Second order elastic anomalies in barium titanate from cubic to tetragonal phase transition

The anomalies of the second order elastic constants have been derived for barium titanate for the phase transition from cubic to tetragonal. The equilibrium values of the components of the order parameter and the strain variables have been obtained from the stability conditions. The fluctuations in the order parameter have been derived from the Landau-Khalatnikov equations. Expression for the shift in the zero point energy in the tetragonal phase is obtained and is shown to be proportional to (T −T _c)². The anomalies for all the second order elastic constants have been derived and relations among them reported. It is shown that the second order elastic anomalies suffer a discontinuity at the transition temperature.     

Light-induced charge transport processes in photorefractive barium titanate doped with rhodium and iron

Absorption, light-induced absorption changes, photo and dark conductivities, bulk photovoltaic currents and two-beam coupling gain coefficients are measured for oxidized, as grown and reduced BaTiO₃:Rh,Fe crystals. Theoxidized sample shows hole conductivity and three photorefractive levels are present: Rh^4+/5+ Fe^4+/5+ and Rh^3+/4+. The measurements indicate that Rh^3+/4+ is responsible for the photo conductivity and that its energy level is located (0.9 ± 0.1) eV above the valence band edge. Theas grown sample also shows hole conductivity and two photorefractive levels are involved: Rh^3+/4+ and Fe 3+/4+. The results yield that the Fe^3+/4+ level is located (0.95 ± 0.05) eV above the valence band edge. In thereduced sample electrons are the dominant charge carriers and two levels are of importance. The shallow one is located (0.39 ± 0.05) eV below the conduction band edge. Here the performance of this crystal is strongly impaired by a pronounced sublinear photo conductivity.Dedicated to O. F. Schirmer on the occasion of his 60th birthday     

Second order elastic anomalies in barium titanate from orthorhombic to rhombohedral phase transition

The anomalies in second order elastic constants have been derived for barium titanate for the phase transition from orthorhombic to rhombohedral state. The equilibrium values of order parameter, strain variables and fluctuations in order parameter have been derived using stability conditions and Landau-Khalatnikov equations respectively. Expression for shift in specific heat is obtained. All the anomalies in second order elastic constants have been derived and relations among them reported. The numerical values of anomalies in the individual constants are calculated and their variation is represented graphically. Changes in elastic constants occur over a range of temperature of the order 10⁻²K.     

掺杂钛酸钡薄膜的合成及性能研究

通过化学溶液沉积法制备的BiFeO3-BaTiO3薄膜在室温下能够同时显现铁电性及铁磁性。在600-700℃的条件下,以Pt/TiOx/SiO2/Si为载体,能够成功得到钙钛矿单相0.7BiFeO3-0.3BaTiO3薄膜。随着结晶温度上升,晶粒持续增长,最终在700℃到达更高的结晶度。由于0.7BiFeO3-0.3BaTiO3薄膜的绝缘电阻较低,它所显现的极化（P）-电场（E）磁滞回线较弱。尽管如此,由于在0.7BiFeO3-0.3BaTiO3薄膜铁的位置上添加锰,高作用场的漏电流有效地减少,最终铁电性质得到了提高。在室温下,添加了摩尔分数5%的锰的0.7BiFeO3-0.3BaTiO3薄膜同时显现铁电极化和铁磁磁化磁滞回线。     

掺杂钛酸钡薄膜的合成及性能研究

通过化学溶液沉积法制备的BiFeO3-BaTiO3薄膜在室温下能够同时显现铁电性及铁磁性。在600-700℃的条件下,以Pt/TiOx/SiO2/Si为载体,能够成功得到钙钛矿单相0.7BiFeO3-0.3BaTiO3薄膜。随着结晶温度上升,晶粒持续增长,最终在700℃到达更高的结晶度。由于0.7BiFeO3-0.3BaTiO3薄膜的绝缘电阻较低,它所显现的极化（P）-电场（E）磁滞回线较弱。尽管如此,由于在0.7BiFeO3-0.3BaTiO3薄膜铁的位置上添加锰,高作用场的漏电流有效地减少,最终铁电性质得到了提高。在室温下,添加了摩尔分数5%的锰的0.7BiFeO3-0.3BaTiO3薄膜同时显现铁电极化和铁磁磁化磁滞回线。     

掺杂钛酸钡薄膜的合成及性能研究

通过化学溶液沉积法制备的BiFeO3-BaTiO3薄膜在室温下能够同时显现铁电性及铁磁性。在600-700℃的条件下,以Pt/TiOx/SiO2/Si为载体,能够成功得到钙钛矿单相0.7BiFeO3-0.3BaTiO3薄膜。随着结晶温度上升,晶粒持续增长,最终在700℃到达更高的结晶度。由于0.7BiFeO3-0.3BaTiO3薄膜的绝缘电阻较低,它所显现的极化（P）-电场（E）磁滞回线较弱。尽管如此,由于在0.7BiFeO3-0.3BaTiO3薄膜铁的位置上添加锰,高作用场的漏电流有效地减少,最终铁电性质得到了提高。在室温下,添加了摩尔分数5%的锰的0.7BiFeO3-0.3BaTiO3薄膜同时显现铁电极化和铁磁磁化磁滞回线。     

Logarithmic temperature variations of the elastic constant of barium titanate near the ferroelectric phase transition

The temperature variations of the elastic constant C₁₁ and the corresponding acoustic attenuation coefficient of barium titanate single crystals grown by two different methods in the paraelectric phase were obtained in a wide temperature by means of Brillouin light scattering. Two phenomenological approaches were adopted to attempt to explain the anomalous change in C₁₁ (denoted as ΔC₁₁) in the paraelectric phase. ΔC₁₁ exhibited a logarithmic variation as log[(T ? T₀)/T₀] in a certain temperature range from T_c to about T_c + 80 °C instead of the other variation of (T ? T₀)^ζ predicted by the mean-field approach. This temperature range was almost the same as the range where precursor dynamics were theoretically predicted to set in. The fact that the logarithmic variation was proposed for uniaxial systems might indicate that the correlated deformation of precursor polar clusters is tetragonal, as was suggested from nuclear magnetic resonance study [7].     

Effects of oxygen deficiency on the acoustic anomalies and phase transition behaviors of barium titanate single crystals

The acoustic behaviors of oxygen-reduced barium titanate (BaTiO_3-δ) single crystals with δ∼0.04 were investigated as a function of temperature by using Brillouin spectroscopy. The longitudinal acoustic mode of the moderately-reduced BaTiO₃ (BTO) showed two pronounced anomalies at approximately 112 °C and −11 °C, which correspond to the cubic-tetragonal and tetragonal-orthorhombic phase transition temperature, respectively. These temperatures were lower by more than 10 °C compared to those of the pure BaTiO₃ suggesting that the disorder introduced by oxygen vacancies lowers the phase transition temperatures. The paraelectric phase of the reduced BaTiO₃ were characterized by substantial softening of the longitudinal acoustic mode and the growth of central peaks centered at zero frequency. These anomalies were observed in a certain temperature range above the Curie temperature, indicating that pretransitional precursor polar clusters exist in the cubic phase and that their dynamics are responsible for the acoustic anomalies caused by electrostrictive coupling between the strain and the polarization. The relaxation time of the precursor polar clusters derived from the central peak exhibited a critical slowing-down behavior showing that their dynamics becomes more sluggish as temperature approaches the Curie point.     

Dispersion of the complex conductivity in doped polycrystalline barium titanate

Possible mechanisms of polarization relaxation in barium titanate are considered. The dispersion of the dielectric permeability and conductivity is investigated. On the assumption of a single relaxation time, the dispersion coefficients are calculated. Assumptions are advanced on the development of interlayer polarization in the samples investigated.Translated from Izvestiya VUZ. Fizika, No. 6, pp. 52–56, June, 1973.     

Shock-induced phase transition and spalling characteristic in pure iron and FeMnNi alloy

This paper provides an investigation of the phase transition and spalling characteristic induced during shock loading and unloading in the pure iron and the FeMnNi alloy. The impact for the pure iron is symmetric and with same-thickness for both the flyer and the target plate. It is found that an abnormal multiple spalling happens in the pure iron sample as the pressure exceeds the $\alpha -\varepsilon $ transition threshold of 13 GPa. In the symmetric and same-thickness impact and reverse impact experiments of the FeMnNi alloy, two abnormal tension regions occur when the pressure exceeds the $\alpha -\varepsilon $ transition threshold of 6.3~GPa, and the reverse phase transition $\varepsilon -\alpha $ begins below 4.2~GP. The experimental process is simulated successfully from the non-equilibrium mixture phase and Boettger's model. Such abnormal spalling phenomena are believed to relate to the shocked $\alpha -\varepsilon $ phase transition. The possible reasons for the abnormal multiple spalling, which occurs during the symmetric and same-thickness impact experiments of pure iron and FeMnNi alloy, are discussed.     

ZnTe结构相变、电子结构和光学性质的研究

运用密度泛函理论体系下的投影缀加波方法, 对闪锌矿和朱砂相结构的ZnTe在高压下的状态方程和结构相变进行了研究, 并分析了相变前后的原胞体积、电子结构和光学性质. 结果表明: 闪锌矿结构转变为朱砂相结构的相变压力为8.6 GPa, 并没有出现类似材料高压导致的金属化现象, 而是表现出间接带隙半导体特性. 相变后, 朱砂相结构Zn和Te原子态密度分布均向低能级方向移动, 带隙变小; 轨道杂化增强, 更有利于Te 5p与Zn 3d间的电子跃迁, 介电常数虚部主峰明显增强, 但宏观介电常数不受压力的影响.     

Defect studies of doped and undoped barium titanate using computer simulation techniques

We present the results of a computer simulation study of defect energies in BaTiO₃. Our calculations are used to propose a defect model consistent with experimental studies. Intrinsic disorder is masked by oxygen vacancies which compensate accidental acceptor impurities. Holes are trapped by acceptors at high po₂ values whereas doubly ionised oxygen vacancies lead to n-type behavior at lower po₂. Whilst holes and oxygen vacancies are weakly bound by barium vacancies, titanium vacancies act as deep traps for holes, although negligible concentrations will be present in lightly doped BaTiO₃ samples. Three types of competing compensation mechanism are proposed for trivalent dopants, enabling donor ‘effectiveness’ to be predicted in satisfactory agreement with experiment. Binding energies for electron and cation vacancy trapping by donor dopants are generally calculated to be lower than those proposed previously to interpret conductivity measurements.     

Influence of 90° domain switching on the physical properties of tetragonal barium titanate single crystals

In this paper, we report the influence of 90° domain switching on the physical properties of tetragonal BaTiO₃ single crystals. It is found that the contribution of 90° domain switching to the piezoelectric response is much larger in magnitude than the contribution of the direct piezoelectric effect under large external mechanical stress. Simultaneously, the interconversion of a and c crystallographic axes and the large actuation strain as high as 1% is induced by 90° domain switching in the tetragonal BaTiO₃ single crystal.     

Dielectric properties and substitution preference of yttrium doped barium zirconium titanate ceramics

The dielectric properties of Ba(Zr_0.25Ti_0.75)O₃+xY ₂O₃ ceramics are investigated. We believe that, integrating with the lattice parameters, there is an alternation of substitution preference of yttrium ions for the host cations in perovskite lattice that is responsible for the Curie point. The T_c rises with the increase of Y ³⁺ doping when the doping content is less than 0.05 at%, owing to the replacement of Y ³⁺ ions for Ba²⁺ ions at the A-site; when the Y ³⁺ content is more than 0.05 at%, Y ³⁺ ions tend to occupy the B-site in perovskite lattice, causing a drop of T_c. Owing to the modifications of Y ³⁺ doping, the loss tangent of BZT ceramics is depressed remarkably, making it a superior candidate to replace widely used lead-contained ceramics.     

Effect of superconductivity on the cubic to tetragonal structural transition due to a two-fold degenerate electronic band

The effect of the BCS superconductivity on the cubic to tetragonal structural transition arising from a two-fold degenerate electronic band is investigated within the mean field approximation. The phase diagram of the two transitions is given for a half filled e_g-band. Modification of the two transitions when they are close together are also discussed.     

过渡金属掺杂钛酸纳米管的电子结构和光学性质研究

采用平面波超软赝势方法计算了过渡金属(TM)(Fe,Co或者Ru)掺杂钛酸纳米管的电子结构及光学性质.对TM取代钛酸纳米管层间间隙位H+的几何结构进行优化,发现掺杂对几何结构的影响较大,其中Co或者Ru掺杂的形成能均较低.此外,掺杂的TM与周围的O原子成键,有形成固熔体的趋势.掺杂后的能带结构分析表明:Fe,Co或者Ru掺杂导致钛酸纳米管禁带宽度减小并且于禁带中引入了新的能级,这主要归因于b1g(dx2-y2)及a1g(dz2)态的出现;部分杂质能级处于半填充状态,成为空穴的俘获中心,减少电子和空穴的复合;掺杂后,价带顶向低能方向移动,使价带中形成的空穴氧化性更强.最后,掺杂的钛酸盐纳米管的吸收光谱显示,Ru掺杂的钛酸纳米管导致其在可见光范围内有更强的吸收.     

Structural and electronic characteristics of pure and doped ZnO varistors

We report here the structural,surface morphology,mechanical,and current-voltage characteristics of Zn1-xMxO ceramic samples with various x and M(0.00≤x≤0.20,M=Ni,Cu).It is found that the considered dopants do not influence the well-known peaks related to the wurtzite structure of ZnO ceramics,while the shapes and the sizes of grains are clearly affected.The average crystalline diameters deduced from the SEM micrographs are between 2.06 μm and 4.8 μm for all samples.The oxygen element ratio is increased by both dopants.Interestingly,the potential barrier can be formed by adding Cu up to 0.20,while it is completely deformed by 0.025 Ni addition.The breakdown field can be enhanced up to 4138 V/cm by 0.025 Cu addition,followed by a decrease with further increase of Cu up to 0.20.On the other hand,a gradual decrease in Vickers microhardness is reported for both dopants,and the values in the Ni samples are higher compared to those in the Cu samples.The electrical conductivity is generally improved by Ni,while the addition of Cu improves it only in the over doped region(≥0.10).These results are discussed in terms of the differences of valency and ferromagnetic ordering.