Structural, magnetic and transport properties of double perovskite compounds (Sr2−3xLa2xBax)FeMoO6

The crystal structure and magnetic properties of a series of ordered double perovskite oxides (Sr_2−3xLa_2xBa_x)FeMoO₆ (0x0.3) have been investigated. X-ray powder diffraction reveals that the crystal structure of the compounds changes from a tetragonal I4/m lattice to a cubic lattice around x=0.2. Though the nominal average size of the A site cation of (Sr_2−3xLa_2xBa_x)FeMoO₆ is designed to be almost independent of x, the refinements of the crystal structure show that the lattice constants increase with x in both the tetragonal and the cubic phase regions due to electron doping. As the x increases, the degree of cationic ordering on the B site is decreased pronouncedly, while the Curie temperature of the compounds is nearly unchanged. The saturation magnetization of the compounds decreases with x and shows a linear dependence on the degree of cation ordering. The resistivity of the parent compound shows a semiconducting behavior below room temperature, but those of the doped samples exhibit a metal–semiconductor transition. A correlation between the resistivity and metal-semiconducting transition temperature (T_M−S) is observed. The resistivity and T_M−S of the compounds decrease with x for x0.2 and increase for x0.2. Magnetoresistance of the compounds is reduced by the La/Ba doping. All these observations can be understood based on the interplay of the electron doping, change in bandwidth and the anti-site defect concentration.     

Electromagnetic Properties of Lanthanum Manganese Oxides Doped with Iron

The mechanism of GMR effects in La(Mn_1–x Fe _x )O₃ (x=0, 0.01, 0.03) oxides has been investigated by means of magnetic measurements, resistivity measurements in magnetic field and Mössbauer spectroscopy. For the x=0.01 sample, the GMR behavior is similar to that of the conventional (La, A)MnO₃ (A: divalent cations) oxides. For the x=0.03 sample, even though the temperature dependence of the Mössbauer spectra was similar to that of the x=0.01 sample, the mechanism for the occurrence of the GMR effect seems to be different. The x=0 sample shows a similar behavior to the x=0.03 sample. A thermal stabilization of spin fluctuation is proposed for the x=0 and x=0.03 sample.     

用XRD和TPR技术考察制备方法和灼烧温度对钙钛矿型结构形成以及性质的影响

本文用柠檬酸络合物分解法、碳酸盐共沉淀分解法、硝酸盐直接分解法和陶瓷法合成了轻、重稀土钙钛矿型氧化物LnMnO_3(Ln=Sm.Er).用X-射线衍射(XRD)和程序升温还原(TPR)技术考察了制备方法和灼烧温度对钙钛矿型结构形成的影响.实验结果表明,用柠檬酸络合物分解法可以在较低的温度下得到单一相的钙钛矿氧化物,而陶瓷法的灼烧温度比其他方法所需的温度要高得多。不同方法得到的产物SmMmO_3对CO氧化反应的活性为:柠檬酸法～碳酸盐法～硝酸盐法>陶瓷法。实验表明,TPR技术在某些体系中可作为灵敏的物相分析辅助手段,它可在制备或反应过程中原位观察体系结构的变化,实验中观察到,稀土氧化物能促进某些过渡金属氧化物在稀土氧化物表面分散,能阻止它们形成晶态.本文还讨论了钙钛矿型氧化物的形成机理,提出了过渡金属氧化物的扩散是形成钙钛矿结构反应速率的决定步骤观点。     

Effect of Pr-Ca substitution on the transport and magnetic behavior of LaMnO3 perovskite

The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO₃ perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good example of these properties is (La_1−2x Pr _x Ca _x )MnO₃ (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the T _c varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for understanding the structural, transport and magnetic properties are discussed in detail.     

双层钙钛矿La1.8Ca1.2Mn2O7磁性相关I-V非线性与电输运性质

采用传统固相反应法制备了双层钙钛矿结构锰氧化物La_1.8Ca_1.2Mn₂O₇陶瓷,并用X射线粉末衍射法,扫描电镜,HL5500PC Hall效应分析仪和综合物性测量系统(PPMS)对其磁、电性质进行了表征.结果表明:经过两次高温烧结可合成具有双层Sr₃Ti₂O₇型四方结构的La_1.8Ca_1.2Mn_2<     

V替代Mn对La_(0.45)Ca_(0.55)MnO_3电荷有序相及自旋玻璃态的影响

用固相反应法制备了La0.45Ca0.55Mn1-xVxO3(x=0.00,0.10)多晶样品.通过X射线衍射谱、质量磁化强度-温度曲线、电子自旋共振谱,研究了V5+替代Mn3+/Mn4+对La0.45Ca0.55MnO3电荷有序相和自旋玻璃态的影响.实验结果表明,当x=0.10时,不仅母体的电荷有序相基本破坏,而且母体在40K左右出现的自旋玻璃态也被融化.电荷有序相被破坏的主要原因是用V5+替代Mn3+/Mn4+后,增加了Mn3+与Mn4+的比例,使铁磁双交换作用优于反铁磁超交换作用;自旋玻璃态的融化是由于V替代Mn后破坏了反铁磁背景下有少量铁磁成分的自旋玻璃态的形成条件.     

Effect of citric acid concentration as emulsifier on perovskite phase formation of nano‐sized SrMnO3 and SrCoO3 samples

In this study the effects of citric acid concentration, used as organic emulsifier, on the perovskite phase formation of the nano strontium manganite or cobaltite samples were studied. Stoichiometric perovskites in the absence and presence of citric acid were prepared by drying a solution containing molar ratio of Sr(NO₃)₂/Mn(NO₃)₂·4H₂O and Sr(NO₃)₂/Co(NO₃)₂·6H₂O = 1 followed by calcination at 900 °C for 5 h. Citric acid concentration, selected to be 0.0, 0.3, 0.6, 1.0, 1.3, 2.5 and 5 times of the total number of moles of the nitrate ions. The results revealed that increase in the citric acid concentration, larger than number of moles of the nitrate ions equivalent, deteriorates the perovskite phase formation. Instead, a new phase of carbonates and metal oxides are appeared. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Critical Points and Re-Entrance in Liquid Crystalline Side-Chain Polymers

Competing nematic and smectic A phases of comb-like polymers are described by combining Maier-Saupe and McMillan type theories with the worm concept of semi-flexible polymers. There are as few material parameters as possible: main chain flexibility and phenomenological coupling constants. New qualitative predictions, the existence of critical points and re-entrant nematic phases, as well as quantitative predictions for comparison with experiments are provided. A new mechanism for thermotropic nematic biaxiality which is specific of side-chain polymeric liquid crystals is found.     

Temperature profiles in a two-component heterogeneous system heated with a cw laser

A formalism has been developed to calculate the steady-state temperature profiles induced by a cw laser in a composite system comprising a thick medium covered with a film of another medium. The analysis is carried out assuming that thermal and optical properties of the media are temperature-independent. A concept of the effective thermal conductivity for the system is introduced in terms of the maximum temperature rise. Temperature-distributions are numerically computed in typical cases for a circular Gaussian beam to illustrate dependence on film thickness, thermal conductivities and absorption coefficients of the media and size of the laser beam.     

Structural stability,phase transition and dielectric properties of Pb(Mg1/2W1/2)1−xMxO3 (M = Zr,Sn, Ti) ceramics

ABSTRACT

Pb(Mg_1/2W_1/2)_1?xM_xO₃ (M?=?Zr,Sn,Ti) ceramics have been prepared by the conventional ceramic process. Their crystallographic, phase transition and dielectric properties have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance analyzer in this paper. The solubility of the tetra-valent cation in Pb(Mg_1/2W_1/2)_1?xM_xO₃ decreases in the sequence of Ti^4+?>?Zr^4+?>?Sn⁴⁺. The doping led to the phase transition from orthorhombic antiferroelectric to cubic paraelectric phase, the microscopic nature of which could be attributed to the contraction of Pb-O₁₂ dodecahedron. A definition of the ordering parameter of the cubic phase was deduced to quantitatively evaluate the B-site ordering degree which decreased with the increase in doping concentration. Anti-site disordering of Mg²⁺ and W⁶⁺ occurred in the high-level doping compositions, which led to the relaxor behavior observed in Ti⁴⁺-doped series with x?=?0.01 composition. Both of the maximum dielectric permittivity and loss in the paraelectric phase increased with the increasing doping concentration.