铌酸钾钠基无铅压电陶瓷的X射线衍射与相变分析

分析了斜方相、四方相铌酸钾钠基无铅压电陶瓷材料的结构和X射线衍射图谱的特点. 对于铌酸钾钠基压电材料斜方相结构, 从构成晶胞的一个单斜原胞进行分析, 计算出X射线衍射谱上每个衍射角附近的衍射峰数目和相对强度. 提出了2θ在20°—60°范围内根据(1 0 2)衍射峰(52°附近)和(1 2 1)衍射峰(57°附近)劈裂的数目区分斜方和四方相的新方法. 对于多晶陶瓷粉末, 可以更简便的由22°(或45°)附近前后峰的相对高低来判断斜方、四方相. 关键词： 铌酸钾钠 无铅压电陶瓷 X射线衍射 相变     

Pressure-dependent phase transition in the ordered BaBi0.7Nb0.3O3 perovskite

BaBi_0.7Nb_0.3O₃, an ordered perovskite, crystallizes in a centrosymmetric rhombohedral structure with the space group R3¯. The refined cell parameters obtained from synchrotron powder X-ray diffraction data for the rhombohedral phase at ambient pressure are a=6.109 (2) Å and α=60.3 (1)°. The pressure-dependent synchrotron powder X-ray diffraction studies show a phase transition around 8.44±1 GPa, where it transforms from rhombohedral structure to a monoclinic structure. The lattice parameters obtained for the monoclinic phase at a pressure of 15±1 GPa are a=5.91 (2) Å, b=6.25 (3) Å and c=8.22 (1) Å with monoclinic angle, β=88 (1)°.     

Pressure-induced phase transition of wulfenite

The in-situ high-pressure structures of wulfenite have been investigated by means of angular dispersive X-ray diffraction with diamond anvil cell and synchrotron radiation. In the pressure up to 22.9 GPa, a pressure-induced scheelite-to-fergusonite transition is observed at about 10.6 GPa. The pressure dependence for the lattice parameters of wulfenite is reported, and the axial compression coefficients Ka₀=－1.36×10^－3 GPa^－1 and Kc₀= －2.78×10^－3 GPa^－1 are given. The room-temperature isothermal bulk modulus is also obtained by fitting the P-V data using the Murnaghan equation of state.

     

Thermal expansion and a new phase transition in pyroelectric LiKSO4

Single crystal macroscopic thermal expansion coefficient measurements have been made on uniaxial lithium potassium sulphate crystal both along and normal to the six fold axis, employing Fizeau’s interferometer method. Measurements were made in the range of −120°C to 500°C. The results show that lithium potassium sulphate exhibits two major anomalies in its expansion coefficients around −95°C and 422°C respectively, the one at −95°C has been observed for the first time. The nature of dimensional changes of the crystal at the upper and lower transition points are opposite in nature. The crystal shows considerable lattice anisotropy. Megaw’s tilt concept has been invoked to explain the relative magnitudes of expansion coefficients alonga andc directions. Structural features responsible for the absence of ferroelectricity in this crystal have been pointed out.     

Structural phase transition in substituted La2CuO4 and Pr2CuO4−y

The structural phase transition from orthorhombic (T) phase to tetragonal (T′) phase in substituted La_2−x R _x CuO₄ (R = Pr, Nd, Sm, Eu and Gd) and T′ to T-phase in Pr_2−x M _x CuO_4−y (M = Sr, Ca) has been studied by X-ray diffraction technique. The T-phase of La₂CuO₄ is transferred to T′ phase abruptly atx=0.8, 0.4, 0.4, 0.3 and 0.4 respectively for substitution of Pr, Nd, Sm, Eu and Gd for La in La₂CuO₄ without evidence of the T* phase. The T′ structure of Pr₂CuO₄ (x = 0.0) gets transformed to the T* structure at 30% Ca doping (x=0.6) and then to the T structure at 50% Ca doping (x=1.0), while for Sr-contentx=0.0, 0.4 and 1.0 it shows T′, T* and T structure respectively.     

Domain population in SrTiO3 below the cubic-to-tetragonal phase transition

High-angle double-crystal X-ray diffractometry (HADOX) has been applied to measure the intensity around three reciprocal lattice points: 400, 040 and 004 of SrTiO₃. Each point splits into two in the tetragonal phase according to the appearance of three kinds of domains; the temperature dependence of the intensity of component peaks at these points was measured under identical conditions. The results show that the domain population varies with temperature in a range of about 20 K below the transition. It is found, however, that the total intensities of 400 and 004 of the tetragonal phase are constant; this is observed by summing up the relevant intensity at the three reciprocal lattice points.     

Scaling law and critical exponent for α0 at the 3D Anderson transition

We use high‐precision, large system‐size wave function data to analyse the scaling properties of the multifractal spectra around the disorder‐induced three‐dimensional Anderson transition in order to extract the critical exponents of the transition. Using a previously suggested scaling law, we find that the critical exponent ν is significantly larger than suggested by previous results. We speculate that this discrepancy is due to the use of an oversimplified scaling relation.     

含有Dzyaloshinskii-Moriya相互作用的自旋1键交替海森伯模型的量子相变和拓扑序标度

利用张量网络表示的无限矩阵乘积态算法研究了含有Dzyaloshinskii-Moriya (DM)相互作用的键交替海森伯模型的量子相变和临界标度行为.基于矩阵乘积态的基态波函数计算了系统的量子纠缠熵及非局域拓扑序.数据表明,随着键交替强度变化,系统从拓扑有序的Haldane相转变为局域有序的二聚化相.同时DM相互作用抑制了系统的二聚化,并最终打破系统的完全二聚化.另外,通过对相变点附近二聚化序的一阶导数和长程弦序的数值拟合,分别得到了此模型相变的特征临界指数a和b的值.结果表明,随着DM相互作用强度的增强, a逐渐减小,同时b逐渐增大. DM相互作用强度影响着此模型的临界行为.针对此模型的临界性质的研究,揭示了量子自旋相互作用的彼此竞争机制,对今后研究含有DM相互作用的自旋多体系统中拓扑量子相变临界行为提供一定的借鉴与参考.     

Crystal shape near the faceting phase transition point

The critical behavior of an equilibrium shape of a crystal with the size much larger than the capillary length is studied near the faceting phase transition point.     

Thermal expansion and structure of orthorhombic CaMnO3

The structure of CaMnO₃ has been studied from room temperature to 800 °C by high-resolution synchrotron X-ray powder diffraction. It is concluded that CaMnO₃ remains orthorhombic in Pbnm over the entire temperature range, with the expansion in the cell volume well fitted to the expression Vol=207.37(2)+0.00563(8)T+1.17(9)×10^-6T². The magnitude of the MnO₆ tilts is estimated from the refined structural parameters and these progressively reduce as the temperate is increased.     

Dielectric studies of the phase transition in {N(CH3)4}2HgCl4

Dielectric properties of single-crystal {N(CH₃)₄}₂HgCl₄ (structure at room temperature; orthorhombic Pmcn) have been measured in a temperature range from 4 K to about 370 K. The dielectric constant along the a-, b-, and c-axes shows a break at 278 K. No other anomalies are detected in the temperature range studied. The transition temperature increases linearly with increasing hydrostatic pressure at a rate of 0.20 K/MPa. Twin boundaries are observed in the low-temperature phase on b-plate specimens. The results indicate that the phase transition in {N(CH₃)₄}₂HgCl₄ is ferroelastic, as in {N(CH₃)₄}₂XBr₄ (X: Mn, Co, Zn).     

Pressure effect on phase transition in ferroelectic polymers

Pressure effects of phase transition behaviour in two kinds of ferroelectric polymers of poly(vinylidene fluoride), PVDF, and copolymers of vinylidene fluoride and trifluoroethylene, (VDF/TrFE), are discussed. In the case of PVDF, several high-pressure treatments including a high-pressure annealing and a uniaxial compression were shown to induce a crystal transformation from a non-polar Form II crystal to a polar Form I crystal, which has ferroelectric characteristics and high piezoelectric activity. In addition, substantial pressure effects on ferroelectric phase transition points as well as crystal structures were observed for (VDF/TrFE) copolymers with different VDF contents. The most significant pressure effects were observed for copolymer samples with unstable ferroelectric structures at atmospheric pressure. From high-pressure X-ray and Raman scattering studies, these pressure effects were suggested to originate from the pressure-induced conformational transition from gauche to trans in the molecular chains.     

Dielectric properties of betaine arsenate crystal near the phase transition

Dielectric measurements for single crystal of betaine arsenate (CH₃)₃NCH₂COO·H₃AsO₄ connected with the ferroelectric phase transition at 119 K were performed. The temperature dependence of electric permittivity was measured at dc electric fields up to 700 kV/m. The results show significant suppression of the dielectric constant by the application of dc field. Deviation from the classical behavior was observed. The electric permittivity was also measured in the paraelectric phase at constant temperature as a function of electric field intensity up to 700 kV/m. The electric permittivity might be well described by the classical relation with additional term including contribution to permittivity coming from clusters. The fit parameters indicate that the polar-clusters carries polarization P₀=0.7- with the clusters size of L=12-20 nm.     

Pressure-induced phase transition of B-type Y_2O_3

The synthesized monoclinic(B-type) phase of Y_2O_3 has been investigated by in situ angle-dispersive x-ray diffraction in a diamond anvil cell up to 44 GPa at room temperature. A phase transition occurs from monoclinic(B-type) to hexagonal(A-type) phase at 23.5 GPa and these two phases coexist even at the highest pressure. Parameters of isothermal equation of state are V_0= 69.0(1) ~3, K_0= 159(3) GPa, K_0= 4(fixed) for the B-type phase and V_0= 67.8(2) ~3, K_0= 156(3) GPa,K'_0= 4(fixed) for the A-type phase. The structural anisotropy increases with increasing pressure for both phases.     

应力作用下GdTiO_3磁序相变的第一性原理研究

采用基于密度泛函理论的第一性原理方法,系统研究了GdTiO_3薄膜在压缩应力和拉伸应力作用下的磁序相变.计算结果表明:1)在LaAlO_3压缩衬底的作用下,GdTiO_3薄膜从铁磁基态转变为G型反铁磁基态.该结果不同于YTiO_3和LaTiO_3在LaAlO_3压缩衬底作用时都呈现A型反铁磁基态的情况.若进一步加大压缩应力,例如在(001)平面施加YTiO_3衬底,此时GdTiO_3薄膜基态才为A型反铁磁态.2)在LaScO_3和BaZrO_3拉伸衬底的作用下,GdTiO_3薄膜的基态仍是铁磁态,但是随着拉伸应力的增大,A型反铁磁态的能量和铁磁态的能量差逐渐缩小,即GdTiO_3薄膜的基态有转变为A型反铁磁态的趋势.3)在外加应力的作用下,GdTiO_3薄膜基态的磁序发生了相变,但是其绝缘性并没有变,说明GdTiO_3薄膜仍为Mott型绝缘体.     

用同步辐射X射线衍射技术分析GaN/Si外延膜的结构与应变

选用有AlN和AlGaN缓冲层的GaN/Si作为测试样品,采用同步辐射X射线衍射(SRXRD)技术对样品外延膜(GaN)的几何结构、晶格常量及其应变进行了分析.结果表明,同步辐射X射线衍射实验可以作为一种有效的技术手段,测试固体结构及应变.     

Lattice energy and phase transition of alkaline earth chalcogenide

In this study, we have investigated the high-pressure structural phase transition of alkaline earth's chalcogenides within the framework of three-body potentials. We are considering short-range repulsive interactions up to the second nearest neighbors. The structural phase transition from the low-pressure NaCl (B1) to the high-pressure CsCl (B2) structure is estimated by Gibbs free energy calculations. The results are satisfactory and in agreement with the available experimental and other theoretical results.     

Structural,spectroscopic and dielectric properties of KVO3

Polycrystalline samples of KVO₃, a member of the pyroxene structural family have been synthesised with high-temperature solid-state reaction technique. Preliminary structural and spectroscopic and detailed dielectric properties have been studied in different conditions. No dielectric anomaly or hysteresis loop has been observed in a wide temperature range (30°C to 450°C). Basic structural and spectroscopic (IR and Raman) studies suggest that at room temperature, KVO₃ is centrosymmetric as reported earlier. This and other observations do not support the recent report of ferroelectric phase transition in the compound.     

晶体相场法研究预变形对熔点附近六角相/正方相相变的影响

应用双模晶体相场模型计算二维相图,并模拟了在熔点附近预变形和保温温度对六角相晶界演化以及六角相/正方相相变的影响.研究发现:在相变初期,当预变形为零、保温温度离熔点很近时在晶界发生缺陷诱发预熔;增大预变形,变形与缺陷的交互作用在熔点附近诱发预熔;随着预变形的进一步增大,变形在畸变处同时诱发液相和正方相,且预变形越大、保温温度越接近熔点,液相生长越明显,反之正方相生长明显.持续保温使得畸变能释放,晶粒最终完全转变为平衡正方相.模拟结果表明:预变形六角相在熔点附近保温时,由于晶界固有缺陷和预变形双重作用使得原子无序度增加,从而在晶界或其他缺陷处产生液相,待能量释放后晶粒再转变成平衡正方相,进而延缓了六角相/正方相相变时间.