Bond valence sum analysis

An empirical method for correlating the oxidation state of a metal ion with the coordination geometry and the bond lengths Bond valence sum analysis has been used in characterizing the structural features of vanadium-dependent haloperoxidases . ... 

Liu, W. and Thorp, FI. FI. (1993). Bond valence sum analysis of metal-ligand bond lengths in metalloenzymes and model complexes. 2. Refined distances and other enzymes. Inorg. Chem. 32, 4102-5. 

Thorp, H. H. (1998). Bond valence sum analysis of metalloenzymes. 3. Predicting bond lengths in adjacent redox states using inner-sphere reorganiza-tional energies. Inorg. Chem. 37, 5690-2. 

For application to YBCO, it has been pointed out that bond-valence calculations are empirical and cannot be used to determine the valences of the elements involved to better than around 10% [11.43]. However, in the case of the boundary structures observed here, the atomic positions can only be determined with an accuracy of 0.1 A, making any errors induced by the bond-valence sum analysis second order. As such, the bond-valence sums can be used to indicate positions where the valence of the elements involved changes considerably, although relating the magnitude of the change to properties must be approached with care. 

The combination of Z-contrast imaging and EELS allows the hole concentration occurring at grain boundaries to be correlated with defined structural features. Using bond-valence sum analysis to interpret the results has highlighted the differences between structural units containing reconstructed atomic columns on different sub-lattice sites. In particular, it has been found that for... 

A stable atomic coordination will also have electrons filling space in all directions. This phenomena is well known and has been considered at least since the introduction of valence shell electron pair repulsion theory [21]. In bond valence theory, this has been formalized as the valence vector sum mle In a stable coordination sphere the sum of the bond valence vectors around an ion is zero [22] (See Sect. 6.2 in Chap. 2 of this volume by Brown [16]). For metal cations at oxide surfaces, this requires bonds to O anions equally in all directions. As lone electron pairs can also fulfill this requirement for O anions, they can be stable with a nonzero valence vector sum, i.e. with bonds distributed unequally through space (See Sect. 7.1 in Chap. 2 of this volume by Brown [16]). Quantitative work on surfaces has so far been limited to bond valence sums. Since bond valence sums ignore these geometric concerns, metal terminated stmctures will be less stable than indicated by a bond valence sums analysis. This is seen in the differences between the Sll and the DFT calculated energies for cation terminated surfaces. Valence vector sums should be as useful for surface stmctures as for bulk stmctures. So far, however, the valence vector sum mle has been applied only qualitatively to surface stmctures, and so we will limit our discussion in this chapter to applying it qualitatively. 

Jansen, L. and Block, R. (1991). On the concept of bond valence sums and its applicability in the analysis ofhigh-T superconductors. Physica C181,149-59. 

I was characterized by powder X-ray diffraction (PXRD), energy dispersive analysis of X-rays (EDAX), chemical analysis, thermogravimetric analysis (TGA) and IR spectroscopy. EDAX analysis indicated the ratio of Mn S to be 3 2. The presence of fluorine was confirmed by analysis and the percentage of fluorine estimated by EDAX in a field emission scanning electron microscope was also satisfactory. Thermogravimetric analysis also confirms the stoichiometry of the compound. Bond valence sum calculations6 and the absence of electron density near fluorine in the difference Fourier map also provide evidence for the presence of fluorine. The sulfate content was found to be 30.8% compared to the expected 32% on the basis of the formula. 

Bond valence sum (BVS) analysis, developed by Brown (43) to calculate metal oxidation states in materials such as high-temperature superconductors and zeolites, has recently been shown by Thorp (44) to be predictive for metalloenzymes and model compounds. On the basis of crystallographic data, the empirical parameters r0 and B are determined. These values can then be used to calculate oxidation states from known coordination environments or coordination numbers from known oxidation states and bond lengths. The requisite equations are... 

Bond valence sum is calculated using interatomic distances and empirical bond valence parameters tabulated for each type of the bond. The analysis was conducted using VaList software [A.S. Wills and I.D. Brown, VaList, CEA, France (1999)], available from ftp //ftp.ill.fr/pub/dif/valist/. 

In structure analysis, bond valence sums can be used to test different models, assign oxidation states, examine the distribution of atoms among different sites, locate missing atoms and examine diffusion paths, as the following examples illustrate. 

Sodium ions have been proposed to bind to N3 of guanine or adenine and the 02 of cytosine or thymine in the minor groove of the DNA dodeeamer referred to as the Dickerson-Drew-B-DNA dodecamer. This sequence, the subjeet of several X-ray erystal structures extending to sub-Angstrom resolution, eontains a run of A T base pairs known as an A-tract. All monovalent cation sites are only partially oceupied in these struetures. Beeause sodium ions are diffieult to distinguish from water molecules using erystallographie teehniques, a bond-valence sum approaeh was employed in the analysis by Williams and eo-workers, who proposed that the DNA minor... 

Rietveld analysis of LSCF6482 was performed using the neutron diffraction data taken at 667 K in the 26 range of 20°-153° by a trigonal R3c perovskite-type structure. La and Sr atoms were placed at the special position 6a 0,0,1 /4 of the R3c symmetry. Co and Fe atoms were put at the 6b 0,0,0 site. O atom was placed at the 18e x, 0, 1/4. In a preliminary analysis, the refined occupancy factor of O atoms at the 18e site g(0) was unity within the estimated standard deviation in the Rietveld analysis Thus, the g(0) was fixed to be unity in the final refinement. Isotropic and anisotropic atomic displacement parameters were used for the cations and anions, respectively. The calculated profile agreed well with the observed one [13]. The refined crystal parameters and reliability factors are shown in Table 6.4 [13]. The averaged valence of the Co and Fe cations was estimated to be 3.4 from the occupancy factor at 667 K, which is consistent with the calculated bond valence sum (BVS) value of 3.3. Here the average value of the bond valence parameter of 1.7118 was used for the... 

The bond valence model also works well for surface analysis, as indicated by the complementary nature of bond valence sums and DFT structural convergence. Figure 1 shows data during a DFT structural minimization of a hydroxylated MgO (111) surface stmcture [20]. The G and DFT calculated energy have nearly... 

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