纳米晶钛酸钡的Sol-gel法制备及其尺寸效应

采用sol-gel法制备了不同粒径的钛酸钡（BT）纳米晶粉. XRD、Raman光谱和DSC测试结果显示,随着退火温度的升高，晶粒长大，晶胞的a轴逐渐减小， c轴逐渐增大.粒径在54 nm左右时，钛酸钡由顺电立方相向铁电四方相结构转变，在立方-四方相变过程中，晶胞略微膨胀.随着粒径的减小，正交-四方相变温度升高，四方-立方相变温度降低， Raman谱峰降低和宽化，粒径为38 nm左右时四方相的特征峰消失.     

弛豫基铁电单晶0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3的制备与 性能研究

用改进的Bridgman法,在加入助熔剂的条件下,生长出最大尺寸为φ30×25mm^3的铌锌酸铅-钛酸铅固溶体单晶[0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3]。无宏观缺陷的晶片的典型尺寸为20×15mm^2。晶体的单晶性及其结构用X射线衍射法加以研究。所得晶体用Laue衍射法定向,取(001)晶片进行性能表征。研究了材料的介电性能,并用偏光显微镜观察了(001)晶片的电畴结构。结果表明,室温下材料的介电常数较大,为2500～5000。随着温度的升高,材料发生四方铁电相—立方顺电相的相变,相变温度为190℃左右。单晶的介电温谱呈现明显的频率色钐现象,同时,介电常数最大值的温度tm随着频率的升高而降低,观察到了电畴结构的不均匀与孪生现象。     

亚微米级四方相钛酸钡低温固相法制备及晶型控制

以八水合氢氧化钡和α-钛酸为原料,在相对低的焙烧温度下,制备出近似球形、亚微米级的钛酸钡。通过XRD、SEM、Raman和FTIR等手段对钛酸钡样品进行表征,样品具有高结晶度,颗粒均一性良好。晶型转变的初步探究表明,立方相为主的钛酸钡可以在400℃发生晶相转变,成为以四方相为主的钛酸钡。     

Pb_(1-x)La_xTi_(1-x/4)O_3纳米晶的结构

纳米材料的表面效应和尺寸效应能够使材料性质发生显著变化 .铁电纳米晶体是制备铁电纳米陶瓷和铁电复合材料的基础 ,薄膜材料亦可看作零维纳米材料在二维平面上扩展生长形成的 ,因此铁电纳米晶体结构研究具有重要意义 .对于纯ＰｂＴｉＯ3纳米晶 ,铁电 顺电相变温度随粒径减小而降低 ,并求得了室温下铁电 顺电相变消失、粒子成为顺电体的临界晶粒尺寸[1,2 ].近年来 ,有关Ｌａ改性钛酸铅 (ＰＬＴ)薄膜和纳米粉制备、结构和性质的研究引起了各国学者的重视[3～ 6 ],但对于其微观结构 ,缺乏系统的研究 .我们研究了ＰＬＴ纳米晶的结构和临界晶粒…     

钛酸铅与偏氟乙烯-三氟乙烯共聚物形成的铁电复合物相变过程的热释电研究

研究了偏氟乙烯-三氟乙烯共聚物与钛酸铅形成的铁电复合物PT/P[VDF(70)-TrFE(30)]的热释电流行为。P[VDF(70)-TrFE(30)]室温下存在两个铁电相,即较无序的铁电相和较有序的铁电相,升温经过各自的相转变点后转变成较有序的顺电相和较无序的顺电相。热释电流谱上出现在95℃和108℃的两个电流峰,分别由两个铁电相的结构陷阱以及部分取向的偶极所贡献。采用Tc以上温度极化并冷却到不同温度的方法可明确鉴别出该试样两个顺电相降温转变过程,转变点分别为62℃和50℃。实验证实陷阱仅存在于铁电相而不存在于顺电相中。钛酸铅的引入虽不导致新电流峰出现,但可使共聚物的Tc降低、电流加大。热释电流方法可非常灵敏地跟踪铁电共聚物的相变过程。     

Li+-CaxPb1-xTiO3湿敏纳米薄膜的激光拉曼光谱、晶格畸变和激光显微组织--应用"软模理论"解谱

测定了标题物Li^ -CaxPb1-xTiO3（缩写为Li-CPT,其中x=0．35,Li/Ti=1．0％,mol／mol,850℃／h)的激光拉曼光谱(LRS)和激光显微组织,并与湿敏性能极差的纯钛酸铅PbTiO3(缩写为PT)以及不同钙掺杂量的钛酸铅CaxPb1-xTiO3(缩写为CPT,x=0．08,0．16,0．24,0.32,0．40)的LRS进行比较,Li-CPT以及CPT的LRS与PT的LRS相比有很大差别,仅在75,125,190,273和585cm^-1出现五个谱峰．应用“软模理论”将其分别归属于PT四方晶相的五个特征单模E（ITO),A1(ITO),E(2TO),B1 E和A1(3TO)的红移及宽化,PT的其它五个特征单模在Li-CPT及CPT的谱图中完全消失或被淹没,对于这种拉曼活性的降低,可以根据“软模理论”与晶格畸变作用的关联加以解释,Pauling离子半径较小的Ca^2 的掺杂,等价取代了半径较大的Pb^2 的晶格位置,降低了四方晶相畸变度δ=c／a和不对称性导致的各向异性,不单消除了薄膜冷却时位移型相变可能引起的破裂,而且改变了晶格中氧八面体“TiO6”及十二面体“PbO12”的几何结构和电荷分布,改变了晶格c轴上M-O(M-Ti,Pb)键的化学环境,直接阻尼了M-O键的伸缩振动软模,减小了电偶矩及其自发极化,使Li-CPT的振动模的拉曼活性大大降低,因此LRS提供了一种表征晶格畸变的有效方法。反过来也佐证了“软模理论”可以用来解释拉曼光谱。     

氢对PbZr0．5Ti0．5O3在氮氢混合气氛退火中铁电性能的影响

采用基于密度泛函理论的第一性原理,针对PbZr0.5Ti0.5O3无氧和含氢的顺电相和铁电相的二层超晶胞,分别计算了 Ti 沿 c 轴位移时体系总能量的变化、电子云密度分布和Ti-O、Zr-O和H-O的重叠布居数.结果表明,含氢铁电相的Ti-O键和Zr-O 键相埘无氢铁电相明显减弱,氢氧之间较强的轨道杂化使它们趋于形成共价键;晶格中氧氧键的钉扎效应使含氢情况下的顺电相能昔始终低于铁电相能量,说明氢的引入阻碍了PbZr0.5Ti0.5O3从立方顺电相到四方铁电相的相变,并推断其为含氧气氛退火过程中PbZr0.5Ti0.5O3铁电性能下降的主要原因之一.所得结果对于深入理解铁电材料在氮氢混合气氛退火后铁电件能下降的微观机制具有参考价值.     

溶胶-凝胶模板法制备钛酸钡纳米管

采用氢氧化钡和钛酸四丁酯为前驱体, 溶胶-凝胶结合模板法制备了钛酸钡纳米管, 通过SEM, TEM, XPS和XRD等表征分析, 钛酸钡纳米管直径100 nm, 钛酸钡为立方相结构, 晶胞参数a＝3.995 Å.     

氢对PbZr0.5Ti0.5O3在氮氢混合气氛退火中铁电性能的影响

采用基于密度泛函理论的第一性原理, 针对PbZr0.5Ti0.5O3无氢和含氢的顺电相和铁电相的二层超晶胞, 分别计算了Ti沿c轴位移时体系总能量的变化、电子云密度分布和Ti—O、Zr—O和H—O的重叠布居数. 结果表明, 含氢铁电相的Ti—O键和Zr—O键相对无氢铁电相明显减弱, 氢氧之间较强的轨道杂化使它们趋于形成共价键; 晶格中氢氧键的钉扎效应使含氢情况下的顺电相能量始终低于铁电相能量, 说明氢的引入阻碍了PbZr0.5Ti0.5O3从立方顺电相到四方铁电相的相变, 并推断其为含氢气氛退火过程中PbZr0.5Ti0.5O3铁电性能下降的主要原因之一. 所得结果对于深入理解铁电材料在氮氢混合气氛退火后铁电性能下降的微观机制具有参考价值.     

超微粒子钛酸铅高温相变的X射线衍射分析

用Ｘ射线衍射仪及其原位高温装置对钛酸铅超微粒子在高温下的铁电相变点进行了测定，并计算了在高温下它的晶胞参数。结果表明：随着温度逐渐升高，钛酸铅晶粒逐步长大，同时它的晶体结构由四方体逐渐向立方体转变，相变点与晶粒尺寸的大小有关，在一定范围内，晶粒尺寸越小相变点越低。当晶粒大小为９８８ｎｍ时，相变点为４９０℃，而当晶粒大小为６１３ｎｍ时，相变点降低为４７５℃。点阵类型为简单立方体，晶胞参数ａ＝ｂ＝ｃ＝０３９６５ｎｍ，空间群为Ｐｍ３ｍ，一个钛酸铅分子占有一个晶胞（Ｚ＝ｌ）。     

BaTiO3的电子结构和光学性质

采用第一性原理赝势平面波方法, 在局域密度近似(LDA)和广义梯度近似(GGA)下分别计算了BaTiO3立方相和四方相的电子结构, 并在局域密度近似下计算了BaTiO3立方相的光学性质. 结果表明, BaTiO3立方相和四方相都为间接带隙, 方向分别为Γ-M和Γ-X, 大小分别为2.02和2.20 eV. 对BaTiO3和PbTiO3铁电相短键上电子布居数的对比分析, 给出了它们铁电性大小的差别. 且在30 eV的能量范围内研究了BaTiO3 的介电函数、吸收系数、折射系数、湮灭系数、反射系数和能量损失系数等光学性质,并基于电子能带结构对光学性质进行了解释. 计算结果与实验数据相符合.     

Vibrational spectra of [(CH3)3NH]3Sb2Cl9, [(CH3)3NH]3Bi2Cl9 and their mixed crystals

The IR and Raman spectra of [(CH₃)₃NH]₃Sb₂Cl₉ (A), [(CH₃)₃NH]₃Bi₂Cl₉ (B) and two of their mixed crystals containing respectively 33% (AB.33) and 42% Bi (AB.42) are analyzed and compared. A and AB.33 show ferroelectric–paraelectric phase transition at 364 K and 344 K, respectively. AB.42 and B are paraelectric in the temperature range between 90 and 365 K. Most of the vibrational modes show continuous changes, with the temperature, in the IR frequencies or intensities with no soft mode behavior. However, characteristic ν(NHCl) and δ(NHCl) vibrations of weakly hydrogen-bonded species are only observed in A and AB.33 below the temperature of the phase transition and are related to the ferroelectricity. The evolution of the IR spectra with the temperature suggests that the ferroelectric properties are connected with the reorientation of the cations which needs a breaking of the weak NHCl hydrogen bonds in the paraelectric phase.     

水热重结晶法制备四方相纳米Ba0.9Ca0.1TiO3晶体的研究

采用水热重结晶法对低温液相法制备的Ba_0.9Ca_0.1TiO₃(B_0.9C_0.1T)浆料进行220 ℃, 3 h的水热处理, 成功制备了四方相B_0.9C_0.1T纳米粉体. 并对水热重结晶和未水热处理的两种粉体进行了TEM, XRD, Raman, FT-IR, TG和介电性能的表征. 研究表明, 经水热重结晶后的B_0.9C_0.1T粉体体系发生从顺电立方相向铁电四方相的转变, 羟基缺陷以及CO₃^2－杂质显著减少, 粉体形貌由外边缘毛躁的球形转变为四边形. 并发现羟基缺陷的减少是影响体系相转变的关键因素.     

复合多铁材料NiFe2O4/BaTiO3的制备及性能

采用溶胶-凝胶与固相反应相结合的方法制备了xNiFe2O4/(1-x)BaTiO3(x=0.1,0.2,0.3,0.4,0.5,0.6)系列复合多铁材料.X射线衍射(XRD)结果表明,复合材料中只含有钙钛矿结构的BaTiO3和尖晶石结构的NiFe2O4,说明共烧过程中两者未发生明显的化学反应,铁电相与铁磁相共存.扫描电子显微镜(SEM)观测结果表明材料内部是异质结构的,高分辨透射电子显微镜(HRTEM)观测结果进一步说明了NiFe2 O4和BaTiO3共存,并且在两种物质的接触处能够看到清晰的界面.这种由BaTiO3和NiFe2 O4组成的复合材料对外同时表现出铁电性和铁磁性.电滞回线结果表明,该复合材料具有铁电性,但存在着一定的漏电.介电频谱表明材料的介电常数随着频率的升高而下降,在低频下达到定值,并且铁磁相的含量对材料的介电性有影响.磁性能测试结果表明材料的磁性源于NiFe2O4,并且磁性随着NiFe2O4含量的增加而增强.     

Phase Diagram and Phase Transitions in the Relaxor Ferroelectric Pb(Fe2/3W1/3)O3-PbTiO3 System

A complete solid solution between relaxor ferroelectric Pb(Fe_2/3W_1/3)O₃ (PFW) and ferroelectric PbTiO₃ (PT), (1−x)PFW-xPT, was synthesized by a B-site precursor method and characterized by X-ray diffraction, differential scanning calorimetry, and dielectric measurements. A phase diagram between PFW and PT has been established. The diffuse phase transition temperature (T_max≈180 K) of PFW was found to continuously increase with the increasing amount of Ti⁴⁺ ions on the B-site. At the same time, the relaxor ferroelectric behavior of PFW is gradually transformed toward a normal ferroelectric state, as evidenced by sharp and nondispersive peaks of dielectric permittivity around T_C for x≥0.25. At room temperature, a transition from a cubic to a tetragonal phase takes place with x increased up to 0.25. A morphotropic phase boundary is located within the composition interval 0.25≤x≤0.35, which separates a pseudocubic (rhombohedral) phase from a tetragonal phase.     

Pressure-induced phase transitions in PbTiO3: a query for the polarization rotation theory

Our first-principles computations show that the ground state of PbTiO3 under hydrostatic pressure transforms discontinuously from P4mm to R3c at 9 GPa. Spontaneous polarization decreases with increasing pressure so that the R3c phase transforms to the centrosymmetric Rc phase at around 27 GPa. The first-order phase transition between the tetragonal and rhombohedral phases is exceptional since there is no evidence for a bridging phase. The essential feature of the R3c and Rc phases is that they allow the oxygen octahedron to increase its volume VB at the expense of the cuboctahedral volume VA around a Pb ion. This is further supported by the fact that neither the R3m nor Cm phase, which keep the VA/VB ratio constant, is a ground state within the pressure range between 0 and 40 GPa. Thus, tetragonal strain is dominant up to 9 GPa, whereas at higher pressures, efficient compression through oxygen octahedra tilting plays the central role for PbTiO3. Previously predicted pressure induced colossal enhancement of piezoelectricity in PbTiO3 corresponds to unstable Cm and R3m phases. This suggests that the phase instability, in contrast to the polarization rotation, is responsible for the large piezoelectric properties observed in systems like Pb(Zr,Ti)O3 in the vicinity of the morphotropic phase boundary.     

新钽酸盐Ba3TiTa4O15的结构与介电性能

在BaO Ta2O5系统通过掺Ti4＋合成了新钽酸盐Ba3TiTa4O15,用粉晶XRD对其结构进行了分析,并测试了其烧结体的介电特性.结果表明:Ba3TiTa4O15在室温下属于填满型四方钨青铜结构,晶胞参数a=1.256 27(6)nm, c=0.395 86(4)nm; Ba3TiTa4O15在－35 ℃从铁电相转变为顺电相.     

Cell Volume Effect on the Ferroelectric Stability of Perovskite Oxides PbTiO3 and BaTiO3 from First Principle Calculation

Electronic structure of ferroelectric PbTiO3 and BaTiO3 is calculated by the full potential linearized augmented plane wave method. The total energy as a function of the displacement of Ti-cation is obtained for PbTiO3 and BaTiO3 at different cell volumes. At experimental cell volume, Ti-displacement lowers the total energy and the ferroelectricity is stable. When the cell volume is reduced to 90%, total energy is increased with Ti-displacement and ferroelectricity will disappear. The cell volume effect is also confirmed by comparison of the density of states of Ti and O at different cell volumes.     

Vibrations and dissociation of molecules in strong electric fields: N2, NACl, H2O and SF6

The static deformations and vibrations of the title compounds with an applied static electric field have been studied by density functional theory calculations. It is found that for diatomic molecules, bond length and vibrational frequency as a function of the field can be fit very well all the way up to the dissociation limit by an analytical formula derived from a Morse potential model including an additional external field term. Polyatomic molecules show more complex behaviour with a single mode becoming soft at the dissociation limit. The frequency of the soft mode near the critical field is again described well by the analytical model. The vibrational analysis shows that in polyatomic molecules dissociation proceeds as a heterolytic fragmentation process, which can break the symmetry of the molecule in the applied field.