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Bond-valence Sums (bond-valence + sum)
Selected AbstractsISAACS, interactive structure analysis of amorphous and crystalline systemsJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2010Sébastien Le Roux ISAACS (interactive structure analysis of amorphous and crystalline systems) is a cross-platform program developed to analyze the structural characteristics of three-dimensional structure models built by computer simulations. The models may have any degree of periodicity (i.e. crystallinity) and local symmetry. The following structural information is computed from the models: total and partial radial distribution functions and structure factors for X-ray or neutron scattering, coordination numbers, bond-angle and near atomic neighbor distributions, bond-valence sums, ring statistics, and spherical harmonics invariants. The information may be visualized conveniently and stored for further use. [source] Ternary fluorides BaMF4 (M = Zn, Mg and Mn) at low temperaturesACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2009Jose Maria Posse Ternary fluorides BaMF4 (M = Zn, Mg, Mn) have been studied in the temperature range from 300 to 10,K using synchrotron and laboratory powder and single-crystal diffraction. The first two compounds are stable down to 10,K, while the third one undergoes a phase transition to an incommensurately modulated structure at approximately 250,K. The modulated phase is stable down to 10,K. The magnetic anomalies at 45 and 27,K observed previously in BaMnF4 are exclusively reflected in the behavior of the , component of the q vector, which assumes an irrational value of approximately 0.395,Å,1 at the temperature corresponding to the onset of the magnetic ordering and then stays constant down to 10,K. Mn,Mn distances do not indicate any structural response to the magnetic ordering. The formation of the modulated phase can be explained on the basis of simple geometrical criteria. The incorporation of the large Mn cation in the octahedral sheets implies an increase of the cavity in which the Ba ion is incorporated. This leads to the formation of the modulated structure to adapt the coordination sphere around Ba in such a way that the bond-valence sums can be kept close to the ideal value of two. With further lowering of the temperature, the charge balance around the Ba ion requires an increasingly anharmonic character of the modulation function of Ba, until finally at 10,K a crenel-like shape is assumed for the modulation of this atom. [source] On the correlations between the polyhedron eccentricity parameters and the bond-valence sums for the cations with one lone electron pair.ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2009Addendum Systematic variations of the bond-valence sums calculated from the poorly determined bond-valence parameters [Sidey (2008), Acta Cryst. B64, 515518] have been illustrated using a simple graphical scheme. [source] On the optimization of bond-valence parameters: artifacts conceal chemical effectsACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2009Xiqu Wang We recently proposed that calculated bond-valence sums, BVS, represent a non-integer structural valence, `structV', rather than the integer-value stoichiometric valence, stoichV. Therefore, the usual attempts to `optimize' bond-valence parameters r0 and b by adjusting them to stoichV are based on the false assumption that numerical values of structV and stoichV are always equal. Bond-valence calculations for several compounds with stereoactive cations SnII, SbIII and BiIII reveal the balanced distribution of the bonding power structV between atoms of each structure. [source] On the correlations between the polyhedron eccentricity parameters and the bond-valence sums for the cations with one lone electron pairACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2008Vasyl Sidey Applicability of the Wang,Liebau polyhedron eccentricity parameter in the bond-valence model [Wang & Liebau (2007). Acta Cryst. B63, 216,228] has been found to be doubtful: the correlations between the values of the polyhedron eccentricity parameters and the bond-valence sums calculated for the cations with one lone electron pair are probably an artifact of the poorly determined bond-valence parameters. [source] The assignment and validation of metal oxidation states in the Cambridge Structural DatabaseACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2000Gregory P. Shields A methodology has been developed for the semi-automatic assignment and checking of formal oxidation states for metal atoms in the majority of metallo-organic complexes stored in the Cambridge Structural Database (CSD). The method uses both chemical connectivity and bond-length data, via ligand donor group templates and bond-valence sums, respectively. In order to use bond-length data, the CSD program QUEST has been modified to allow the coordination sphere of metal atoms to be recalculated using user-defined criteria at search time. The new methodology has been used successfully to validate the +1, +2 and +3 oxidation states in 743 four-coordinate copper complexes in the CSD for which atomic coordinates are available in ca 99% of structures using one or other method, and both succeed for >86% of structures. [source] Manganoan rockbridgeite Fe4.32Mn0.62Zn0.06(PO4)3(OH)5: structure analysis and 57Fe Mössbauer spectroscopyACTA CRYSTALLOGRAPHICA SECTION C, Issue 3 2006Günther J. Redhammer The structure of the basic iron phosphate rockbridgeite [iron manganese zinc tris(phosphate) pentahydroxide] was reinvestigated with special emphasis on the cation distribution deduced from new X-ray and 57Fe Mössbauer data. Rockbridgeite is orthorhombic, space group Cmcm, and shows three different Fe sites, one with symmetry, another with m symmetry and the third in a general position. One phosphate group has the P atom on a site with m symmetry, while the other has the P atom at a site with mm symmetry. Two Fe sites are fully occupied by ferric iron, while Mn3+ and Fe2+ are situated at a third, principally Fe, site. Structural data, bond-valence sums and polyhedral distortion parameters suggest a new interpretation of the rockbridgeite 57Fe Mössbauer spectrum. [source] | ||||||||||