Showing papers in "Acta Crystallographica in 2005"

A study of charge density in copper

Abstract: Very accurate low-order structure factors have been measured in copper, magnesium and strontium titanate using quantitative convergent beam electron diffraction (QCBED) The charge density distribution in these materials has been studied using the measured structure factors The results have also been compared to ab initio density functional theory (DFT) calculationsIn the case of copper, we combined our low-order structure factors with higher order γ-ray structure factors, in order to obtain a larger experimental data set for maximum entropy and multipole analysis The results show that bond formation induces a large change in the 3d orbital radial function As expected for metallic bonding, no asphericity of the orbitals has been observed These results are in perfect agreement with DFT calculationsFor magnesium it was shown that the anisotropic displacement parameters could be determined accurately from the low order QCBED data, if structure factors from DFT calculations were used as a static lattice reference This data set was combined with X-ray structure factor measurements and used to test some commonly used DFT functionals and self interaction correction (SIC) schemes It was found that the local density approximation combined with the SIC of Lundin and Eriksson (2001) gave the best agreement with experiments Using this functional no non-nuclear maximum was found in beryllium, but not in magnesium

Ca2CuTe4O10Cl2, a new synthetic tellurium(IV) oxochloride

Abstract: Complex transition metals oxyhalides containing a lone pair element, such as tellurium (IV), form an attractive research field because there is a high probability of finding new low-dimensionally arranged compounds and, particularly, a low-dimensionally arranged transition metals substructures, leading to interesting physical properties. Tellurium (IV) can drive the formation of many unusual structures because of its stereochemically active lone pair electrons, E. It commonly takes a coordination of three or four oxygen atoms to form either a TeO3E square pyramid or a TeO3+1E trigonal bipyramid. These lone pairs are very important players involved in lowering the dimensionality of crystal structures. Previous studies in transition metal tellurium (IV) oxohalide quarternary systems revealed a family of compounds, many of which exhibit interesting properties e.g. magnetic frustration. The unique point of this thesis is to employ alkaline earth elements (AE) to augment this ability of lone pair elements to lower the dimensionality of the transition metal arrangements. By this double usage of “chemical scissors” (a lone pair element used in conjunction with alkaline earth elements) we obtained new types of low-dimensionally arranged compounds.This thesis is focused on the syntheses and characterization of a series of compounds in the pentanary (five components) system AE-TeIV-TM-O-X (AE=alkaline earth metal, TM=transition metal and X=halogen), in which nine new compounds were found. The crystal structures of each of these compounds were determined by the single crystal X-ray diffraction data.