0.5O₃ (KTN) are investigated. The imaginary part of dielectric function ε₂(ω), the optical absorption coefficient I(ω) and the reflectivity R(ω) of KTN in the cubic (paraelectric) and tetragonal (ferroelectric) phases are calculated. The origin of the different behavior of the optical spectra between the two phases is discussed.     

KTa0.4Nb0.6O3 nanoparticles synthesized through solvothermal method

Potassium tantalate niobate (KTa_0.4Nb_0.6O₃, KTN) nanoparticles of perovskite structure were successsfully synthesized by a solvothermal method. The KTN nanoparticles synthesized at 250 °C for 8 h with 1 to 4 M KOH concentration using isopropyl alcohol [(CH₃)₂ CHOH] as the solvent was composed of a single phase of cubic perovskite structure. Futhermore, the KTN powers synthesized at the same conditions besides of using (CH₃)₂CHOH/H₂O as a solvent compose of a single phase of tetragonal perovskite structure. The nanoparticles exhibit a mixture of cubic and prism-like shapes with lengths of 100 nm to 500 nm and average cross sections of 200×200 nm². The solvent dependence of the powder formation is discussed. X-ray diffraction and electron diffraction results show that the powders have the needed tetragonal perovskite structure. The band gap of KTN nanoparticles is determined to be 3.26 eV from the optical absorption spectra.      

Time Dependences of Dielectric and Acoustic Properties in PbFe0.5Nb0.5O3 and PbFe0.5Nb0.5O3–7PbTiO3 Single Crystals

Physics of the Solid State - The time changes of permittivity, damping and velocity of sound are studied for PbFe0.5Nb0.5O3 and PbFe0.5Nb0.5O3–7PbTiO3 single crystals in the electric fields...     

Downward self‐polarization of lead‐free (K0.5Na0.5)(Mn0.005Nb0.995)O3 ferroelectric thin films on Nb:SrTiO3 substrate

Spontaneously appearing macroscopic polarization (self‐polarization) in ferroelectrics without an electrode or an external electric field would enable the freedom to design many ferroelectric heterostructures and devices. The (K_0.5Na_0.5)(Mn_0.005Nb_0.995)O₃ (KNMN) thin film was grown on Nb:SrTiO₃ single‐crystal substrate and the resultant self‐polarization behavior due to strain relaxation was investigated. The lattice mismatch and difference in TEC between the KNMN thin film and the Nb:SrTiO₃ substrate creates a compressive strain. The compressive strain gradient may be the main cause for the observed downward self‐polarization. The downward self‐polarization of the KNMN thin film can be explained by the strong inhomogeneous compressive strain caused by strain relaxation. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Spontaneous phase transition from relaxor to macrodomain ferroelectric state in single-crystal PbSc0.5Nb0.5O3-BaSc0.5Nb0.5O3 solid solutions

The cooling of Pb_1?x Ba_xSc_0.5Nb_0.5O₃ solid solutions with x≤0.04 leads to a spontaneous transition from a relaxor to a macrodomain ferroelectric state, accompanied by anomalous variation of the dielectric and optical properties of the material. As the barium content in the system increases, the relaxor state becomes more stable and eventually “freezes” at x≈0.05. The crystals with x=0.06 exhibited the appearance of a macrodomain ferroelectric phase induced both by an external electric field with a strength of 1.5 kV/cm and by an internal electric field formed in the course of dielectric aging.     

Dielectric response of K(Ta,Nb)O3 thin films

The dielectric response of K(Nb,Ta)O₃ films grown on (001) MgAl₂O₄ (100) by pulsed laser deposition from a mosaic target of KTa_0.63Nb_0.37O₃ and KNO₃ has been examined. In particular, the effects of growth temperature (650–800 °C), growth pressure (1–100 mTorr O₂), and annealing conditions on the tunability, dielectric constant, and dielectric losses in interdigitated capacitor device structures fabricated with these films are described. Annealing treatments lead to a reduction in the loss tangents for most of the films considered. Figure of merit calculations indicate that the best dielectric response (tunability=37%, tan =0.022) is achieved for films grown at 750 °C in an oxygen pressure of 100 mTorr and then annealing at 1000 °C for 2 h in air.     

Phase transitions in lead scandoniobate PbSc0.5Nb0.5O3

Crystals of PbSc_0.5Nb_0.5O₃ are investigated by x-ray diffraction and optical spectroscopy. It is found that the phase transformations in PbSc_0.5Nb_0.5O₃ are governed by two mechanisms: displacive ferroelectric phase transitions and consistent rotations of oxygen octahedra.     

Ferroelectric phase transitions in Pb0.96Ba0.04Sc0.5Nb0.5O3 and Pb0.94Ba0.06Sc0.5Nb0.5O3 solid-solution single crystals

The optical properties of Pb_0.96Ba_0.04Sc_0.5Nb_0.5O₃ (PBSN-4) and Pb_0.94Ba_0.06Sc_0.5Nb_0.5O₃ (PBSN-6) single-crystal solid solutions were studied for the first time. It was shown that the spontaneous phase transition occurring in PBSN-4 with no electric field present is accompanied by a sharp minimum in optical transmission, which indicates the percolation nature of the transition. No sharp changes were observed in the temperature dependence of optical transmission in PBSN-6 single crystals with no electric field applied. However, a very weak electric field, ～0.4 kV/cm, is sufficient to induce the ferroelectric state in PBSN-6 single crystals. It was shown that the destruction of the induced ferroelectric state is a first-order phase transition which is accompanied by an anomalously narrow peak in the small-angle light scattering intensity (or by a minimum in optical transmission) and occurs through the percolation mechanism.     

Defect induced fluctuations in the paraelectric phase of KTa0.94Nb0.06O3

First order Raman lines observed in the paraelectric phase of KTa_0.94Nb_0.06O₃ are shown to be the result of odd symmetry microdistortions or fluctuations with a correlation length that extends to several unit cells. The possibilities that the lines result from impurity induced or disorder induced Raman scattering are considered and discarded. From simple theoretical considerations it is shown that the integrated intensity of distortion induced first order Raman lines, normalized by the corresponding phonon occupation numbers, is proportional to the average distortion intensity. This proportionality is independent of the correlation length of the distortion field provided it is larger than a certain minimum.Using this relation and the experimentally measured integrated intensity of the TO₂ line we find that the average distortion intensity is proportional toT/(T _c—T). This result is compared to two models which qualitatively predict the existence of microdistortions or fluctuations in the paraelectric phase. The experimentally observed temperature dependence is found to be consistent with the defect induced fluctuations model and inconsistent with continuous phase transitions models.     

Electrical characteristics of BaTiO3/ Bi0.5K0.5TiO3 multilayered thin films synthesized via metalloorganic decomposition

Multilayered BaTiO₃(BTO)/Bi_0.5K_0.5TiO₃ (BKT) thin films have been fabricated on Pt/Ti/SiO₂/Si substrates using a metalloorganic decomposition process. XRD investigation of the resultant BTO/BKT multilayered thin films shows that they retain a perovskite-related structure type. They also exhibit a well-defined, polarization–electric field hysteresis loop with a measured remnant polarization (2P_r) of 5 µC/cm² at an applied electric field of 250 kV/cm. The measured dielectric constant and dielectric loss at 10 kHz is 470 and 0.07 respectively. These multilayer BTO/BKT films maintain an excellent fatigue-free character even after 10⁹ switching cycles. The mechanism associated with the enhancement of the electrical properties of the synthesized BTO/BKT films is also discussed.     

Exafs measurements on Nb3Ge thin films

We have measured the Extended X-ray Absorption Fine Structure (EXAFS) spectra for superconducting thin films of Nb₃Ge prepared at variable deposition temperature by sputtering and electron beam coevaporation. As the deposition temperature is varied from 1050 K to 875 K, the film structure changes continuously from the A15 phase to a quasi-amorphous phase. This amorphous phase is characterized by a mean Ge-Nb distance of 2.66 A, as compared to the A15 distance, 2.87 A. The mean coordination of the Ge sites is reduced from 12 to 8±2, and the mean square spread in Ge-Nb distances (thermal + spatial) has decreased by only 0.01 A².     

Thickness-dependent tunability for Ti-doped K(Ta,Nb)O3 thin films

Alumina thin films were deposited on hot work tool steel AISI H11 at a growth temperature of 500 to 600 °C by plasma assisted chemical vapour deposition and were studied with respect to the structure and composition by X-ray diffraction and electron-probe microanalysis, respectively. The electrical power density at the cathode was varied from 2.7 to 6.6 W/cm². Within the investigated process window the following characteristic phases could be identified: γ-alumina and α-alumina as well as mixtures thereof. The alumina phase formation was found to be strongly influenced by deposition temperature and electrical power density at the substrate. It is shown that constitution changes due to a reduction in substrate temperature can be avoided by increasing the electrical power density at the cathode, which leads to an increase in both ion flux and ion energy at the substrate surface. PACS 52.77.Dq; 68.55.Nq; 68.55.Ac; 61.10.Nz     

High-temperature impedance spectroscopy of BaFe0.5Nb0.5O3 ceramics doped with Bi0.5Na0.5TiO3

Bi_0.5Na_0.5TiO₃ (BNT)-doped BaFe_0.5Nb_0.5O₃ (BFN) ceramics were synthesized by a two-step solid-state reaction. Temperature dependence of dielectric properties measured at different frequencies was investigated over broad temperature and frequency ranges. Impedance spectroscopy and universal dielectric response were employed to study the relaxation behavior and conductivity mechanism of the ceramics in a frequency range from 40 Hz to 100 MHz and a temperature range from 300 K to 800 K. The complex plane impedance data revealed the bulk and grain boundary contributions toward conductivity processes in the form of semicircular arcs. The high-temperature conductivity of ceramics is attributable to thermally activated second ionized oxygen vacancy.     

四方铁电体PbFe0.5Nb0.5O3精细结构的第一性原理研究

采用全电势线性缀加平面波法(FPLAPW),计算了PbFe0.5Nb0.5O3的总能量,从而确定了它的四方铁电稳态结构,即B位离子(Fe和Nb)在四方铁电相的平衡构型.计算结果表明,Fe相对于氧八面体沿[001]方向的位移为0.022nm,而Nb的偏心位移为0.0020nm,其中铁电非稳性主要归因于前者 关键词： 铁电体 密度泛函理论 平面波法     

Theoretical study on the fine structure of PbFe0.5Nb0.5O3

The full potential linearized augmented plane wave method within the generalized gradient approximation was used to determine the tetragonal fine structure for PbFe_0.5Nb_0.5O₃, i.e., the equilibrium configuration of B-site compound cations (Fe and Nb). It was found that the displacement of atom Fe along [001] direction is 0.022 nm and that of Nb is 0.0020 nm. The ferroelectric instability of complex compound PbFe_0.5Nb_0.5O₃ is mainly coming from the displacement of atom Fe along [001] direction.     

Si基Bi3.25La0.75Ti3O12铁电薄膜的制备与特性研究

采用Sol-Gel工艺低温制备了Si基Bi3.25La0.75Ti3O12铁电薄膜.研究了退火温度对薄膜微观结构、介电特性与铁电性能的影响.500℃退火处理的Bi3.25La0.75Ti3O12薄膜未能充分晶化,晶粒细小且有非晶团聚,介电与铁电性能均较差.高于550℃退火处理的Bi3.25La0.75Ti3O12薄膜表面平整无裂纹,晶粒均匀,无焦碌石相或其他杂相,薄膜为多晶生长,具有较好的介电与铁电性能,4V电压下的漏电流密度低于2×10-8A/cm2.随退火温度升高,晶化程度的提升和晶粒尺寸的增大使薄膜的剩余极化增大而矫顽电场降低.600℃退火处理的Bi3.25La0.75Ti3O12薄膜显示了优于Bi4Ti3O12薄膜的铁电性能,其剩余极化Pr和矫顽电场Ec分别达到17.5μC/cm2和102kV/cm.     

Concentration dependence of the compositional order-disorder phase-transition temperature in the solid solution PbSc0.5Nb0.5O3-PbSc0.5Ta0.5O3

The degree of compositional order S of crystals of the complete series of solid solutions (1−x)PbSc_0.5Nb_0.5O₃-xPbSc_0.5Ta_0.5O₃ grown under identical conditions is determined by x-ray examination. The form of the dependence S(x) and the variations of S for different compositions, after high-temperature annealing, indicate that the concentration dependence of the compositional order-disorder phase-transition temperature is close to linear in the given system of solid solutions. Fiz. Tverd. Tela (St. Petersburg) 41, 1828–1830 (October 1999)