钙钛矿锰氧化物中电荷有序现象研究进展

电荷有序现象在混价过渡金属氧化物中早有研究, 钙钛矿锰氧化物中由于电荷有序与自旋有序、轨道有序、晶格畸变等相互关联而特别引人注目. 本文综述了有关电荷有序现象的早期 研究与近期进展, 重点介绍了电荷有序转变附近的一些物理异常现象与影响电荷有序态稳定的有关因素.     

Optical Spectroscopy of Perovskite-Type Manganites

Review is given on late optical studies on colossal magnetoresistance (CMR) manganites with perovskite or related structures. In La_1–x Sr _x MnO₃ which undergoes a ferromagnetic transition, optical conductivity spectrum () shows a change from a gap-like feature to metallic but highly diffussive one with increasing spin-polarization. Such a remarkable spectral weight transfer is also observed in a layered manganite, La_2–2x Sr_1+2x Mn₂O₇ (x = 0.4), with a mid-infrared peak structure due to inter-orbital transition. In Pr_1–x Ca _x MnO₃ (x = 0.4) which undergoes the charge ordering phase transition, () shows an anisotropic feature reflecting charge/orbital ordering pattern at 10 K. The charge-ordered state is transformed into a ferromagnetic metallic state by a magnetic field of 6.5 T at 30 K, which is manifested in a huge change of optical spectra over a wide photon-energy region (0.05 eV–3 eV).     

Magnetotransport Properties and Mechanism of the A-Site Ordering in the Nd–Ba Optimal-Doped Manganites

The crystal structure and magnetotransport properties of the Nd_0.70Ba_0.30-MnO_3.00−δ manganites have been investigated. The stoichiometric A-site ionic disordered Nd_0.70Ba_0.30MnO_3.00 sample is orthorhombic (SG=Imma) ferromagnet below T _C ≈151 K. It has metal-insulator transition at T _MI ≈140 K and peak of magnetoresistance ∼50% in field of 9 kOe. The anion-deficient partly A-site ionic ordered Nd_0.70Ba_0.30MnO_2.60 sample is ferrimagnet with T _N ≈130 K and has nanometric ∼440 nm grains. The oxygen annealed stoichiometric partly A-site ordered Nd_0.70Ba_0.30MnO_3.00 sample consists of two perovskite phases: (i) A-site ionic completely ordered tetragonal (SG=P4/mmm) NdBaMn_2.00O_6.00 with a Curie point of ∼310 K and (ii) superstoichiometric orthorhombic (SG=Pnma) NdMnO_3.00+γ with a Curie point of ∼120 K. The magnetoresistance of this sample at the room temperature is about 7% in a field of 9 kOe. Considerable changes of the magnetic properties are interpreted as due to the ordering of Nd³⁺ and Ba²⁺ cations as well as the formation of nanometric grains.      

同步辐射技术在核材料研究中的应用

先进同步辐射光源具有高通量、高相干性、高脉冲重复率等优点,将基于其的X射线衍射、小角散射、成像、谱学等表征方法与原位环境(如温度场、应力场、气氛、溶液介质等)实验装置配合,可为系统表征与评价核能系统用材料与部件的服役行为和损伤机制提供重要技术手段.核材料在高温/应力/介质/中子辐照等复杂多场环境下的服役损伤行为长期以来...     

同步辐射小角X射线散射及其在材料研究中的应用

小角X射线散射(small angle X-ray scattering,SAXS)是研究物质内部一纳米到数百纳米甚至到微米尺度级别微观结构的有力工具.近年来随着我国同步辐射技术的不断发展,同步辐射SAXS技术被越来越多地应用到各种材料的研究领域.然而,由于SAXS图谱是倒空间的信号,并不像显微镜那么直观,也不如X射线...     

同步辐射小角X射线散射研究PAN原丝制备过程中孔结构的演变

采用同步辐射小角X射线散射研究了PAN原丝制备过程中纤维孔结构的演变。结果表明,在水洗工艺中,纤维孔隙较多、较大,孔径分布较宽,近似圆形;在热水牵伸工艺中,纤维孔隙仍较多、较大,孔径分布较宽,近似椭圆形,长轴约17 nm~21 nm,短轴约4 nm~11 nm;在干燥致密化工艺中,孔隙急剧减少、减小,孔径分布较窄,沿纤维轴向约7 nm~9nm,垂直纤维轴向约2 nm;经过蒸汽牵伸,孔隙又增多、增大,孔径分布变宽,孔隙沿纤维轴向被牵伸得很长,近似梭形;但是随后的松弛热定型又使孔隙减小,孔径分布变窄。     

同步辐射原位X射线散射技术在纳米与能源材料中的应用

同步辐射原位X射线散射技术可以实现对材料结构进行多尺度的、无损的、高时间空间分辨率的表征,动态地揭示材料微观结构在不同外界环境下的演变过程。X射线散射基础理论已经相对成熟。第三代同步辐射光源大幅提高了X射线散射技术的时空分辨率,进一步拓宽X射线散射技术的应用场景。当前同步辐射原位X射线散射技术的难点主要集中于实验装置设计和大数据处理。概述了X射线散射技术的主要分类和基本的实验方法,主要介绍了不同分类的同步辐射原位X射线散射技术在纳米材料(纳米颗粒生长和纳米颗粒自组装)与能源材料(以钙钛矿薄膜材料为代表)研究中的应用。最后结合当前国内外先进同步辐射光源的发展现状,展望了同步辐射原位X射线散射技术未来发展的方向和应用前景。     

Local Order and Structure in Mn-Substituted Manganites Studied by EXAFS

We report extended X-ray absorption fine structure measurements at the Mn K edge on La_0.25Ca_0.75Mn_0.92Cr_0.08O₃ samples. Macroscopically, the Mn substitution induces a short-range charge-ordered state in spite of the long-range one present in the parent manganite La_0.25Ca_0.75MnO₃. We show how on a local scale the structural EXAFS parameters still confirm the presence, with only a slight reduction, of the features characteristic of the charge ordered un-substituted compounds.     

碳化硼固相烧结微观结构演化的同步辐射CT观测

利用同步辐射CT (SR-CT) 技术,在碳化硼陶瓷样品烧结过程中对其进行实时投影成像,并应用滤波反投影算法和数字图像处理技术,得到了样品在整个烧结过程中内部微结构演化的二维和三维重建图像,实现了对陶瓷固相烧结过程实时、无损的观测.通过重建图像清晰观测到了陶瓷样品在烧结三个阶段中颗粒接触、烧结颈形成、晶粒和气孔长大、气孔球化并收缩等烧结现象;统计了样品在不同烧结时刻的孔隙率,得到了孔隙率随烧结时间和烧结时间对数的变化曲线,并根据曲线分析了样品在不同烧结时刻致密化速率的变化,得到了烧结中期孔隙率和时间对数的线性关系.实验结果和现有烧结理论相吻合,并为进一步完善烧结理论以及建立扩散和本构模型提供了有效的实验数据.     

Unusual eg 3d x2?y2 Orbital Ordering and Low-Energy Excitations in the CE Structure of NaMn7O12

Similarly to the half-doped manganese oxides with perovskite structure like La_0.5Ca_0.5MnO₃, the double perovskite compound NaMn₇O₁₂ contains an equal number of Mn³⁺ and Mn⁴⁺ ions in the corner-sharing network of MnO₆ octahedra and exhibits a CE order of the charge and spins of these ions at low temperature. Though, in NaMn₇O₁₂ the order is complete, the charge and spin ordering transitions are sharp and the system is free of disorder, phase coexistence or structural inhomogeneities thanks to the absence of chemical substitutions. Here we discuss two unusual features of the CE structure of NaMn₇O₁₂: (1) the e _g 3d x ²−y ² orbital ordering expected from the direct crystallographic observation of compressed Mn³⁺O₆ octahedra below the charge ordering transition; (2) the existence of a large amount of low-energy excitations evidenced by the low temperature behavior of the specific heat. We propose a picture of nearly degenerate spin and orbital configurations of the CE structure arising from the peculiar orbital ordering.     

聚硅氧烷基纳米多孔薄膜双分形结构的同步辐射小角X射线散射分析

以聚硅氧烷基材料为预聚体,采用两种核壳型致孔剂,以旋涂工艺分别制备两组聚硅氧烷基纳米多孔薄膜,采用北京同步辐射装置(BSRF)光源进行了小角X射线散射测试,在掠入射模式(入射角αi=0.2°)下得到了两组不同孔隙率纳米多孔薄膜的二维散射数据,在此基础上分析了薄膜的分形特征,发现所制备的薄膜除试样A1外,均存在双分形结构...     

Synthesis, Magnetic, and Transport Properties of Highly Doped with Cu or Ru Manganites

Polycrystalline samples with nominal compositions Pr_0.5Sr_0.5Mn_0.5Cu_0.5O₃ and Pr_0.5Sr_0.5Mn_0.5 Ru_0.5O₃ were synthesized and their magnetical and transport properties were investigated. It was shown that Pr_0.5Sr_0.5Mn_0.5Cu_0.5O₃ is an insulator with an AFM state accompanied by weak FM at low temperatures. The transport mechanism in this highly doped with Cu manganite can be explained by a conventional thermal activation. The Pr_0.5Sr_0.5Mn_0.5Ru_0.5O₃ material has a PM-FM transition beginning at T _C 325 K and a FM-AFM one—at about 160 K. The low magnetization values and temperature and field dependencies of the magnetization suggest a phase separation of anomalous FM state and AFM state below the FM-AFM transition. The carrier transport in the highly doped with Ru manganites can be described by a small polaron hopping model.     

同步辐射装置——上海光源及其应用

扼要介绍了第三代同步辐射装置——上海光源的主要特性、构造、第一期工程完成的七条X射线光束线站的主要性能指标、功能及应用。     

Photoemission Study of Perovskite-Type Manganites with Stripe Ordering

We have studied the chemical potential shift and changes in the electronic density of states near the Fermi level (E _F) as functions of temperature and carrier concentration in Pr_1−x Ca _x MnO₃ (PCMO) and Nd_1−x Sr _x MnO₃ (NSMO) by measurements of photoemission spectra. Suppression of the chemical potential shift as a function of carrier concentration has been observed in PCMO and NSMO near and in the composition range where the CE-type antiferromagnetic charge-ordered (CO) phase appears at low temperatures. This result indicates that there is charge self-organization on a microscopic scale such as stripe formation in this composition range. In the ferromagnetic metallic phase of NSMO, we found a large temperature-dependent chemical potential shift at low temperatures and attributed this to double-exchange mechanism. Suppression of the temperature-dependent chemical potential shift near Curie temperature was observed, possibly associated with the formation of correlated polarons. In the valence band near the E _F of PCMO, spectral weight was transferred with hole doping, leading to a finite intensity at E _F even in the paramagnetic insulating phase above the CO phase for x≳0.3, and as the temperature was lowered, a clear gap opening was observed in the CO phase.     

复合稀土锰氧化物材料电阻温度特性

用复合方法制备了五种以La0 8Ca0 2 MnO3 和La0 8Pb0 2 MnO3 为基料不同质量比的复合样品 ,测量了它们低温区电阻随温度变化曲线 ,讨论了复合样品的转变温度TP 随La0 8Ca0 2 MnO3 的质量分数变化的原因 ,并分析了低温 10 0K左右处复合样品的电阻极小值现象     

Magnetic and Electrotransport Properties of the Anion-Deficient Manganites with Perovskite Structure

The chemical phase content and microstructure as well as magnetic and magnetotransport properties for the doped anion-deficient La³⁺_1-xSr²⁺_xMn³⁺O^2–_3-x/2 orthomanganites have been experimentally studied. The high-quality anion-deficient ceramic with controlled chemical phase content and microstructure has been obtained from the parent stoichiometric one at the 800 C in vacuum. The average grain size is ~5 m. It is established that the samples in the region of 0 x 0.125 are O-orthorhombic perovskites whereas in the 0.175 x 0.30—rhombohedric. As doping level increases along with the oxygen vacancies number the samples in ground state undergo a transition from the antiferromagnet A-type (x=0) through the inhomogeneous magnetic state (0 < x 0.175 to the cluster spin glass one (0.175 < x 0.30. The temperature of magnetic moment freezing is ~ 45 K. All the samples are semiconductors and show considerable magnetoresistance over a wide temperature range with peak for x = 0.175 only. Fitting of the electrical resistivity to T^–1 and T^–1/4 realized. The concentration dependences of the activation energy, spontaneous magnetization, coercivity as well as magnetic transition temperatures for the anion-deficient La³⁺_1-xSr²⁺_xMn³⁺O^2–_3-x/2 ortomanganites have been established. Obtained experimental results are interpreted in terms of the oxygen vacancies, isotropic superexchange Mn³⁺-O–Mn³⁺ interactions and phase separation models.     

Consequences of Orbital Degeneracy in Insulating and Doped Manganites

We derive a spin-orbital model for manganites which includes the complete e _g and t _2g superexchange of S=2 spins at Mn³⁺ ions and predicts that the same type of antiferromagnetic and orbital ordering is stabilized in insulating La₃Mn₂O₇ as in LaMnO₃ by a purely electronic mechanism dominated by the e _g-superexchange. At finite doping we consider a model of t–J variety which describes a metal–insulator transition due to a competition between lattice polarons and kinetic energy in the metallic ferromagnetic ground state.     

Engineering Transport in Manganites by Tuning Local Nonstoichiometry in Grain Boundaries

Interface‐dominated materials such as nanocrystalline thin films have emerged as an enthralling class of materials able to engineer functional properties of transition metal oxides widely used in energy and information technologies. In particular, it has been proven that strain‐induced defects in grain boundaries of manganites deeply impact their functional properties by boosting their oxygen mass transport while abating their electronic and magnetic order. In this work, the origin of these dramatic changes is correlated for the first time with strong modifications of the anionic and cationic composition in the vicinity of strained grain boundary regions. We are also able to alter the grain boundary composition by tuning the overall cationic content in the films, which represents a new and powerful tool, beyond the classical space charge layer effect, for engineering electronic and mass transport properties of metal oxide thin films useful for a collection of relevant solid‐state devices.