钙钛矿锰氧化物(La_(1-x)Gd_x)_(0.5)Sr_(0.5)MnO_3的磁电阻效应

采用传统的高温固相法制备了多晶样品(La1-xGdx)0.5Sr0.5MnO3(x=0,0.1,0.2,0.3,0.4),利用X射线衍射仪(XRD)、超导量子磁强计(SQUID)、标准四端引线法分别对样品结构、磁性、电性以及磁电阻效应进行了研究。研究表明:Gd的少量替代并没有引起结构变化;随着Gd含量的增加,所有样品的居里温度TC和金属-绝缘体转变温度Tp都降低了;在TC附近发现了磁电阻效应,同时在低温下发现了更大的磁电阻;并且Gd的少量替代可使磁电阻MR增大。     

La0.67Ca0.33Mn1-xAlxO3体系的结构和输运特性研究

研究了Al离子的Mn位替代对巨磁阻材料La0.67Ca0.33MnO3体系的结构和输运特性的影响。结果表明,随Al3 替代量的增加,在整个替代范围内,晶胞体积表现出单调减小的规律。体系的电阻率急剧增加,绝缘体 金属转变温度TIM向低温方向移动,且与Al3 替代量存在线性关联。对少量Al3 替代量,在T>TIM的高温区域体系的输运特性满足热激活模型,在T相似文献   

YBa_2Cu_(3-x)Fe_xO_(7-δ)陶瓷样品的超声特性研究

本文研究了YBa_2Cu_(3-x)Fe_xO_(7-δ)(0≤x≤0.4)高温氧化物多晶超导体系的电阻,超声声速和超声衰减随温度的变化特性。对于超导样品,声速-温度曲线出现三次明显异常,首先是在超导转变温度附近声速呈现一极小值,表明样品中出现声子软化现象,其次分别在170K和240K附近声速发生两次大的突变,同时超声衰减曲线也相应地出现两个峰,而不超导的样品(x=0.40)只有170K和240K两处出现超声异常,而低温下无明显的声子软化现象,这说明,高温超导相变的发生会导致样品的弹性性质发生变化,而240K处体系发生的相变同高温超导电性无必然的联系,文中还初步讨论了170K超声异常的起因。     

复合氧化物LaMn1-xFexO3(x=0-1)的XPS研究

用XPS方法研究了具有ABO3结构的LaMn1-xFexO3（x＝0－1）氧化物的氧化还原性能、表面组成和吸附氧种．样品经还原和再氧化处理后，Mn2p和Fe2p结合能的变化对Fe和Mn之间发生的氧化还原提供了明显的证据．可表示如下：Me4＋＋Fe（3－δ）→Mn（4－δ）＋＋Fe3＋通过计算机用三种氧物种（OⅠ，OⅡ和OⅢ）对O1s峰进行拟合，确定了每种氧物种的状态．同时，以氧物种含量随还原、再氧化的变化，确定了发生在表面上的氧化还原反应同OⅠ和OⅡ吸附氧物种有关．在此基础上，对吸附位与氧之间的电子转移过程进行了讨论．     

(1-x)La_(0.67)Ca_(0.33)MnO_3/xCuO复合体系与La_(0.67)Ca_(0.33)Mn_(1-x)Cu_xO_3电输运行为比较

用不同实验方法制备了名义组分为(1-x)La_(0.67)Ca_(0.33)MnO_3/xCuO(LCMO/CuO)和La_(0.67)Ca_(0.33)Mn_(1-x)Cu_xO_3(LCMCO)两组样品,在宽的温度范围内研究了样品的电输运行为随Cu含量x的变化关系,发现这两组样品表现出不同的行为.对于LCMCO,随x的增加,金属-绝缘体转变温度T_p迅速降低,当x=5.5%,样品表现出绝缘体导电行为;而LCMO/CuO复合样品,当x≤6%时,随x增加,Tp逐渐下降,x≥6%时,T_p不再继续降低,所有样品几乎表现出相同的电输运行为.另外,这两组样品均表现出较好的低场磁电阻效应(LFMR),在0.3 T下样品的最大磁电阻分别达到了～76%和88%.基于样品结构以及制备过程的分析,我们认为LFMR效应的增强主要是因为颗粒边界上形成的Cu相关自旋无序层引起的.     

MMgF4(M=Ca,Sr, Ba)体系中Eu和Tb的电荷迁移

于不同气流中, 合成了MMgF4:xEu,yTb复合氟化物磷光体。该体系中Eu^3^+和Eu^2^+共存。Tb的存在影响Eu的价态存在形式。ESR测试表明, 随Tb的掺入浓度增加, Eu^2^+的浓度呈规律性变化。随Eu的掺入, 样品的XPS谱中出现了四价铽的Tb3d5/2特征伴峰。认为Eu^3^+和Tb^3^+之间存在电荷迁移平衡。即Eu^3^++Tb^3^+=Eu^2^++Tb^4^+。通过半定量手段研究了SrMgF4中这一衡的平衡常数。     

LiNi1-xAlxO2(x=0～1.0)固溶体结构的XRD研究

利用溶胶凝胶法制备了新型锂离子二次电池正极材料LiNi_1-xAl_xO₂(x=0,0.1～1.0)固溶体,X射线衍射结果表明,在0≤x≤1范围内,材料均为具有α-NaFeO₂型结构的LiNi_1-xAl_xO₂固溶体单相.随着Al固溶量的增加,晶胞参数发生变化,a轴缩短,c/a比增大,晶胞体积V₀减小,材料的层状属性更加明显.     

La0.7-xDyxSr0.3MnO3(0.00≤x≤0.30)体系低温磁行为和电阻率的反常

By measuring M-T curves, ρ-T curves and MR-T curves of the samples under different temperatures, the influence of Dy doping (0.00 ≤ x ≤0.30) on the magnetic and electric properties of La0.7-xDyxSr0.3MnO3 has been studied. The experimental results show that, with the increase of the Dy content, the system undergoes a transition from long range ferromagnetic order to the cluster-spin glass state and further to antiferromagnetic order. For the samples with x=0.20 and 0.30, their magnetic behaviors are abnormal at low temperature, and their resistivities at low temperature have a minimum value. These peculiar phenomena not only come from the lattice effect induced by doping, but also from extra magnetic coupling induced by doping.     

[Mn(phen)3](ClO4)2(H2O)·0.5(azpy)的合成和晶体结构

锰在生物体系的新陈代谢过程中有重要作用。锰有机配合物的研究成为生物无机化学研究领域的一个热点[1]。本文报道用高氯酸锰、1,10 邻菲咯啉和4,4 偶氮联吡啶[2 4]合成的锰配合物[Mn(phen)3](ClO4)2(H2O)·0 5(azpy)的晶体结构。1　实验部分1 1　Mn(Ⅱ)配合物的合成将0 0724g(0 2mmol)Mn(ClO4)2·6H2O溶于8mL水,缓慢加入含有0 1190g(0 6mmol)1,10 邻菲咯啉(phen)和0 0277g(0 15mmol)4,4′ 偶氮联吡啶(azpy)的15mL甲醇溶液,搅拌0 5h,室温下静置,一周后得[Mn(phen)3](ClO4)2(H2O)·0 5(azpy)晶体。元素分析(计算值)/%:C54 19(54 …     

La0.5>RE0.3Sr0.2FeO3－δ (RE = Nd、Ce、Sm)体系双稀土阴极材料的制备与电性能

采用甘氨酸-硝酸盐法(GNP)合成了La0.5RE0.3Sr0.2FeO3－δ(RE=Nd、Ce、Sm)系列复合氧化物粉体. 用X射线衍射(XRD)和TG-DSC分析了样品钙钛矿物相的形成过程, 用Archimedes排水法测量体积密度并计算烧结样品的相对密度, 用四端子技术测量电导率. 结果显示, 掺Nd的样品1200 ℃烧结2 h成为单一立方钙钛矿结构, 掺Ce样品有明显的CeO2立方相析出, 掺Sm样品主相为钙钛矿结构伴有微弱的杂峰. 1250 ℃烧结2 h的La0.5Nd0.3Sr0.2FeO3－δ在600 ℃时电导率高达100 S•cm-1以上, 明显高于La0.5Ce0.3Sr0.2FeO3－δ及La0.5Sm0.3Sr0.2FeO3－δ样品的电导率, 预示着La0.5Nd0.3Sr0.2FeO3－δ可能是一种良好的中温固体氧化物燃料电池(SOFC)阴极材料.     

Ultrasonic Study on Jahn-Teller Distortions in La1=3Sr2=3CoO3

The longitudinal ultrasonic velocity (Vl), attenuation (ffl), magnetization and resistivity of single phase polycrystalline La1=3Sr2=3CoO3 were measured as a function of temperature from 20 K to 300 K. The resistivity shows metallic behavior in the whole temperature range and a kink at 235 K was observed, which coincides with the ferromagnetic transition temperature (Tc). As the temperature cools down from Tc, the Vl softens conspicuously at beginning and reaches a minimum at 120 K. After that the Vl dramatically stiffens below 120 K accompanied by a wide attenuation peak. The analysis of the results suggests that these ultrasonic anomalies may correspond to local lattice distortions via the Jahn-Teller effect of intermediate spin Co3+.     

Photoluminescence Study on Charge Transfer Behavior of Poly(3‐hexylthiophene) and PCBM Blends

Charge transfer behavior of Poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl eser (PCBM) in solutions and in films were examined by photoluminescence (PL) spectroscopy. PL study in solutions indicated that separation distance between P3HT and PCBM affected charge transfer efficiency more seriously than the interface area issue between P3HT and PCBM. P3HT/PCBM film showed very effective photo‐induced charge transfer before post‐thermal annealing on the bi‐layer P3HT/PCBM film. Charge transfer efficiency was gradually diminished by the annealing‐induced phase separation between P3HT and PCBM as revealed by increasing PL emission intensity of P3HT.     

Hydrothermal Synthesis and Magnetic Properties ofSmCr0.5M0.5O3(M=Fe and Mn) Micro-plates

Nanostructured B-site Fe and Mn doped SmCrO₃ was prepared by mild hydrothermal growth. The as-prepared crystals are mainly micrometer-scale plates, ranging from rhombus(SmCr_0.5Fe_0.5O₃) to elongated he- xagonal(SmCr_0.5Mn_0.5O₃), and finally to well-edged rectangular(SmCr_0.17Mn_0.5Fe_0.33O₃) plates. Fe and Mn doped SmCrO₃ crystals are indexed into Pbnm space group. The cell parameters of SmCr_0.5Fe_0.5O₃ are slightly smaller than that of pristine SmCrO₃. Binding energy analysis of Cr, Mn and Fe in SmCr_0.17Mn_0.5Fe_0.33O₃ sample indicates that they all possess +3 oxidation states. Temperature dependent magnetization of the as-prepared samples presents obviously stronger ferromagnetic interactions than the undoped counterparts. This work represents a remarkable development for hydrothermal synthesis into fabricating perovskite oxide crystals with uniform distribution of doping ions.     

Structure, magnetism and transport of the perovskite manganites Ln0.5Ca0.5MnO3 (Ln=Ho, Er, Tm, Yb and Lu)

It was found that the manganese perovskite oxides Ln_0.5Ca_0.5MnO₃ (Ln=Ho, Er, Tm, Yb and Lu) have an orthorhombic structure (space group Pnma). The Mn-O-Mn angles were calculated to be ∼148-150°, revealing an existence of a large crystallographic distortion in these oxides. Electrical resistivity measurements indicated both an insulating nature and a small magnetoresistance effect, both of which are owing to narrow bandwidths of the Mn-3d electrons arising from the crystallographic distortion. DC magnetization measurements showed the three characteristic temperatures, which could be assigned to charge-order, antiferromagnetism of Mn moments, and possible glassy states. All of these temperatures were decreased for the heavier Ln ions, which is explained in connection with both a difference of ionic radii of Ln³⁺ and Ca²⁺, and a lowering of electron transfer. The charge-ordering transition was not clearly observed only for Lu_0.5Ca_0.5MnO₃ containing the smallest lanthanide ion, plausibly due to a large randomness of magnetic interactions arising from the ionic radii difference of Lu³⁺ and Ca²⁺. In addition, preliminary measurements of AC dielectric response suggested that these manganites belong to a so-called multiferroic system.     

X-ray-induced persistent photoconductivity in La0.9A0.1MnO3 (A=Sr, Ca)

We observed persistent photoconductivity (PPC) in La-based perovskite manganese oxides, La_0.9Sr_0.1MnO₃ (LSMO) and La_0.9Ca_0.1MnO₃ (LCMO), under X-ray irradiation. The PPC in LSMO was attributed to the collapse of the charge- and orbital-ordered (CO and OO, respectively) states, similar to that reported in previous papers. This PPC effect was different from that observed in a similar compound with a slightly different composition. This difference was explained in terms of doped carriers. This observation of PPC in LCMO is the first result of the PPC in a compound in which the ground state is not the CO phase; it was attributed to the collapse of the OO ground state. We proposed that OO ground state is a prerequisite for the occurrence of PPC in these compounds.     

Electrical conduction nature and phase transition in CaTi1−x FexO3−δ (x = 0.1–0.5)

Transport numbers for oxygen ions and protons are measured by an emf method in the system CaTi_1?x Fe_xO_3?δ (x = 0.1–0.5) in the oxidizing and reducing atmospheres in the temperature interval 973–1173 K. It is shown that the compounds under study are mixed ion-electron conductors at small iron concentrations and electron conductors, at large iron contents. The proton conductivity in the compounds is very poor and does not exceed 0.5% in air. On the basis of the temperature dependences of transport numbers for ions and linear expansion, it is established that the CaTi_0.9Fe_0.1O_3?δ system has a phase transition of a second order in a reducing environment at 1020–1050 K. The total and partial electron conductivities of CaTi_0.9Fe_0.1O_3?δ are studied as a function of the partial pressure of oxygen at 1173 K. The nature of electroconduction in CaTi_1?x Fe_xO_3?δ is discussed.     

A Fe Mössbauer study of charge ordering and phase separation in the rare-earth manganates, Nd0.5Ca0.5MnO3 and Nd0.5Sr0.5MnO3

Mössbauer studies of 2% ⁵⁷Fe-doped Nd_0.5Ca_0.5MnO₃ and Nd_0.5Sr_0.5MnO₃ have been carried out over the 4.2-300 K range after ensuring that such doping does not change their basic properties. The charge-ordering transition in these manganates is marked by abrupt changes in the quadrupole splitting. In the case of Nd_0.5Ca_0.5MnO₃, two phases manifest themselves on cooling below the charge-ordering transition temperature. The evolution of the spectra as a function of temperature shows that long-range magnetic order does not occur sharply. The observed evolution with temperature is different in the two materials studied. In Nd_0.5Ca_0.5Mn_0.98⁵⁷Fe_0.02O₃, it resembles that of a disordered magnetic material, whereas the temperature dependence of line shape of Nd_0.5Sr_0.5Mn_0.98⁵⁷Fe_0.02O₃ is typical of a superparamagnetically relaxed magnetic system. Although both the manganates show well-resolved magnetic hyperfine spectra at 4.2 K, the lines are slightly broad indicating possible coexistence of phases at low temperatures. A weak paramagnetic signal is also seen in the spectra of both the manganates at 4.2 K.     

Crystal structure of Ln1/3NbO3 (Ln=Nd, Pr) and phase transition in Nd1/3NbO3

The crystal structure of the A-site deficient perovskite Ln_1/3NbO₃ (Ln=Nd, Pr) at room temperature has been determined, for the first time, as orthorhombic in space group Cmmm using high-resolution neutron powder diffraction. Pertinent features are the alternation of unoccupied layers of A-sites and layers partly occupied by Ln cations, as well as out-of-phase tilting of the NbO₆ octahedra around an axis perpendicular to the direction of the cation/vacancy ordering. The phase transition behaviour of Nd_1/3NbO₃ has also been studied in situ. This compound undergoes a continuous phase transition at around 650 °C to a tetragonal structure in space group P4/mmm due to the disappearance of the octahedral tilting. The analysis of spontaneous strains shows that this phase transition is tricritical in nature.     

Hydrothermal Synthesis of Pb_(1-1.5x)La_xZr_(0.5)Ti_(0.5)O_3 (x = 0.01-0.06) Nanocrystallites

Lanthanum doped lead zirconate titanate (PLZT) ceramics display excellent electrooptic and photostriction properties because of the existence of cavities in the perovskite structure1-3. The PLZT powders are conventionally prepared by solid state reaction4-5, and the wet chemical methods such as sol-gel techniques are then introduced6-7. However, the homogeneity, morphology and size of the particles, which greatly affect the sinterability and the property of the resulting ceramics, are dif…     

Electronic Structures of LixV2O5 (x=0.5 and 1): A Theoretical Study

The electronic properties of α‐Li_xV₂O₅ (x=0.5 and 1) are investigated using first principle calculations based on density functional theory with local density approximation. Different intercalation sites for Li in the V₂O₅ lattices are considered, showing different influences on the electronic structures of Li_xV₂O₅. The lowest total energy is found when Li is only intercalated along the c axis between two bridging oxygen ions of sequential V₂O₅ layers. The intercalation of Li into V₂O₅ does not change the electron transition property of V₂O₅, which is an indirect band gap semiconductor, but leads to a reduction of vanadium ions and an increase of the Fermi level of Li_xV₂O₅ arising from the electron transfer from the Li 2 s orbital to the initially empty conduction band of the V₂O₅ host.