Synthesis of CaCu3Ti4O12 ceramic via a sol-gel method

The novel nano-ultrafine powders for the preparation of CaCu3Ti4O12 ceramic were prepared by the sol-gel method and citrate auto-ignition method. The obtained precursor powders were pressed, sintered at 1000 degrees C to fabricate microcrystal CaCu3Ti4O12 ceramic. The microcrystalline phase of CaCu3Ti4O12 was confirmed by X-ray powder diffraction (XRD). The morphology and size of the grains of the powders and ceramics under different heat treatments were observed using scanning electron microscopy (SEM). The relative dielectric constant of the ceramic sintered at 1000 degrees C was measured with a magnitude of more than 10(4) at room temperature, which was approaching to those of Pb-containing complex perovskite ceramics, and the loss tangent was less than 0.20 in a broad frequency region. The relative dielectric constant and loss tangent were also compared with that of CaCu3Ti4O12 ceramic prepared by other reported methods. (c) 2006 Elsevier B.V. All rights reserved.

Dielectric relaxation and giant dielectric constant of Nb-doped CaCu3Ti4O12 ceramics under dc bias voltage

CaCu3Ti(4-x)Nb(x)O(12) (x = 0, 0.01, 0.08, 0.2) ceramics were fabricated by a conventional solid-state reaction method. The ceramics showed the body-centered cubic structure without any foreign phases and the grain size decreases with Nb doping. Two Debye-type relaxations were observed for the Nb-doped samples at low frequency and high frequency, respectively. The complex electric modulus analysis revealed that the surface layer, grains and grain boundaries contributed to the dielectric constant. The low-frequency dielectric constant relative to the surface layer decreased to a minimum and then increased with the dc bias voltage at 100 Hz, which were well explained in terms of a model containing two metal oxide semiconductors in series, confirming the surface layer in the ceramics. The shift voltage V-B corresponding to the minimal capacitance increased with increase of the composition x. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

钙钛矿过渡金属氧化物电、磁性质的理论研究

钙钛矿过渡金属氧化物已有大量实验和理论研究。本论文采用一般梯度近似（GGA）和GGA+U（U表示原位的库仑相互作用）下的第一性原理密度函数方法研究了双层，四层和含氧空位的钙钛矿过渡金属氧化物的晶体结构、电子结构以及电、磁性质。 从对双层钙钛矿Sr2FeMoO6和Sr2CoMoO6的研究，我们发现Sr2FeMoO6的四方相比立方相稳定，而且两种结构下它都显半金属特性；对于Sr2CoMoO6，原位的库仑相互作用决定了它的半导体性质。此外，我们还研究了实验上备受争议的Ba2YIrO6和Ba2LaIrO6在立方 Fm-3m, 菱形 R-3和单斜 P21/n三种结构下的相对稳定性。结果表明第一性原理与半经验的键价模型得到的结论相同，即Ba2YIrO6和Ba2LaIrO6的最稳定结构分别是单斜 P21/n和菱形R-3。 不同Mn-O-Mn角度下YBaMn2O5的电子结构和磁结构的计算结果表明，当Mn-O-Mn 角度处于实验所测的157.8o时，G-型反铁磁结构比A-型稳定，与实验结果相符。随着角度的增加，大约在170出现了磁结构转变。当角度大于170时，A型反铁磁结构比G型稳定，即YBaMn2O5从G型过渡到A型。此外，我们还研究了YBaMn2O5在不同磁结构以及不同角度下的导电性。 通过对四层钙钛矿化合物CaCu3M4O12 （M是3d过渡金属离子：Ti, V, Cr, Mn, Fe, Co）的能带结构计算研究了M离子的电子构型对其磁结构和导电性的影响。结果表明随着M电子数的增加，该系列化合物磁结构为：在CaCu3Ti4O12（Ti4+：d0）中Cu-Cu为反铁磁性耦合，即该物质为反铁磁体；在CaCu3M4O12 (M= V4+：d1, Cr4+：d2, Mn4+：d3, Fe4+：d4；dn，0CaCu3Ti4O12（Ti4+：t2g0），转变为半金属性的CaCu3V4O12（V4+：t2g1）和CaCu3Cr4O12（Cr4+：t2g2），再过渡为半导体性的CaCu3Mn4O12（Mn4+：t2g3），其后过渡为半金属性的CaCu3Fe4O12（Fe4+：t2g3eg1）以及金属性的CaCu3Co4O12（Co4+：t2g5）。 通过对CaCu3Cr2Sb2O12的研究以及它与CaCu3Cr4O12的类比，我们发现由于在 八面体位置有序地掺入了非磁性离子Sb5+， Cr-Cr铁磁性相互作用大大减弱，这在很大程度上使Cr3+的3d电子定域。从而导致CaCu3Cr2Sb2O12显半导体特性，不同于CaCu3Cr4O12的半金属性质。 对这一系列化合物的计算结果的分析讨论使我们了解到M元素的电子构型（原子序数）以及非磁性离子Sb5+和它与M在八面体位置的有序性对化合物电学和磁学性质的影响，从中得到了一些规律性因素。基于这一点，我们成功地预测了CaCu3Fe4O12，CaCu3Co4O12和CaCu3Fe2Sb2O12的反应路径和合成条件，稳定性以及电、磁性质。 计算了CaCu3Ru4O12（Ru4+：4d4）基态的电子结构。总能量和能带结构显示该化合物是顺磁性金属，与实验结果相一致。在CaCu3Ru4O12中，除了接近费米能级Ru-O dpπ相互作用导致Ru-O-Ru网络导电以外，还存在Ru 4d电子与定域的Cu 3d电子通过O 2p轨道形成的杂化相互作用，该作用促使定域的Cu 3d电子通过与Ru 4d电子耦合也参与导电。因而CaCu3Ru4O12具有比CaRuO3更强的导电性，在CaRuO3中仅有Ru-O-Ru网络参与了导电。