Empirical Bond Valence Research Articles

From predicting to correlating the bonding properties of iron sulfide phases

Iron sulfides have emerged as a fascinating class of materials in electromagnetics and catalysis areas, which however are challenging in first-principles modeling because of the strongly-correlated interactions between Fe 3d and S 3p electrons. Here, we assess the performances of 14 density functionals on the structural, electronic, and magnetic properties of five iron sulfides. The PBE + U with Ueff = 2.0 eV has the overall best performance. After evaluating functionals and obtaining reliable properties, our final goal is from predicting to correlating in order to do high throughput screening for the systems since the complex structures and phases of iron sulfides, to put it in another way, to hunting a reliable descriptor for predicting their properties. In the work, we demonstrate that the crystal orbital Hamilton population (COHP) and Bader charge of Fe atoms presents a good correlation with the empirical bond valence. Our results open a new avenue to effectively investigate phases and properties for various structures of iron sulfides. Indeed, the correlations between COHP/Bader charge and bond valence can be extended to other systems.

Geometry of trigonal boron coordination sphere in boronic acids derivatives - a bond-valence vector model approach.

The systematic analysis of the geometry of three-coordinate boron in boronic acid derivatives with a common [CBO2] skeleton is presented. The study is based on the bond-valence vector (BVV) model [Zachara (2007). Inorg. Chem. 46, 9760-9767], a simple tool for the identification and quantitative estimation of both steric and electronic factors causing deformations of the coordination sphere. The empirical bond-valence (BV) parameters in the exponential equation [Brown & Altermatt (1985). Acta Cryst. B41, 244-247] rij and b, for B-O and B-C bonds were determined using data deposited in the Cambridge Structural Database. The values obtained amount to rBO = 1.364 Å, bBO = 0.37 Å, rBC = 1.569 Å, bBC = 0.28 Å, and they were further used in the calculation of BVV lengths. The values of the resultant BVV were less than 0.10 v.u. for 95% of the set comprising 897 [CBO2] fragments. Analysis of the distribution of BVV components allowed for the description of subtle in- and out-of plane deviations from the `ideal' (sp(2)) geometry of boron coordination sphere. The distortions specific for distinct groups of compounds such as boronic acids, cyclic and acyclic esters, benzoxaboroles and hemiesters were revealed. In cyclic esters the direction of strains was found to be controlled by the ring size effect. It was shown that the syn or anti location of substituents on O atoms is decisive for the deformations direction for both acids and acyclic esters. The greatest strains were observed in the case of benzoxaboroles which showed the highest deviation from the zero value of the resultant BVV. The out-of-plane distortions, described by the vz component of the resultant BVV, were ascertained to be useful in the identification of weak secondary interactions on the fourth coordination site of the boron centre.

Is it True That the Normal Valence-Length Correlation Is Irrelevant for Metal-Metal Bonds?

The most intriguing feature of metal-metal bonds in inorganic compounds is an apparent lack of correlation between the bond order and the bond length. In this study, we combine a variety of literature data obtained by quantum chemistry and our results based on the empirical bond valence model (BVM), to confirm for the first time the existence of a normal exponential correlation between the effective bond order (EBO) and the length of the metal-metal bonds. The difference between the EBO and the formal bond order is attributed to steric conflict between the (TM)n cluster (TM=transition metal) and its environment. This conflict, affected mainly by structural type, should cause high lattice strains, but electron redistribution around TM atoms, evident from the BVM calculations, results in a full or partial strain relaxation.

Efficient automatic screening for Li ion conductive inorganic oxides with bond valence pathway models and percolation algorithm

Fast ion conductive solid oxide electrolytes are urgently needed because of the development of batteries, fuel cells, and sensors. Ab initio density functional theory can predict ionic conductivities with high accuracy, although it often requires large computational resources and time. In this paper, we use empirical bond valence relations [Adams et al., Phys. Status Solidi A 208, 1746 (2011)] and a percolation algorithm for fast, efficient, fully automated evaluation of migration energies for Li ion conduction in 14 olivine-type LiMXO4 compounds. The results showed a high correlation coefficient with the ab initio density functional theory (DFT) approach, indicating that our method could be attractive for identifying fast ion conductors in databases of numerous candidates.

Transition-metal substitution in PbAlBO4: Synthesis, structural and spectroscopic studies of manganese containing phases

Mullite-type metal borates PbAlBO4 with planar BO3 groups are potential candidates for nonlinear optical properties. We successively substituted aluminum with manganese in the composition PbAl1−xMnxBO4, which would help tuning the crystal chemistry and related physical properties. The end member PbAlBO4 was synthesized by conventional solid state reaction; the other members of the PbAl1−xMnxBO4 series were prepared by glycerin method. The fundamental parameter approach was applied in the Rietveld refinements to describe the X-ray powder diffraction profiles. The cell parameters and interatomic bond distances were studied with respect to the chemical composition x. The MO6 octahedral distortion linearly increased with the averaged MO bond distance, which is correlated with the increasing amount of manganese atoms in the structure. The change of the average crystal size and microstrain were explained in terms of Al/Mn ratio in the solid solution. While the empirical bond-valence sums (BVS) of one oxygen atom, bridging the edge-sharing MO6 octahedron, were found to be under-bonded, the other one was over-bonded. The stereoactivity of the 6s2 lone electron pair of the Pb2+ cation was characterized in terms of the absolute value of eccentricity parameter (|Φi|). Both bond valence parameter (r0i) and BVS of Pb showed a linear correlation with |Φi|. Deconvolution of the absorption feature of the infrared powder spectra between 1000cm−1 and 1400cm−1 required four fitted bands, which were assigned to BO stretching vibrations of the trigonal planar BO3 groups. With increasing Al3+ dilution in PbAl1−xMnxBO4 almost all observed modes shifted toward lower wavenumbers confirming the solid solution. The absorptions caused by the vibrations of the heavy atom Pb were observed in the far-IR region.

Theoretical Justification for Bond Valence -- Bond Length Empirical Correlations

Bond valence – bond length empirical correlations are of great interest in chemistry, biology, geology and materials science because they offer a quick and convenient way of checking and evaluating molecular structures. Linus Pauling’s relationship is the most commonly used, but is a two-parameter fit where R0 and b must be optimized. In this study, a simplified quantum-mechanical approach was used to derive Pauling’s empirical bond valence – bond length relationship. A covalency factor was also introduced to account for the difference in “softness” between cation and anion (resulting in increased orbital overlap). An expression for the b parameter was determined that yields values that are in agreement with experimental data. The derived relationship for the b parameter allows an independent determination of b using orbital exponents and electronegativity values for the cation and anion.

Distortion modes and related ferroic properties of the stuffed tridymite-type compoundsSrAl2O4andBaAl2O4

The structural and associated ferroic properties of the stuffed tridymite-type compounds ${\text{BaAl}}_{2}{\text{O}}_{4}$ and ${\text{SrAl}}_{2}{\text{O}}_{4}$ have been investigated by means of ab initio calculations. Structures and energy landscapes have been analyzed in terms of symmetry-adapted distortion modes. Despite their rather different room-temperature symmetries, a triply-degenerate unstable antiferrodistortive rigid unit mode (RUM) of the ${\text{Al}}_{2}{\text{O}}_{4}$ tetrahedral framework is shown to be the dominant instability for both compounds. An orthorhombic configuration resulting from a single wave $(1\mathbf{q})$ distortion mode competes with a hexagonal configuration resulting from three superposed such waves $(3\mathbf{q})$. The very small energy difference between these two configurations in the case of ${\text{BaAl}}_{2}{\text{O}}_{4}$ would explain the recent electron microscope observation of orthorhombic symmetry on the nanoscale. A second unstable mode of polar character (also a RUM for the tetrahedral framework) is also present. While this second instability is too weak to condense in ${\text{BaAl}}_{2}{\text{O}}_{4}$, in the case of ${\text{SrAl}}_{2}{\text{O}}_{4}$ it is fundamental to make the $1\mathbf{q}$ configuration prevail. The condensation of this additional instability is the cause of the symmetry reduction in ${\text{SrAl}}_{2}{\text{O}}_{4}$ to monoclinic. In contrast with previous literature, ${\text{SrAl}}_{2}{\text{O}}_{4}$ is a proper ferroelectric and a pseudoproper ferroelastic, with an uncommon bilinear coupling between its spontaneous polarization and shear monoclinic strain. One expects direct switching of spontaneous polarization through a shear stress, and conversely switching of the ferroelastic spontaneous shear strain through an electric field. This property must be related to the elasticoluminescent and electroluminiscent properties reported in Eu-doped ${\text{SrAl}}_{2}{\text{O}}_{4}$. The calculated energy maps have been systematically compared with variations in the so-called global instability index, introduced within the empirical bond valence model. The similarity of the variation in both quantities (for the most unstable distortion modes) reported in other systems is also observed here.

Hydroxyl groups in nonmetamict chevkinite-(Ce): a crystal chemical discussion

The minerals of chevkinite group were commonly considered to be anhydrous minerals. The infrared absorption spectrum of natural nonmetamict chevkinite-(Ce) from the aegirine-alkali granite, Mianning, Sichuan Province, China, exhibited two broad peaks in the 3600–2800 cm −1 region owing to the OH stretching. The corresponding H 2O content required for the charge balance in formula was 1.27%. The O-H ··· O bond lengths maight cover from 0.2658 to 0.2794 nm by the correlated OH stretching energies. An electrostatic charge balance for chevkinite-(Ce) based on the assigned site-population from chemical data was calculated without the hydrogen contribution. The resulting empirical bond-valence sum on O6, O8, O2, O3, O5, and O4 ranged from 1.73 to 1.95 vu. The partial substitution of O by OH may occur in four atom sites: O6, O2, O4, and O5. The small differences in the bond-valence sums between the supposed donors and acceptors may mean a mixed donor/acceptor role of the involved oxygen atoms. The IR spectral features between 3394 and 3035 cm −1 consisted of various hydrous species at different structural sites and orientations. The OH groups in the chevkinite-(Ce) appeared to be involved in local charge imbalance in the structure and to be present when the mineral crystallized hydrothermally.

Comparison of empirical bond-valence and first-principles energy calculations for a complex structural instability

A simple bond-strain parameter, the so-called global-instability index, defined within the bond-valence model reproduces with striking accuracy the basic features of the ab initio total-energy surface around the energy minimum of $\mathrm{Sr}{\mathrm{Bi}}_{2}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$, but only for the degrees of freedom within the subspace of energetically unstable modes. These results imply that ab initio total-energy calculations and bond-valence criteria could be combined in the study and prediction of structural instabilities. The quantitative agreement between total-energy and bond-valence maps might be related to the irrelevance of cation-cation and anion-anion interactions within this subspace.

Structure of heavy metal sorbed birnessite. Part III: Results from powder and polarized extended X-ray absorption fine structure spectroscopy

The local structures of divalent Zn, Cu, and Pb sorbed on the phyllomanganate birnessite (Bi) have been studied by powder and polarized extended X-ray absorption fine structure (EXAFS) spectroscopy. Metal-sorbed birnessites (MeBi) were prepared at different surface coverages by equilibrating at pH 4 a Na-exchanged buserite (NaBu) suspension with the desired aqueous metal. Me/Mn atomic ratios were varied from 0.2% to 12.8% in ZnBi and 0.1 to 5.8% in PbBi. The ratio was equal to 15.6% in CuBi. All cations sorbed in interlayers on well-defined crystallographic sites, without evidence for sorption on layer edges or surface precipitation. Zn sorbed on the face of vacant layer octahedral sites (□), and shared three layer oxygens (O layer) with three-layer Mn atoms (Mn layer), thereby forming a tridentate corner-sharing (TC) interlayer complex (Zn-3O layer-□-3Mn layer). TCZn complexes replace interlayer Mn 2+ (Mn inter 2+) and protons. TCZn and TCMn inter 3+ together balance the layer charge deficit originating from Mn layer 4+ vacancies, which amounts to 0.67 charge per total Mn according to the structural formula of hexagonal birnessite (HBi) at pH 4. At low surface coverage, zinc is tetrahedrally coordinated to three O layer and one water molecule ( [IV]TC complex: (H 2O)- [IV]Zn-3O layer). At high loading, zinc is predominantly octahedrally coordinated to three O layer and to three interlayer water molecules ( [VI]TC complex: 3(H 2O)- [VI]Zn-3O layer), as in chalcophanite ( [VI]ZnMn 3 4+O 7·3H 2O). Sorbed Zn induces the translation of octahedral layers from − a/3 to + a/3, and this new stacking mode allows strong H bonds to form between the [IV]Zn complex on one side of the interlayer and oxygen atoms of the next Mn layer (O next): O next…(H 2O)- [IV]Zn-3O layer. Empirical bond valence calculations show that O layer and O next are strongly undersaturated, and that [IV]Zn provides better local charge compensation than [VI]Zn. The strong undersaturation of O layer and O next results not only from Mn layer 4+ vacancies, but also from Mn 3+ for Mn 4+ layer substitutions amounting to 0.11 charge per total Mn in HBi. As a consequence, [IV]Zn,Mn layer 3+, and Mn next 3+ form three-dimensional (3D) domains, which coexist with chalcophanite-like particles detected by electron diffraction. Cu 2+ forms a Jahn-Teller distorted [VI]TC interlayer complex formed of two oxygen atoms and two water molecules in the equatorial plane, and one oxygen and one water molecule in the axial direction. Sorbed Pb 2+ is not oxidized to Pb 4+ and forms predominantly [VI]TC interlayer complexes. EXAFS spectroscopy is also consistent with the formation of tridentate edge-sharing ( [VI]TE) interlayer complexes (Pb-3O layer-3Mn), as in quenselite (Pb 2+Mn 3+O 2OH). Although metal cations mainly sorb to vacant sites in birnessite, similar to Zn in chalcophanite, EXAFS spectra of MeBi systematically have a noticeably reduced amplitude. This higher short-range structural disorder of interlayer Me species primarily originates from the presence of Mn layer 3+, which is responsible for the formation of less abundant interlayer complexes, such as [IV]Zn TC in ZnBi and [VI]Pb TE in PbBi.

Synthesis and Structure Determination of ETS-4 Single Crystals

We report the hydrothermal synthesis of 10 μm × 100 μm × 10 μm single crystals of the microporous titanosilicate sodium-ETS-4. The crystallization of large needle-shaped Na-ETS-4 crystals is preceded by the formation and subsequent dissolution of the titanosilicate GTS-1. Large Na-ETS-4 crystals are obtained with purity higher than 97%. The structures of Na-ETS-4 and its Sr ion-exchanged form are solved from single-crystal X-ray diffraction data. It is suggested from the structure solution and further supported by the results of the empirical bond valence model that the bridging unit of ETS-4 contains five-coordinate titanium(IV). Infrared spectroscopy is also used to support the crystallographic results, with regard to the presence of charge-balancing protons and five-coordinate titanium. ETS-4 is, thus, one of only two synthetic titanosilicate materials containing five-coordinate titanium. We also discuss some structural differences between ETS-4 and its natural analogue, zorite.

Correlating Atomic Scale Experimental Observations to Develop Three-Dimensional Structural Models for Grain Boundaries in Oxides

: Determining the three-dimensional atomic structure of grain boundaries is a crucial first step toward understanding how these defects control the overall bulk properties of materials. In this report we discuss the correlation of experimental atomic resolution Z-contrast images and electron energy loss spectrometry (EELS) to achieve this goal. Initial structural analysis is afforded through empirical bond-valence potentials. This structure is then refined using multiple scattering analysis of the energy loss spectra. These techniques are demonstrated in the analysis of a 27 degrees MgO [001] tilt grain boundary. Through this analysis, we were able to determine specific atomic locations of Ca dopants found present at this grain boundary.

121Sb Mössbauer Study of Sb2S3–As2S3–Tl2S Glasses

The local environment of antimony atoms in vitreous samples of the ternary system Sb2S3–As2S3–Tl2S is studied by121Sb Mössbauer spectroscopy. The results have been interpreted by means of EBV (empirical bond valence) and EBLV (empirical bond length valence) calculations. Variations in isomer shifts are attributed to changes in the environment of antimony atoms. The isomer shift is found to be a very sensitive parameter reflecting variations in the mean coordination number and in the mean Sb–S bond length. The results presented here confirm the random substitution of As by Sb in the binary system As2S3–Sb2S3and the competitive effects—breaking of As–S–As bridges by Tl and formation of As–S–Sb bridges—in the ternary system Sb2S3–As2S3–Tl2S, in agreement with previous XAFS studies.

Further evidence for Tl 3+ in Tl-based superconductors from improved bond strength parameters involving new structural data of cubic Tl 2O 3

We report on refined structural parameters for cubic Tl 2O 3 (avicennite, space group Ia3, a=10.5344 (3) A ̊ ) using Guinier powder diffraction data. The final reliability factor of the structure analysis was R( F)=0.015. Empirical bond valence sums of the two different Tl 3+ ions in the structure were calculated from bond lengths. These values deviate negatively from the actual 3+ valence by the same amount that has been frequently observed for Tl-based superconductors. Therefore, a least squares fit of the Tl 3+ bond strengths s on the bond lengths d according to s=( d d 0 ) -N has been performed giving improved parameters d 0=2.04 (1) A ̊ , N=6.4(2) and as result persuasive evidence forthe 3+ valence Tl in high- T c superconductors, even though the Tl site is occupied to some extent by Cu 1+.

On the concept of “bond valence sums” and its applicability in solid state chemistry and physics

A critical analysis is made of the current frequent use of “bond valence sums” (BVS) in the analysis of solid state phenomena, in particular those involving high-temperature super-conducting cuprates. It is shown that explicit expressions for the empirical bond valence concept can be derived on the basis of the Born-Mayer model of ionic solids. Whereas the “identity” of a cation-anion bond in terms of constant parameters ( R (0) and B) is preserved to good approximation upon changing the environment, bond lengths are not invariant under such changes. This invalidates conclusions drawn regarding ionic charges from measured bond lengths and applies in particular to the high- T c cuprates. The concept of “pressure-induced charge transfer” is shown to be both qualitatively in conflict with experimental observations and quantitatively unrealistic by several orders of magnitude. We also analyse time-dependent phenomena in the cuprates and the observed changes in core-level energies with varying chemical composition.

On the concept of “bond valence sums” and its applicability in the analysis of high- Tc superconductors

A critical analysis is made of the at present frequent use of “bond valence sums” (BVS) in the analysis of high-temperature superconducting cuprates. It is shown that explicit expressions for the empirical bond valence concept can be derived on the basis of the Born-Mayer model of ionic solids. Whereas the “identity” of a cation-anion bond in terms of constant parameters ( R( 0) and B) is preserved to good approximation upon changing the environment, bond lengths are not invariant under such changes. This invalidates conclusions drawn regarding the (cation) valence from measured bond lengths and applies, in particular, to the high- T c cuprates (Cu valence). The concept of “pressure induced charge transfer” is shown to be a quantitatively unrealistic by several orders of magnitude.