材料化学教学 材料化学

1、材料化学教学材料化学教学 材料化材料化学学high dielectric constantmagnetic insulatorO vacancies and polarons stable structure Exist large off-stoichiometric oxygenredox propertiesCeO2The property of CeO2各掺杂体系单元表现的磁矩和各掺杂磁性离子对磁矩的贡献 可见掺杂体系的单元总磁矩和磁性离子贡献的磁矩随掺杂元素d轨道电子数的增加呈先增加后减小的趋势，在Mn 3d5时达到最大，这符合d轨道电子的排布规律。这随明掺杂磁性离子的d电子主要参与

2、铁磁性耦合，s电子则明显的参与成j键. 掺杂的磁性离子与其在氧化物中的电子态显著不同：只有4s电子发生完全的电荷转移参与成键，这也决定了其在掺杂体系中大致的化合价；而3d电子主要参与了与自陷电子的交换耦合没有明显的电荷转移。 在v，Cr，Mn，Fe，Ni离子掺杂的Ce02体系中，如果没有Vo的存在，则所有掺杂体系中的磁性离子都是通过氧原子产生了反铁磁超交换作用，因此都没有铁磁性。下面只考虑引氧空位的体系。只有V0引入TM掺杂的Ce02中才能导致磁性离子之间的铁磁性耦合，并且这种耦合可以扩展到几个原子距离的长度。这一结论既证明了氧空位诱导的铁磁性交换机制，又是对该理论的进一步补充Progress

3、ThisHoweverSome researchersTiwari et al the Co-doped CeO2 displays RTFM with high magnetic moment（8.2B/Co）and Curie temperature (725 K ). The giant magnetization was attributed to unquenched orbital magnetic moments of the dopant Coclaimed ferromagnetism in thin films, nan-opowders, and nanocubes fo

4、rms of nondoped CeO2 by irradiation and other manipulationsthe theoretical predictions deny the possibility of RTFM ordering in it，While the reports of ferromagnetism in the TM doped oxides are suspected in terms of metallic clus-ter formationstudy of the bulk CeO2 to focus on potential role of VO i

5、n the RTFM mechanism re-heated in air for the same conditionForth stepThird step Second stepFist step High purity ceria powderThe lattice parameter a = 5.4107 for CeO2 is in good agreement with the standard value The evacuation leads to a small expansion in the cell volume (V = 158.40 3 for CeO2 and

6、 V = 159.64 3 for evacuated sample )V0 ,V1, U1 represent Ce3+, U3,Ce4+,respectly(Color online)The Ce 3d XPS spectra for pure CeO2,the vacuum-annealed CeO2: Vac, the air-heated CeO2: Htd, and the difference spectrum (CeO2:H) (CeO2)2Ce4+Lattice + 4O2Lattice VO+O+2Ce3+Lattice + 3O2LatticeO deficiency i

7、s not confined tothe surface layers and rather uniformly created inremoved out after annealingOthers result Shah et al, Co-doped bulk CeO2 samples exhibit ferromagnetic ordering linked with the VObut However, our findings show a contradiction with their prediction regarding the nondoped CeO2 Liu et

8、al. who showed Ms to be independent of VO in CeO2 nanopowders Opinion: the defect induced ferromagnetism depends on locations of VO in the fluorite structureBut, the probing technique used in lacks sensitivity to detect small changes in the stoichiometryLi et al, claim that the RTFM does not relate

9、to the VO but to the mixed valent Ce3+/ Ce4+ pairs at the surface layersBut , the role of VO cannot be ruled out because our findings confirm that occurrence of Ce mixed valence in CeO2: Vac is accompanied by creation of VO as a subsequent electronic phenomenonBound magnetic polaron (BMP)model -F-ce

10、nter exchange mechanism the F2+ center with no trapped electrons; the F+ center with one trapped electron; the F0 center with two trapped electrons. F-中心中心 在离子晶体中，辐射过程释放出来的荷电空穴和电子均可被带有适当电荷的空位所俘获，从而形成各种不同的色心 当电子被负离子空位俘获时，则形成F心，F心的能级位于带隙中（如右图），F心再俘获一个电子形成F心碱金属卤化物碱金属卤化物MX的各类色心的各类色心 电磁学 ，两个临近的磁偶极趋于指向相反的

11、方向，因此，它们的磁场会互相抗拒，互相抵销。但是，由于单独自旋产生的磁场很小，这效应很微弱，形成的排列很容易就会被热涨落（thermal fluctuation）摧毁。在有些物质裏，由于一种称为交换相互作用（exchange interaction）的特别量子力学效应，自旋与自旋彼此之间方向的改变，会导致临近电子静电排斥力的改变。在近距离，交换相互作用会比偶极-偶极磁相互作用强劲很多。因此，对于铁磁性物质，临近电子的自旋趋于指向同样的方向 根据氧空位诱导的铁磁性交换理论，两个Co 3d轨道产生铁磁性耦合需借助于两离子之间的Vo。而此时处于自陷态的Vo中局域电子在Co离子正电荷的作用下其离域性更强，从而获得了更大的玻尔轨道半径使近邻的两个Co离子得以产生铁磁性耦合Co-Vo-Co