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Stefan Adams - One of the best experts on this subject based on the ideXlab platform.
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Bond softness sensitive Bond Valence parameters for crystal structure plausibility tests
IUCrJ, 2017Co-Authors: Haomin Chen, Stefan AdamsAbstract:Based on a description of Bond Valence as a function of Valence electron density, a systematic Bond softness sensitive approach to determine Bond-Valence parameters and related quantities such as coordination numbers is elaborated and applied to determine Bond-Valence parameters for 706 cation–anion pairs. While the approach is closely related to the earlier softBV parameter set, the new softNC1 parameters proposed in this work may be simpler to apply in plausibility checks of crystal structures, as they follow the first coordination shell convention. The performance of this softNC1 Bond-Valence parameter set is compared with that of the previously derived softBV parameter set that also factors in contributions from higher coordination shells, and with a benchmarking parameter set that has been optimized following the conventional choice of a universal value of the Bond-Valence parameter b. The results show that a systematic adaptation of the Bond-Valence parameters to the Bond softness leads to a significant improvement in the Bond-Valence parameters, particularly for Bonds involving soft anions, and is safer than individual free refinements of both R0 and b from a limited number of reference cation environments.
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understanding ionic conduction and energy storage materials with Bond Valence based methods
2014Co-Authors: Stefan Adams, Prasada R RaoAbstract:The analysis and prediction of ion transport in solids from static and dynamic structure models has become an interesting application for the Bond Valence approach. Specific adaptations of the Bond Valence approach for this application area are discussed, and the resulting predictions are compared to those from alternative screening approaches. A particular advantage is that the Bond-Valence-based approach can be applied to both crystalline and glassy solids and that the level of computational effort can be easily adjusted to the level of detail required in the prediction from static pathway models for screening purposes to Bond-Valence-based molecular dynamics simulations for analyzing the coupling between the migration of the mobile species and rearrangements in the immobile substructure.
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Practical Considerations in Determining Bond Valence Parameters
Structure and Bonding, 2013Co-Authors: Stefan AdamsAbstract:Based on an investigation of empirical links of the Bond Valence method to observable quantities, especially the electron density at the Bond critical point as well as absolute electronic potential and hardness values in the frame of the hard and soft acids and bases concept, it is ascertained that Bond Valence can be understood as a functional of Valence electron density. Therefrom a systematic approach for deriving Bond Valence parameters and related quantities such as coordination numbers and Bond breaking energies is discussed that together allow for a conversion of the Bond Valence method to a simple effective atomistic forcefield.
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transport pathways for mobile ions in disordered solids from the analysis of energy scaled Bond Valence mismatch landscapes
Physical Chemistry Chemical Physics, 2009Co-Authors: Stefan AdamsAbstract:Structure–property relationships provide valuable guidelines for a systematic development of functional materials. Here an augmented Bond-Valence approach is worked out that is linked directly to the energy scale. This energy-scaled Bond-Valence approach is then used to identify ion-conduction pathways and to establish structure–property relationships in complex disordered solids using lithium silicate glasses as model systems. Representative local structure models of glassy solid electrolytes as a basis for the pathway analysis are derived from molecular dynamics simulations. Predictions of the Bond-Valence model from a static structure model are compared to a complete trajectory analysis, showing a high degree of agreement. The method yields consistent results when changing the simulation force field and is applicable to a wide range of glasses.
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Improved Bond Valence models for ion transport pathways in glasses
MRS Proceedings, 2008Co-Authors: Stefan AdamsAbstract:ABSTRACTStructure property function relationships provide valuable guidelines for a systematic development of functional materials. It is demonstrated how an augmented Bond Valence (BV) approach helps to establish such relationships in solid electrolytes. In principle it permits to identify mobile species, transport pathways and provides estimates for ion mobilities. In this work we discuss ion conduction pathways in glassy Lithium metasilicate as an illustrative example. The required representative local structure model is derived from Molecular Dynamics simulations, which provides the opportunity to compare the Bond-Valence-based predictions from a static structure model with a comprehensive analysis of a complete simulation trajectory. It is shown that understanding the Bond Valence mismatch as an effective Morse-type interaction opens up a way for systematically analyzing ion transport pathways and for a generally applicable method with improved reliability to predict ion transport characteristics in solid electrolytes from the structure model.
Vasyl Sidey - One of the best experts on this subject based on the ideXlab platform.
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Universal 'Bond Valence versus Bond length' correlation curve for manganese-oxygen Bonds.
Acta Crystallographica Section B Structural Science Crystal Engineering and Materials, 2014Co-Authors: Vasyl SideyAbstract:The oxidation-state independent `Bond Valence (s) versus Bond length (r)' correlation curve for manganese-oxygen Bonds has been closely approximated using the modified two-parameter Tromel s = f(r) function [Tromel (1983). Acta Cryst. B39, 664-669], s = [(r0 - l)/(r - l)](2), where r0 = 1.763 (2) A and l = 1.148 (9) A. The r0 and l refinable parameters of the above function can be regarded as the alternative Bond-Valence parameters intended for use in the modern Bond-Valence model [Brown (2009). Chem. Rev. 109, 6858-6919] in cases where the traditional Bond-Valence parameters (r0; n) and (r0; b) fail.
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Bond Valence parameters for Mn+m―O (5 ≤ m ≤ 7) Bonds
Zeitschrift für Kristallographie - Crystalline Materials, 2013Co-Authors: Vasyl SideyAbstract:The oxidation state specific and universal sets of the Bond Valence parameters (r 0 ; b) have been determined for Mn +m ―O (5 ≤ m ≤ 7) Bonds. The oxidation state specific r 0 and b values obtained for Mn +m ―O (5 ≤ m ≤ 7) Bonds by using the standard calculation scheme are b = 0.37 A (constant) and r 0 = 1.780 A, 1.804 A and 1.818 A for m = 5, 6 and 7, respectively. The universal Bond Valence parameters obtained for the above Bonds in two different calculation schemes are r 0 = 1.757 A and b = 0.26 A.
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Exact solution of the Bond-Valence sum rule for a set of coordination shells.
Acta Crystallographica Section B Structural Science, 2012Co-Authors: Vasyl SideyAbstract:The calculation scheme for determining the Bond-Valence parameters (r(0) and b) resulting in the exact solution of the Bond-Valence sum rule for a given set of coordination shells is presented.
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a simplified empirical model for approximation of the Bond Valence Bond length correlation for h o Bonds
Acta Crystallographica Section B-structural Science, 2011Co-Authors: Vasyl SideyAbstract:The `Bond Valence (s)-Bond length (r)' correlation reported earlier for H-O Bonds [Brown (2009). Chem. Rev. 109, 6858-6919] has been closely approximated using the function s = (α - βr)(1/γ), where α = 1.185 (10), β = 0.321 (8) and γ = 0.36.
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On the correlations between the polyhedron eccentricity parameters and the Bond-Valence sums for the cations with one lone electron pair. Addendum.
Acta Crystallographica Section B Structural Science, 2009Co-Authors: Vasyl SideyAbstract:Systematic variations of the Bond-Valence sums calculated from the poorly determined Bond-Valence parameters [Sidey (2008), Acta Cryst. B64, 515–518] have been illustrated using a simple graphical scheme.
Andrew M Rappe - One of the best experts on this subject based on the ideXlab platform.
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sr induced dipole scatter in ba x sr 1 x tio 3 insights from a transferable Bond Valence based interatomic potential
Physical Review B, 2019Co-Authors: Robert B Wexler, Andrew M RappeAbstract:In order to design next-generation ferroelectrics, a microscopic understanding of their macroscopic properties is critical. One means to achieve an atomistic description of ferroelectric and dielectric phenomena is classical molecular dynamics simulations. Previously, we have shown that interatomic potentials based on the Bond Valence molecular dynamics method can be used to study structural phase transitions, ferroelectric domain nucleation, and domain wall migration in several perovskite oxides and fixed-composition binary and ternary alloys. Most modern devices, however, use variable-composition perovskite oxide alloys such as ${\mathrm{Ba}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{TiO}}_{3}$ (BST). In this paper, we extend our Bond Valence approach to BST solid solutions and, in so doing, show that the potential parameters for each element are transferable between materials with different $x$. Using this potential, we perform Bond Valence molecular dynamics simulations investigating the temperature and composition dependence of the lattice constants, Ti displacements, and ferroelectric polarization of BST and find that our predictions match experiments and first-principles theory. Additionally, based on a detailed analysis of local dipole distributions in rhombohedral BST, we demonstrate that substitution of Sr for Ba scrambles dipoles, reduces global polarization, and enhances the order-disorder character of the ferroelectric-paraelectric phase transition.
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reinterpretation of the Bond Valence model with Bond order formalism an improved Bond Valence based interatomic potential for pbtio 3
Physical Review B, 2013Co-Authors: Shi Liu, Ilya Grinberg, Hiroyuki Takenaka, Andrew M RappeAbstract:We present a modified Bond-Valence model of PbTiO$_3$ based on the principles of Bond-Valence and Bond-Valence vector conservation. The relationship between the Bond-Valence model and the Bond-order potential is derived analytically in the framework of a tight-binding model. A new energy term, Bond-Valence vector energy, is introduced into the atomistic model and the potential parameters are re-optimized. The new model potential can be applied both to canonical ensemble ($NVT$) and isobaric-isothermal ensemble ($NPT$) molecular dynamics (MD) simulations. This model reproduces the experimental phase transition in $NVT$ MD simulations and also exhibits the experimental sequence of temperature-driven and pressure-driven phase transitions in $NPT$ simulations. We expect that this improved Bond-Valence model can be applied to a broad range of inorganic materials.
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reinterpretation of the Bond Valence model with Bond order formalism an improved Bond Valence based interatomic potential for pbtio 3
Physical Review B, 2013Co-Authors: Shi Liu, Ilya Grinberg, Hiroyuki Takenaka, Andrew M RappeAbstract:We present a modified Bond-Valence model of PbTiO${}_{3}$ based on the principles of Bond-Valence and Bond-Valence vector conservation. The relationship between the Bond-Valence model and the Bond-order potential is derived analytically in the framework of a tight-binding model. An energy term, Bond-Valence vector energy, is introduced into the atomistic model and the potential parameters are reoptimized. This model potential can be applied both to canonical-ensemble (NVT) and isobaric-isothermal ensemble (NPT) molecular dynamics (MD) simulations. This model reproduces the experimental phase transition in NVT MD simulations and also exhibits the experimental sequence of temperature-driven and pressure-driven phase transitions in NPT simulations. We expect that this improved Bond-Valence model can be applied to a broad range of inorganic materials.
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development of a Bond Valence based interatomic potential for bifeo3 for accurate molecular dynamics simulations
Journal of Physics: Condensed Matter, 2013Co-Authors: Shi Liu, Ilya Grinberg, Andrew M RappeAbstract:We present an atomistic potential for BiFeO3 based on the principles of Bond-Valence (BV) and Bond-Valence vector (BVV) conservation. The validity of this model potential is tested for both canonical ensemble (NVT) and isobaric–isothermal ensemble (NPT) molecular dynamics (MD) simulations. The model reproduces the ferroelectric-to-paraelectric phase transition in both NVT and NPT MD simulations and the temperature dependence of the local structure in BiFeO3. The calculated domain wall energies for 71°, 109°and 180° walls agree well with density functional theory results. The success of our simple model potential for BiFeO3 indicates that BV and BVV conservation provides a firm basis for the development of accurate atomistic potentials for complex oxides.
Barry R. Bickmore - One of the best experts on this subject based on the ideXlab platform.
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Bond Valence and Bond energy
American Mineralogist, 2017Co-Authors: Barry R. Bickmore, Owen Craven, Matthew C. F. Wander, Hannah Checketts, Joshua Whitmer, Christopher A. Shurtleff, David Yeates, Kiersten Ernstrom, Charles Andros, Hannah ThompsonAbstract:The relationship between Bond Valence and structural energy has never been fully explored, although several predictive models have assumed some simple relationship between the two. Some of these models relate energy only to Bond Valence, while others also take into account other factors, such as Bond character. We examined periodic trends in Bond dissociation energies as a function of their ionicity, covalency, and metallicity, defined in terms of the electronegativity values of the atoms involved. A statistical model was optimized to describe these trends, allowing us to generate rough Bond energy vs. Bond Valence curves. The shapes of these curves vary dramatically as a function of Bond character, and are strongly influenced by the lone-pair Bond-weakening effect. The curve shapes can be used to rationalize several chemical trends, including the preferred structures of compounds with different Bond types, the preValence of peroxide and persulfide minerals, preferred Bond lengths in oxides, and the p K a values of (hydr)oxy-acids. The last is perhaps the most important, because some Valence-based acidity models are in current use, despite the fact that some aspects of their rationale are unclear.
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AIM analysis and the form of the Bond-Valence equation
American Mineralogist, 2014Co-Authors: Matthew C. F. Wander, Barry R. Bickmore, Hannah Checketts, Charles Andros, Larissa Lind, John Hunt, Tyler GoodellAbstract:The Bond-Valence model (BVM) posits an inverse relationship between Bond Valence (essentially Bond order) and Bond length, typically described by either exponential or power-law equations. To assess the value of these forms for describing a wider range of Bond lengths than found in crystals, we first assume that the Bond critical point density (ρ b , reported in e − /A 3 ) is at least roughly proportional to Bond Valence. We then calculate ρ b -distance curves for several diatomic pairs using electronic structure calculations (CCSD/aug-cc-pVQZ) and Atoms-In-Molecules (AIM) analysis. The shapes of these curves cannot be completely described by the standard exponential and power-law forms, but are well described by a three-parameter hybrid of the exponential and power-law forms. The ρ b -distance curves for covalent Bonds tend to exhibit exponential behavior, while metallic Bonds exhibit power-law behavior, and ionic Bonds tend to exhibit a combination of the two. We next use a suite of both experimental and calculated (B3LYP/Def2-TZVP) molecular structures of oxo-molecules, for which we could infer X-O Bond Valences of ~1 or ~2 v.u., combined with some crystal structure data, to estimate the curvature of the Bond Valence-length relationship in the high-Valence region. Consistent with the results for the ρ b -distance curves, the standard forms of the Bond Valence-length equation become inadequate to describe high-Valence Bonds as they become more ionic. However, some of these systems demonstrate even higher curvature changes than our three-parameter hybrid form can manage. Therefore, we introduce a four-parameter hybrid form, and discuss possible reasons for the severe curvature. Although the addition of more parameters to the Bond Valence-length equation comes at a cost in terms of model simplicity and ease of optimization, they will be necessary to make the BVM useful for molecular systems and transition states.
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The use of cation-cation and anion-anion Bonds to augment the Bond-Valence model
American Mineralogist, 2014Co-Authors: Matthew C. F. Wander, Barry R. Bickmore, Charles Andros, Matthew Davis, W. Joel Johansen, Larissa LindAbstract:The Bond-Valence model has, for several decades, been widely used for creating quantitative structure-activity relationships (QSARs), crystal structure modeling, and verification of proposed structures. Certain limitations of the model, such as the neglect of interactions between cations and between anions, have prevented it from being more broadly applied. In this work, we use cation-cation and anion-anion Bonds to augment the existing Bonding model in the systems H-Al-Si-O and K-Al-Si-O. The Bond Valence-length curves for these interactions employ the same mathematical form as ionic Bonds, but make only a small contribution to the overall Bonding in ionic materials. In the systems examined here, oxygen-oxygen interactions were much more important than those between cations for producing accurate Bond-Valence sums. Both anion-anion and cation-cation Bonding could prove important, however, for our ultimate goal of producing Valence-based force fields for use in molecular dynamics simulations. Rolling these interactions into the Bond-Valence framework would produce behavior similar to hard-sphere repulsive or van der Waals terms, but would more flexibly account for the complete Bonding environment. The overall improvement in Valence sums was robust, was maintained outside the calibration set, and was invariant to elemental substitution. We conclude that this minor alteration of the Bond-Valence approach will significantly improve Bond-Valence models in conjunction with other recent extensions of the approach.
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Electronic structure effects in the vectorial Bond-Valence model
American Mineralogist, 2013Co-Authors: Barry R. Bickmore, Charles Andros, Matthew F.c. Wander, Joel H. Edwards, Josh Maurer, Kendrick Shepherd, Eric R. Meyer, W. Joel Johansen, Rose A. Frank, Matthew DavisAbstract:The vectorial Bond-Valence model (VBVM) describes the spatial distribution of Bonds to each atom in a system in terms of the vector sum of the incident Bond Valences. It has been applied in the past to cations not subject to electronic structure effects (e.g., lone-pair or Jahn-Teller effects) in which case the expectation is that the vector sum will be approximately zero. Here we analyze 178 simple-oxide crystal structures and show that the vectorial Bond-Valence sum is a predictable function of the atomic Valence (oxidation state) of each atom and the Valence of the strongest Bond to atoms for which second-order Jahn-Teller and lone-pair effects play a role in determining molecular geometry. Outliers are uniformly metastable or unstable under ambient conditions, suggesting that deviation from ideal vectorial Bond-Valence sums might be used as a proxy for some aspect of structural potential energy. These results are all strictly in harmony with the VSEPR model of molecular geometry, but may allow for more quantitative prediction.
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Bond Valence methods for pka prediction ii Bond Valence electrostatic molecular geometry and solvation effects
Geochimica et Cosmochimica Acta, 2006Co-Authors: Barry R. Bickmore, Kevin M Rosso, Christopher J Tadanier, Eric J Bylaska, Darrin DoudAbstract:Abstract In a previous contribution, we outlined a method for predicting (hydr)oxy-acid and oxide surface acidity constants based on three main factors: Bond Valence, Me–O Bond ionicity, and molecular shape. Here, electrostatics calculations and ab initio molecular dynamics simulations are used to qualitatively show that Me–O Bond ionicity controls the extent to which the electrostatic work of proton removal departs from ideality, Bond Valence controls the extent of solvation of individual functional groups, and Bond Valence and molecular shape control local dielectric response. These results are consistent with our model of acidity, but completely at odds with other methods of predicting acidity constants for use in multisite complexation models. In particular, our ab initio molecular dynamics simulations of solvated monomers clearly indicate that hydrogen Bonding between (hydr)oxo-groups and water molecules adjusts to obey the Valence sum rule, rather than maintaining a fixed Valence based on the coordination of the oxygen atom as predicted by the standard MUSIC model. However, we also show how our method for pKa prediction could be improved using ab initio molecular dynamics simulations of solvated surfaces.
Ferdinando Bosi - One of the best experts on this subject based on the ideXlab platform.
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Bond Valence at mixed occupancy sites. I. Regular polyhedra.
Acta Crystallographica Section B Structural Science Crystal Engineering and Materials, 2014Co-Authors: Ferdinando BosiAbstract:Bond Valence sum calculations at mixed occupancy sites show the occurrence of systematic errors leading to apparent violations of the Valence Sum Rule (Bond Valence theory) in regular and unstrained Bonding environments. The systematic deviation of the Bond Valence from the expected value is observed in the long-range structure, and is discussed from geometric and algebraic viewpoints. In the Valence–length diagram, such a deviation arises from discrepancies between the intersection points of the long-range Bond Valences and the theoretical Bond Valences with the Valence–length curves of involved cations. Three factors cause systematic errors in the Bond Valences: difference in atomic Valences, Bond Valence parameters Ri (the length of a Bond of unit Valence) and Bond Valence parameters bi (the Bond softness) between the involved cations over the same crystallographic site. One important consequence strictly related to the systematic errors is that they lead to erroneous Bond strain values for mixed occupancy sites indicating underBonding or overBonding that actually does not exist.
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Mean Bond-length variation in crystal structures: a Bond-Valence approach
Acta Crystallographica Section B Structural Science Crystal Engineering and Materials, 2014Co-Authors: Ferdinando BosiAbstract:The distortion theorem of the Bond-Valence theory predicts that the mean Bond length 〈D〉 increases with increasing deviation of the individual Bond lengths from their mean value according to the equation 〈D〉 = (D' + ΔD), where D' is the length found in a polyhedron having equivalent Bonds and ΔD is the Bond distortion. For a given atom, D' is expected to be similar from one structure to another, whereas 〈D〉 should vary as a function of ΔD. However, in several crystal structures 〈D〉 significantly varies without any relevant contribution from ΔD. In accordance with Bond-Valence theory, 〈D〉 variation is described here by a new equation: 〈D〉 = (DRU + ΔDtop + ΔDiso + ΔDaniso + ΔDelec), where DRU is a constant related to the type of cation and coordination environment, ΔDtop is the topological distortion related to the way the atoms are linked, ΔDiso is an isotropic effect of compression (or stretching) in the Bonds produced by steric strain and represents the same increase (or decrease) in all the Bond lengths in the coordination sphere, ΔDaniso is the distortion produced by compression and stretching of Bonds in the same coordination sphere, ΔDelec is the distortion produced by electronic effects. If present, ΔDelec can be combined with ΔDaniso because they lead to the same kind of distortions in line with the distortion theorem. Each D-index, in the new equation, corresponds to an algebraic expression containing experimental and theoretical Bond Valences. On the basis of this study, the ΔD index defined in Bond Valence theory is a result of both the Bond topology and the distortion theorem (ΔD = ΔDtop + ΔDaniso + ΔDelec), and D' is a result of the compression, or stretching, of Bonds (D' = DRU + ΔDiso). The deficiencies present in the Bond-Valence theory in explaining mean Bond-length variations can therefore be overcome, and the observed variations of 〈D〉 in crystal structures can be described by a self-consistent model.
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Bond-Valence constraints around the O1 site of tourmaline
Mineralogical Magazine, 2013Co-Authors: Ferdinando BosiAbstract:The stabilities of possible Y (R 3+ + R 2+ + Li + ) clusters around the W anion (O1 site) of the tourmaline structure were checked using the Bond-Valence approach. Arrangements involving R 3+ = Al 3+ or Fe 3+ and R 2+ = (Fe, Mn, Mg) 2+ were all found to be stable. Structural data show a strong linear correlation between the mean formal Valence (MFV) of the Y cations and the long-range average Bond Valence sum (BVS) at the O1 site, as estimated from Bond-Valence parameters. This correlation is observed for all chemical compositions of tourmaline, except for fluor-buergerite where the O3 site is dominated by oxygen anions. Results show that the long-range site populations of the Y and O1 sites are related to each other by Valence constraints described by the empirical and theoretical equations: BVS(O1) = [0.99 MFV( Y ) − 1.20] and MFV(O1) = [1.00 MFV( Y ) − 1.00], respectively. The systematic deviation of the empirical equation from the ideal one is ascribed to the occurrence of Bond strain involving the O1 site. An important implication of the correlation between MFV( Y ) and BVS(O1) is that the (OH) content at the O1 site may be estimated by the equation W (OH) = 2 – [1.01 BVS(O1)] – 0.21 – F.