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Reasonably homogeneous samples were obtained by following the formula Y_(0.334)Ba_(0.666)CuO_(3-y) andusing citric acid as a complex-forming agent to improve the dispersion of the metal oxides. We havealso found that the variant condition in preparative procedure leads to variant deviation from tetrago-nality and that more pronounced deviation gives better superconducting properties. The crystal structure determined from X-ray powder diffraction diagrams confirms the stoichiom-etry of the metal ions and gives the positions of the oxygen atoms. The orthorhombic or pseudo-tetragonal unit cell with a=3.893, b= 3.813 and c= 11.687 A contains YBa_2Cu_3O_7. It is composed ofthree perovskite cubes with three copper atoms at their corners, the two barium atoms at the centers ofthe top and bottom cubes, and the yttrium at the center of the unit cell. The oxygen atoms sit on centersof cube edges. The positions (0 0 1/2) are vacant. The vacancies represented by (0(1/2)0) cause the ar-rangement of atoms to lose tetragonal symmetry, and have far-reaching effect for this phase. The yttriumand barium atoms are coordinated in a cuboctahedron respectively with four equatoria oxygen atoms at(0 0 1/2) and two at (0 1/2 0) removed. In the light of bond valence theory we can infer from the in-teratomic distances Y-O, Ba-O and Cu-O that the occupancy of the oxygen atoms at (1/2 0 0) is wellbelow unity while the positions (0 0 1/2) are vacant, the formula of the phase is YBa_2Cu_3O_(7-d), thefraction of Cu(Ⅱ) and (Ⅲ) for the set of copper at (0 0 0) is respectively d/0.5 and (1-d/0.5), and0.2相似文献
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The dispersion threshold level of zinc oxide on silica gel was determined by X-ray quantitative phase analysis. And the results of XPS experiments show that the surface of the sample is enriched with ZnO and there is a limit for the ZnO amount dispersed on the surface. The value of this limit is corresponding with the threshold level obtained by XRD. The measurements of surfaee acidity show that there are some stronger acid sites on the surface of ZnO/SiO_2, they have not been found on the ZnO or SiO_2 individually, and the total amount of acid increases remarkably. The acidity depends on the ZnO concentration in the surface phase and reaches the maximum at the threshold value.In this paper, we propose a new point of view for the origin of the acidity on surface of the ZnO/SiO_2 samples. ZnO is well dispersed on the surface of SiO_2 gel in a monolayer state. In this situation, some new surface structure units emerge as a result of interaction between dispersed ZnO and the carrier, which are responsible 相似文献
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MoO3在低比表面载体α-Al2O3上的分散容量测定 总被引:1,自引:1,他引:1
固体化合物如无机氧化物、盐类及有机物等在高比表面载体上呈现自发分散,这是一种相当普遍的现象,在催化剂和吸附剂及材料科学研究方面已有许多研究报导['-'].然而,相应固体化合物在情性小比表面载体上的分散还没有人研究过.小比表面载体的结构比高比表面载体稳定,结构较易确定·本文通过表面灵敏的X射线光电子能借(XPS)和X射线衍射方法(XRD)首次测定了Moo。在小比表面。-AI。O。上的分散容量,发现它与在高比表面载体7-AI。Oa上单位面积的分散容量不同,说明a-AI。O。与7-AI。O。的表面结构不同,为研究小比表面… 相似文献