PZT四方相区介电常数εr与晶格畸变关系的研究

从Haun的PZT热力学理论出发，给出了PZT四方相区介电常数ε_r与晶格畸变c/a关 系的基本方程，并深入地根据相关实验资料和物理图像的合理推论，建立了关于晶格畸变c/ a与介电常数ε_r相互依赖的基本关系式.然后，根据该基本关系式仔细地研究了PZT四 方相区晶格畸变c/a对介电常数ε_r影响的物理图像.与相关实验资料比较表明，基本 关系式得到的介电常数ε_r随晶格畸变c/a的变化规律,与相关实验结果相比较，两者 吻合得 关键词： 晶格畸变 r')" href="#">介电常数ε_r 四方相 PZT     

Microscopic coexistence of superconductivity and magnetism in Ba(1-x)K(x)Fe2As2

It is widely believed that, in contrast to its electron-doped counterparts, the hole-doped compound Ba(1-x)K(x)Fe(2)As(2) exhibits a mesoscopic phase separation of magnetism and superconductivity in the underdoped region of the phase diagram. Here, we report a combined high-resolution x-ray powder diffraction and volume-sensitive muon spin rotation study of Ba(1-x)K(x)Fe(2)As(2) showing that this paradigm does not hold true in the underdoped region of the phase diagram (0≤x≤0.25). Instead we find a microscopic coexistence of the two forms of order. A competition of magnetism and superconductivity is evident from a significant reduction of the magnetic moment and a concomitant decrease of the magnetoelastically coupled orthorhombic lattice distortion below the superconducting phase transition.     

The contribution of Neutron and synchrotron radiation topography to the investigation of first-order magnetic phase transitions

Neutron and white beam synchrotron topography have proved to be valuable and complementary tools for the investigation of the coexistence of magnetic phases in single crystals. Neutron topographs indicate unambiguously which phase occupies a given region of the sample, whereas synchrotron radiation topographs give information about the coexistence-related lattice distortion and the dynamics of the phase transition. A few examples illustrate these capabilities. 1) Helimagnetic-ferromagnetic coexistence was followed in Tb and MnP by varying either the magnetic field or the temperature. The observed interfaces appear to result from competition between the elastic and magnetostatic terms of the total energy, the last term being dominant in the field-driven case. 2) The reorientational transition of hematite was shown to correspond, when spread over a wide temperature range, to a succession of abrupt transitions, probably related to an inhomogeneous incorporation of small amounts of impurities during growth. A similar feature also explains the occurrence of small ferromagnetic islands which remain in the helimagnetic phase of Tb.     

Structural properties of Pb2MnW(1-x)Re(x)O6 double perovskites

Pb2MnW(1-x)Re(x)O6 samples have been synthesized and their structure determined by powder x-ray diffraction. These samples undergo a first order structural phase transition between 413 and 445 K depending on the composition. Above this temperature, the samples are cubic. Below the transition temperature, solid solutions are found for x ≤ 0.2 and x ≥ 0.5. The W-rich samples adopt an orthorhombic cell whereas the Re-rich compounds are monoclinic. In the intermediate region, 0.2 < x < 0.5, both phases coexist. X-ray absorption spectra did not reveal significant changes in the local structure for Pb, Mn or Re atoms across the structural phase transition. All the atoms exhibit distorted environments in the whole series. In the case of Pb and W(Re) atoms, the local distortion remains in the high temperature phase. Samples with x ≤ 0.2 also show a sharp discontinuity in the dielectric permittivity at the phase transition temperature indicating the presence of a concomitant electrical ordering in the bulk grains. Such an anomaly in the dielectric constant is not observed for the x ≥ 0.5 samples, compatible with the lack of dipole ordering for this composition range. The different electrical behaviours also explain the differences in the entropy content for the two types of transition.     

Phase transitions on the (111) surface of diamond type semiconductors

The reconstruction and the phase transition are discussed on the (111) surfaces of Si and Ge by the use of the lattice gas model on a triangular net. A Monte Carlo simulation calculation is carried out of the lattice gas model. It is shown that the characteristic diffuse scattering of electrons in the (1x1) phase of Ge(111) can be reproduced by the model. Moreover, the underlying mechanism is discussed in terms of particle arrangements divided into irregular hexagon-like (2x2) domains which yield the diffuse patterns in the (1x1) phase. For the Si(111) surface, on the other hand, the calculation concludes the absence of stacking faults in the (1x1) state. The interrelation between the reconstructions of the surfaces is also discussed.     

傅里叶变换轮廓术中相位失真的预矫正方法

介绍了一种基于傅里叶变换轮廓术的三维面形测量系统中相位失真的预矫正方法。由于投影系统和成像系统的空间三角位置关系、投影仪的发散照明和两套系统蕴含的光学畸变,投影一幅相位与空间坐标成理想线性关系的标准正弦光场,拍摄到的条纹相位和空间坐标不再呈线性分布,引起相位失真,甚至会影响系统测量精度。该方法借鉴反向条纹投影的思想,计算拍摄光场的非线性相位分布与理想的线性相位分布之间的关系,预先矫正,反算出一个新的待投影光场。实验结果表明这种方法能有效地减小该类相位失真所导致的测量误差,获得了更好的测量结果．     

Phase Transformations in the Fe-Co System

X-ray diffraction, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and magnetic studies of Fe₅₀Co₅₀, Fe₆₅Co₃₅ and Fe₃₅Co₆₅ alloys have been performed after different isothermal heat treatments of the alloys at 500–1200°C. It is shown that in the A2 region of the diagram the separation into fcc Co and bcc Fe occurs in a thin surface layer, and partially in the bulk (5–10%). The conclusion is made that B2 ordering of the alloy quenched starts with the precipitation of globular particles consisting of the species, which are in excess relative to the stoichiometric composition of the B2 phase. Prior to the B2 ordering onset, the separation-ordering phase transition occurs; this is accompanied by changes in the electronic structure of the valence band of the Fe₅₀Co₅₀ alloy. The so-called 550°C anomaly is the result of the phase transformation, which proceeds according to a martensitic mechanism and leads to a small increase of the lattice parameter. The conclusion is made that the dependence of the magnetic properties of Fe-Co alloys on B2 ordering is not clearly expressed.     

冲击相边界传播过程中梯度材料的形成

 采用一种基于简单混合物模型的本构，研究了不可逆相变材料中一维的冲击相边界的传播规律，发现在突加载荷并连续卸载的应力边界条件下，样品内部有梯度材料形成。分析了边界条件对材料梯度变化的影响，并且用基于特征线的数值方法计算了不同的边界条件下材料的梯度分布，从而提出利用冲击相边界的传播制备梯度材料的可能性。最后对这种方法的利弊做了探讨。     

Lattice distortion and multiple spin density wave state in CsCl type AuMn

The relation between the tetragonal lattice distortion and the multiple-Q antiferromagnetic ordering and possible phase diagram in the temperature vs. composition plane of CsCl type ordered alloys Au_xMn_1−x (x≈0.5) are discussed on the basis of Landau's phenomenological theory.     

水杨酸镧的流变相合成、表征及掺杂Tb3+的水杨酸镧的发光性质

用流变相反应法合成了水杨酸镧和掺杂Tb3 的水杨酸镧 ,通过元素分析 ,TG ,DTA ,X射线粉末衍射及红外光谱分析确定了水杨酸镧的组成、晶体结构和羧酸根与金属离子的配位方式 ,测定了掺杂Tb3 的水杨酸镧的激发和发射光谱 ,并对其发光特性进行了讨论。X射线粉末衍射表明该化合物为具有层状结构的单斜晶系 ,晶胞参数为 :a =2 1 6 0 1 0 ,b =1 3 80 1 5 ,c =3 81 0 3 ,β=97 1 1° ,V =1 1 2 7 2 3 ,Z =2 ,ρcal =1 6 2 1g·cm-3 ,ρexp =1 6 5 3g·cm-3 。掺杂Tb3 的水杨酸镧固体样品在紫外光激发下可观测到很强的三价铽离子的绿色发光 ,其中5D4→ 7F5的跃迁发射最强。     

Domains induced by elasticity

Near first order phase transitions, the coupling between order parameter and lattice can lead to inhomogeneous structures. This can be understood as a rotation of the coexistence region in the phase diagram, thereby allowing to cross this region by variation of an intensive thermodynamic variable. The properties of critical and tricritical points are changed, the crossover exponents being γ and α, respectively.     

小波相位分析测量成像径向畸变

为了测量光学成像像面各个像素的径向畸变大小,提出将小波变换载频条纹相位分析应用于径向畸变测量。采用正弦载波条纹作为测量模板,把径向畸变转化为径向调制相位。应用条纹相位分析导出径向调制相位和径向畸变的转化关系。采用小波频率估计和相位估计提取变形条纹的相位,由于变形条纹中心点是零畸变,中心点的瞬时频率和相位可以计算参考条纹的基频相位。两种基频相位之差就是与所有像素径向位置畸变分布对应的三维调制相位——称为径向畸变分布。利用校正公式和立方卷积插值算法对彩色畸变图像进行校正,给出详细的理论分析和实验结果。     

Phase diagrams of Ising fluids with Yukawa-Lennard-Jones interactions from an integral equation approach

An integral equation approach is developed to investigate phase coexistence properties of Ising spin fluids with Yukawa ferromagnetic and Lennard-Jones nonmagnetic interactions in the presence of an external field. The calculations are carried out on the basis of the Duh and Henderson closure with a specific Duh-like partitioning of the total potential. The coupled set of the Ornstein-Zernike equation, the closure relation and the external field constraint are solved using an efficient numerical algorithm. The phase diagrams are evaluated in a wide range of varying the external field and the ratio of strengths of Yukawa to Lennard-Jones interactions. Different types of the phase diagram topology as well as various external field dependencies of critical temperatures and densities are identified. The complexity with respect to simple Lennard-Jones fluids is explained by coupling between spatial and spin degrees of freedom in the system. A comparison of the obtained theoretical results with simulation data is made and a good agreement is observed.     

二氧化碳-甲烷混合气体水合物四相区实验研究

以水合物的形式封存CO₂和置换海底的天然气（CH₄）水合物需要对CO₂-CH₄混合水合物的四相平衡状态及数据有清楚的了解。本文通过实验和模型计算对不同组分的CO₂-CH₄混合水合物的较高四相区（Q2）相平衡进行了测定和表述。实验温度范围为273.16～297 15 K,压力范围分为0～10 MPa。四相区的温度压力范围分别是283.51到287.04 K和4.74到8.37 MPa,甲烷的摩尔组份为0～0.225。结果揭示了相平衡温度和压力随着甲烷组分而变化情况以及四相区的范围和临界点,同时还给出了CO₂-CH₄混合气体水合物在四相状态下的融化开始和融化结束点。实验结果与热力学模型计算得出的CO₂-CH₄混合气体水合物相平衡结果进行比较,两者很好吻合,四相平衡区域的存在范围得以明确。     

Phase diagrams of lattice systems with residual entropy. II. Low temperature expansion

For some lattice systems with an infinite number of ground states, it is shown that the pressure and the coexistence surfaces of several phases admit asymptotic expansions aroundT=0. In particular, it follows that the coexistence surfaces are differentiable atT=0, and at low temperatures the stable states are those with maximal residual entropy. The results are applied to construct the phase diagrams for several spin-1 models.     

双层钙钛矿(La_(1-x)Gd_x)_(4/3)Sr_(5/3)Mn_2O_7(x=0,0.05)的相分离

采用传统固相反应法制备(La1-xGdx)4/3Sr5/3Mn2O7(x=0,0.05)多晶样品,并通过测量样品的磁化强度与温度的变化曲线(M-T曲线)、电子自旋共振谱(ESR谱)和电阻率与温度的变化曲线(ρ-T曲线),研究了x=0和x=0.05样品的相分离现象.研究结果表明,两样品在低温部分出现了反铁磁与铁磁相互竞争的现象,体现出团簇自旋玻璃态的特征.x=0和x=0.05样品分别在125—375 K和100—375 K范围内观察到类Griffiths相,同时发现掺杂使得三维铁磁有序温度(T3Dc0≈125 K和T3Dc1≈100 K)降低,而对类Griffiths温度(TG≈375 K)没有明显影响.在TG温度以上两样品均表现出纯顺磁特性.其电特性表明,x=0样品在整个测量范围内出现两次绝缘-金属转变,这是由钙钛矿锰氧化物共生现象所致.而x=0.05样品只出现一次绝缘-金属转变,表明掺杂能抑制共生现象的产生.通过对ρ-T曲线的拟合发现两样品在高温部分的导电方式基本都遵循三维变程跳跃的导电方式.     

Structural and microwave properties of (Ba,Sr)TiO3 films grown by pulsed laser deposition

The relationship between the structure and the microwave dielectric properties of epitaxial Ba_0.5Sr_0.5TiO₃ (BST) films has been investigated. Single-phase BST films (40-160 nm) have been deposited onto (100) MgO substrates by pulsed laser deposition. As-deposited films show a significant tetragonal distortion. The in-plane lattice parameters (a) are always larger than the surface normal lattice parameters (c). The tetragonal distortion depends on the thickness of the films and the post-deposition annealing conditions. Films annealed at 900 °C show less tetragonal distortion than the as-deposited film and the films annealed at higher temperatures. The distortion in the film is due to stress caused by the lattice mismatch and thermal expansion coefficient differences between the film and the substrate. The dielectric constant and its change with dc bias voltage of BST films on MgO at microwave frequencies increase with increasing annealing temperature from 900 °C to 1200 °C, which corresponds to an increase in the tetragonal distortion.     

Generic Two-Phase Coexistence and Nonequilibrium Criticality in a Lattice Version of Schlögl’s Second Model for Autocatalysis

A two-dimensional atomistic realization of Schlögl’s second model for autocatalysis is implemented and studied on a square lattice as a prototypical nonequilibrium model with first-order transition. The model has no explicit symmetry and its phase transition can be viewed as the nonequilibrium counterpart of liquid-vapor phase separations. We show some familiar concepts from study of equilibrium systems need to be modified. Most importantly, phase coexistence can be a generic feature of the model, occurring over a finite region of the parameter space. The first-order transition becomes continuous as a temperature-like variable increases. The associated critical behavior is studied through Monte Carlo simulations and shown to be in the two-dimensional Ising universality class. However, some common expectations regarding finite-size corrections and fractal properties of geometric clusters for equilibrium systems seems to be inapplicable.     

Effect of Ba and Ho co-doping on crystal structure,phase transformation,magnetic properties and dielectric properties of BiFeO3

Multiferroics having composition Bi_0.80-xBa_0.20Ho_xFeO₃ (BBFO, BBHFO5, BBHFO10, BBHFO15 and BBHFO20 for x?=?0.0, 0.05, 0.10, 0.15 and 0.20 respectively) were synthesized by method of solid state reaction. The crystal structure has been studied using X-ray diffraction technique. The X-ray patterns show enormous transform in crystal structure at concentration x?=?0.20. The Rietveld refinement of XRD patterns indicates that at concentration x?=?0.0 sample have rhombohedral structure with R3c space group while for the concentration x?=?0.05, 0.10, 0.15 and 0.20, the mixed phase including rhombohedral R3c and triclinic P1 space groups were obtained with best fitting. This phase transformation in crystal structure is observed due to mismatching of ionic radii of doped ions and parent ions. Magnetic properties of all samples were carried out by using vibrating sample magnetometry. M-H hysteresis loops shows that with doping of Ba and Ho antiferromagnetic BiFeO₃ (BFO) transforms into ferromagnetic. The dielectric and ferroelectric measurements were carried out which shows that dielectric constant, dielectric loss and ferroelectric properties are enhanced with co-doping of Ho in comparison of the pristine BFO due to structure deformation and decrease in oxygen vacancies with higher concentration of Ho. Significant improvement has been observed in dielectric constant and remnant magnetization values with increasing content of Ho and decrease in the dielectric loss.