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The crystallographic and the magnetic structures of the composite compound Nd2Co7 at 300 K are investigated by a combined refinement of X-ray diffraction data and high-resolution neutron diffraction data. The compound crystallizes into a hexagonal Ce2Ni7-type structure and consists of alternately stacking MgZn2-type NdCo2 and CaCu5-type NdCo5 structural blocks along the c axis. A magnetic structure model with the moments of all atoms aligning along the c axis provides a satisfactory fitting to the neutron diffraction data and coincides with the easy magnetization direction revealed by the X-ray diffraction experiments on magnetically pre-aligned fine particles. The refinement results show that the derived atomic moments of the Co atoms vary in a range of 0.7 μB-1.1 μB and the atomic moment of Nd in the NdCo5 slab is close to the theoretical moment of a free trivalent Nd3+ ion, whereas the atomic moment of Nd in the NdCo2 slab is much smaller than the theoretical value for a free Nd3+ ion. The remarkable difference in the atomic moment of Nd atoms between different structural slabs at room temperature is explained in terms of the magnetic characteristics of the NdCo2 and NdCo5 compounds and the local chemical environments of the Nd atoms in different structural slabs of the Nd2Co7 compound. 相似文献
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The R/Ba-ordered and R-site mixed compound Y0.5La0.5BaMn2O6 is synthesized, in which (Y, La) and Ba are regularly arranged, while Y and La randomly occupy the R-site. Y0.5La0.5BaMn2O6 has a tetragonal unit cell with a space group of P4/mmm. A structural transition between tetragonal and orthorhombic is observed at about 325 K by X-ray powder diffraction (XRD). Thermal magnetic measurement shows the occurrence of an antiferromagnetic transition at the temperature TN~190 K. Anomalies in magnetization, resistivity and lattice parameters observed around 340 K indicate a charge/orbital order transition accompanying the structural phase transition. The R-site randomness effect is discussed to interpret the different properties of Y0.5La0.5BaMn2O6 between NdBaMn2O6 and SmBaMn2O6. 相似文献
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Cu2SnSe4化合物具有本征的低热导率和可调控的电导率,同时不含稀贵元素、无毒和价格低廉,具有作为中温区热电材料的潜力.本文通过高能球磨结合放电等离子烧结制备了Cu2SnSe4以及Cu掺杂的Cu2+xSnSe4块体材料(0.2≤z≤1).研究了Cu掺杂填充Cu/Sn位置上1/4本征空位对Cu2+xSnSe4热电性能的影响,发现Cu/Sn中1/4空位能够被Cu完全填满(x=1),且Cu掺杂能够大幅度地提升(可达两个数量级)样品的电导率,从而显著提高了功率因子.同时,发现在大Cu掺杂量范围(0.1≤x≤0.8)内,Cu2+xSnSe4电导率增长与掺杂量增加呈线性关系,且载流子迁移率随Cu掺杂量的增加而增加.进一步的研究发现,载流子在Cu2+xSnSe4中的电输运行为遵循电子-声子耦合的小极化子模型. 相似文献
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稀土(R)富过渡族金属(T)化合物是探索新型稀土永磁材料的主要研究对象,其磁性能与晶体结构密切相关。本文系统综述了我们近年来针对二元化合物不存在的情况下,通过第三组元M的稳定作用,合成和稳定具有CaCu5型衍生结构的化合物Rm-nT5m+2n(包括R(T,M)7,R2(T,M)17,R3(T,M)29和R(T,M)12),具有NaZn13型结构的化合物R(T,M)13,以及m层RT5和n层RT3B2组合的化合物Rm+nT5m+3nB2n等方面的主要研究结果。重点讨论了这些化合物的晶体结构、形成条件和磁性能,阐述了加入M对化合物的稳定作用机制,及M的晶体学占位对磁性能的影响。研究结果表明:M的加入不仅能够起到稳定化合物的作用,往往还可以显著改善化合物的磁性能。 相似文献
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The crystal structures and physical properties of the solid solution compound Ba<sub>5</sub>Y<sub>8-x</sub>Mn<sub>4</sub>O<sub>21-1.5x</sub>(x = 0,1)
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New oxometallides with the formula Ba5Y8 xMn4O21 1.5x(x = 0,1) are prepared through an atmospherecontrolled solid-state reaction.Two single-phase samples with Ba/Y/Mn atomic ratios 5/8/4(Y8) and 5/7/4(Y7) are obtained.The crystal structures and the physical properties of the compounds are investigated by X-ray powder diffraction,magnetization,conductivity,and dielectricity measurements.The Ba5Y8 xMn4O21 1.5x compound is demonstrated to be a Y-deficient solid solution.The solid solution compound Ba5Y8 xMn4O21 1.5x crystallizes into tetragonal symmetry with the space group I4/m.Detailed structure analysis by Rietveld refinement of the X-ray powder diffraction data reveals that the Y vacancies occur preferentially at the Y(2) site.Thermal magnetization measurements indicate the presence of antiferromagnetic interaction between Mn ions in the compounds,and temperature-dependent resistivity measurements show that insulator-semiconductor transitions occur around 175 K and 170 K for the Y8 and Y7 samples,respectively.Strong frequency dependences of the dielectric constant are observed above ~175 K for the two compounds. 相似文献
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采用熔融-淬火方法制备了Cu_(2.95)Ga_xSb_(1-x)Se_4(x=0,0.01,0.02和0.04)样品,系统地研究了Ga在Sb位掺杂对Cu_3SbSe_4热电性能的影响.研究结果表明,少量的Ga掺杂(x=0.01)可以有效提高空穴浓度,抑制本征激发,改善样品的电输运性能.掺Ga样品在625 K时功率因子达到最大值10μW/cm·K~2,比未掺Ga的Cu_(2.95)SbSe_4样品提高了约一倍.但是随着Ga掺杂浓度的进一步提高,缺陷对载流子的散射增强,同时载流子有效质量增大,导致载流子迁移率急剧下降.因此Ga含量增加反而使样品的电性能恶化.在热输运方面,Ga掺杂可以有效降低双极扩散对热导率的贡献,同时掺杂引入的点缺陷对高频声子有较强的散射作用,因此高温区的热导率明显降低.最终该体系在664 K时获得最大ZT值0.53,比未掺Ga的样品提高了近50%. 相似文献
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Different magnetodielectric effects were observed in Bi1-xGdxFeO3 ceramics depending on gadolinium content. A positive one was observed in the samples with x ≤0.10 at 295K and 16K, and a negative one in the sample with x = 0.4 at 16 K. Structure analysis by x-ray diffraction (XRD) reveals that the samples crystallize in the R3c structure (ferroelectrics) for x<0.08 and in the Pbnm structure (paraelectrics) for x≥0.3 at room temperature. Temperature-dependent dielectric response and x-ray diffraction confirm the occurrence of a structural transition in the Pbnm phase at low temperature for the samples with x ≤0.4. While the positive magnetodielectric effects can be attributed to a coupling of magnetic and crystallographic structures of the R3c phase, the observed negative magnetodielectric effect in the Pbnm phase can be associated with a low-temperature modification of the Pbnm structure. The observed dualsigned magnetodielectric effects suggest that the Bi1-xGdxFeO3 oxides are a good prototype for understanding the magnetodielectric coupling mechanism in this kind of materials. 相似文献