Crystal-chemical Parameters Research Articles

Crystal-Chemical and Biological Controls of Elemental Incorporation into Magnetite Nanocrystals.

Magnetite nanoparticles possess numerous fundamental, biomedical, and industrial applications, many of which depend on tuning the magnetic properties. This is often achieved by the incorporation of trace and minor elements into the magnetite lattice. Such incorporation was shown to depend strongly on the magnetite formation pathway (i.e., abiotic vs biological), but the mechanisms controlling element partitioning between magnetite and its surrounding precipitation solution remain to be elucidated. Here, we used a combination of theoretical modeling (lattice and crystal field theories) and experimental evidence (high-resolution inductively coupled plasma-mass spectrometry and X-ray absorption spectroscopy) to demonstrate that element incorporation into abiotic magnetite nanoparticles is controlled principally by cation size and valence. Elements from the first series of transition metals (Cr to Zn) constituted exceptions to this finding, as their incorporation appeared to be also controlled by the energy levels of their unfilled 3d orbitals, in line with crystal field mechanisms. We finally show that element incorporation into biological magnetite nanoparticles produced by magnetotactic bacteria (MTB) cannot be explained by crystal-chemical parameters alone, which points to the biological control exerted by the bacteria over the element transfer between the MTB growth medium and the intracellular environment. This screening effect generates biological magnetite with a purer chemical composition in comparison to the abiotic materials formed in a solution of similar composition. Our work establishes a theoretical framework for understanding the crystal-chemical and biological controls of trace and minor cation incorporation into magnetite, thereby providing predictive methods to tailor the composition of magnetite nanoparticles for improved control over magnetic properties.

Incommensurate Modulation and Competing Ferroelectric/Antiferroelectric Modes in Tetragonal Tungsten Bronzes

The nanoscale structure of Sr0.61Ba0.39Nb2O6, a classic uniaxial relaxor ferroelectric crystallizing with the tetragonal tungsten bronze (TTB) structure and exhibiting an incommensurate modulation, has been determined by atomistic refinements using combined data from variable-temperature neutron total scattering, extended X-ray absorption fine structure, and three-dimensional single-crystal diffuse scattering. We found the modulation to arise from the intergrowth of structural slabs featuring distinct types of octahedral rotations directed to minimize octahedral deformations. This modulation involves displacements of the A cations (Sr and Ba) but exerts no significant effect on the polar displacements of Nb. Our results demonstrated the coexistence of competing polar Γ3– and antipolar Γ2– modes for off-centering of Nb above the nominal ferroelectric transition temperature of 350 K. The nanoscale-correlated antipolar distortions, which exhibit their largest amplitudes at 425 K, are suppressed below the transition. We identified coupling between the polar off-centering of Nb on its two symmetrically distinct sites as a fundamental characteristic controlling the ferroelectric-to-relaxor crossover in TTBs. We then used it to propose a simple microscopic interpretation for the previously established empirical trend that links crystal–chemical parameters in these systems to their polar response.

Effect of Montmorillonite Layer Charge on the Thermal Stability of Bentonite

AbstractThe thermal stability of bentonite is vitally important for its application in the casting field and the layer charge of montmorillonite (Qm) is one of its central crystal-chemical parameters. As the main component of bentonite, the influence of Qm on montmorillonite properties and behavior needs to be considered if bentonite is to be used in high-temperature environments. The objective of the current study was to investigate the relationship between Qm and the thermal stability of Chinese bentonite samples collected from Wuhu, Anhui Province (marked as WH); Xinyang, Henan Province (marked as XY); and Santai, Sichuan Province (marked as ST) below. The values of Qm were obtained using the O (11) method, and the structural properties of the bentonite samples were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermogravimetry-differential scanning calorimetry (TG-DSC), and field emission scanning electron microscopy (FESEM). The results showed that, in the samples investigated, Qm was inversely related to the thermal stability of bentonite. The Qm value (electrons per half unit cell, e/huc) was greatest for sample ST (0.725 e/huc), followed by sample XY (0.470 e/huc), and by sample WH (0.354 e/huc). The dehydroxylation temperature changed related to Qm; the sample with the largest Qm value was WH (701°C), followed by sample XY (684°C), and sample ST (630°C). After the samples were calcined at 600°C, sample WH had the best montmorillonite structural integrity with the greatest degree of reusability (78.21%); while the montmorillonite structures of samples XY and ST were destroyed, and their reusabilities were only 9.48 and 6.01%, respectively.

On the crystal chemistry of inorganic nitrides: crystal-chemical parameters, bonding behavior, and opportunities in the exploration of their compositional space.

The scarcity of nitrogen in Earth's crust, combined with challenging synthesis, have made inorganic nitrides a relatively unexplored class of compounds compared to their naturally abundant oxide counterparts. To facilitate exploration of their compositional space via a priori modeling, and to help a posteriori structure verification not limited to inferring the oxidation state of redox-active cations, we derive a suite of bond-valence parameters and Lewis acid strength values for 76 cations observed bonding to N3−, and further outline a baseline statistical knowledge of bond lengths for these compounds. Examination of structural and electronic effects responsible for the functional properties and anomalous bonding behavior of inorganic nitrides shows that many mechanisms of bond-length variation ubiquitous to oxide and oxysalt compounds (e.g., lone-pair stereoactivity, the Jahn–Teller and pseudo Jahn–Teller effects) are similarly pervasive in inorganic nitrides, and are occasionally observed to result in greater distortion magnitude than their oxide counterparts. We identify promising functional units for exploring uncharted chemical spaces of inorganic nitrides, e.g. multiple-bond metal centers with promise regarding the development of a post-Haber–Bosch process proceeding at milder reaction conditions, and promote an atomistic understanding of chemical bonding in nitrides relevant to such pursuits as the development of a model of ion substitution in solids, a problem of great relevance to semiconductor doping whose solution would fast-track the development of compound solar cells, battery materials, electronics, and more.

Influence of rare-earth doping on the structural and dielectric properties of orthoferrite La0.50R0.50FeO3 ceramics synthesized under high pressure

Abstract The effect of co-doping on the structural and dielectric properties of the La0.50R0.50FeO3 orthoferrites for a full number of substitutions (R = Ce–Lu) has been experimentally and theoretically studied using X-ray diffraction, SEM, EDS, and dielectric spectroscopy methods. The La0.50R0.50FeO3 ceramics with R = Nd, Sm, Gd, Dy, and Er, prepared by a solid-state reaction method under cold pressing at high pressure P = 4 GPa, have been taken as a basis. Structural parameters in the La0.50R0.50FeO3 orthoferrites have been obtained using Rietveld refinement of X-ray experimental data with their further approximation by linear dependence. Monotonic change in crystal lattice parameters, orthorhombic distortion degree, global instability index, and Goldschmidt tolerance factor have been observed for all samples doped by different R-cations. With increase in the radius of R-cation, a growth in an average grain size has been detected. The investigation of dielectric properties in a wide frequency range of 10−1–1010 Hz and their theoretical description with determining main mechanisms of dielectric polarization made it possible to establish regularities among a type of doping R-cation, crystal-chemical parameters and dielectric properties. The studied ceramic La0.50R0.50FeO3 samples demonstrate colossal dielectric constant ∼ 104 and relatively small dielectric loss tangent in a low-frequency range. The obtained results can be useful for further application of orthoferrite-based materials as sensor and switching devices.

Influence of layer charge on hydration properties of synthetic octahedrally-charged Na-saturated trioctahedral swelling phyllosilicates

Smectite hydration impacts dynamical properties of interlayer cations and thus the transfer and fate of H2O, contaminants, and nutriments in surficial environments where this ubiquitous clay mineral is often one of the main mineral components. The influence of key crystal-chemical parameters, such as the amount of charge or the presence of fluorine, rather than hydroxyl groups, in smectite anionic framework, on hydration, organization of interlayer species, and related properties has been described for tetrahedrally substituted trioctahedral smectites (saponites). Despite the ubiquitous character of octahedrally substituted smectites, that make most of the world bentonite deposits, the influence of charge location on smectite hydration properties has not received similar attention. A set of octahedrally substituted trioctahedral smectites (hectorites) with a common structural formula NaxMg6-xLixSi8.0O20(OH)4 and a layer charge (x) varying from 0.8 to 1.6 was thus synthesized hydrothermally. The distribution of charge-compensating Na+ cations and of associated H2O molecules was determined experimentally from the modeling of X-ray diffraction data obtained along water vapor desorption isotherms. Consistent distributions of charge-compensating cations and of associated H2O molecules were also computed from GCMC simulations as a function of layer charge. Interlayer H2O contents [2.5–5.5 and 8.0–10.0 H2O molecules per O20(OH)4 for 1W and 2W hydrates, respectively] are similar in all Na-saturated smectite samples, independent of the location and amount of their layer charge. In contrast to synthetic saponite, for which stability of most hydrated layers was increased by increasing layer charge, the stability of synthetic hectorite hydrates is only marginally affected by layer charge. Consistently, the layer-to-layer distance of Na-saturated hectorite 2W (and 1W) layers is independent of layer charge (15.10–15.65 Å and 12.0–12.6 Å, respectively). The contrasting hydration behavior of synthetic Na-saturated saponite and hectorite is likely due to different electrostatic attraction between the 2:1 layer and interlayer cation, the charge undersaturation of O atoms at the surface of hectorite 2:1 layer being more diffuse compared to saponite. Combined with previous results on saponites, the present data and sample set provides key constraints to assess the validity of force fields simulating clay-water interactions for an unmatched variety of smectite with contrasting locations and amounts of layer charge deficits.

Synthesis and Transport Properties of BIFEVOX-Based Composite Materials

The structure and electrical transport properties of Bi4V1.5Fe0.5O11 – δ (BIFEVOX) and Bi4V1.5Fe0.5O11 – δ + xBi2O3 composites have been studied as functions of composition and temperature. X‑ray diffraction data have been used to evaluate crystal-chemical parameters of individual compounds. The elemental and phase compositions of the materials have been determined by scanning electron microscopy and X-ray diffraction, and phase transitions of individual phases have been identified. The electrical conductivity of the materials has been determined by impedance spectroscopy.

Characterization and assessment of the potential toxicity/pathogenicity of fibrous glaucophane

In California, the metamorphic blueschist occurrences within the Franciscan Complex are commonly composed of glaucophane, which can be found with a fibrous habit. Fibrous glaucophane's potential toxicity/pathogenicity has never been determined and it has not been considered by the International Agency for Research on Cancer (IARC) as a potential carcinogen to date. Notwithstanding, outcrops hosting fibrous glaucophane are being excavated today in California for building/construction purposes (see for example the Calaveras Dam Replacement Project - CDRP). Dust generated by these excavation activities may expose workforces and the general population to this potential natural hazard. In this work, the potential toxicity/pathogenicity of fibrous glaucophane has been determined using the fibre potential toxicity index (FPTI). This model has been applied to a representative glaucophane-rich sample collected at San Anselmo, Marin County (CA, USA), characterized using a suite of experimental techniques to determine morphometric, crystal-chemical parameters, surface reactivity, biodurability and related parameters. With respect to the asbestos minerals, the FPTI of fibrous glaucophane is remarkably higher than that of chrysotile, and comparable to that of tremolite, thus supporting the application of the precautionary approach when excavating fibrous glaucophane-rich blueschist rocks. Because fibrous glaucophane can be considered a potential health hazard, just like amphibole asbestos, it should be taken into consideration in the standard procedures for the identification and assessment of minerals fibres in soil and air samples.

Structure peculiarities of binary state diagrams of vanadium with neighboring elements of Mendeleev’s periodic table

Data on search for correlations between a structure of state diagrams of binary systems based on V-Me (Me = Ti, Zr, Hf, Nb, Ta, Cr, Mo, W) and crystal-geometric and crystal-chemical parameters were presented. Manifestation of both positive and negative deviations in experimental concentration dependences of the atomic volume in solid solutions in systems V-Me (Me = Ti, Nb, Ta, Cr, Mo) from Zen’s law was established. It was found that in the systems with unbounded regions of solid solutions, the deviation from Zen’s law is insignificant. A correlation was found between the types of state diagrams and the difference in atomic volumes of alloy-forming elements in systems V-Me (Me = Ti, Nb, Ta, Cr, Mo).

Synthesis, Crystal and Thermal Properties of Solid Solution Zn2−2xCu2xSiO4 with Willemite Structure

Solid solution Zn2−2xCu2xSiO4 with the willemite structure has been synthesized by the proposed solid-phase synthesis procedure. It is shown that the maximum annealing temperature is 1030 ± 1°C because of the decomposition of CuO. The extent of the Zn2−2xCu2xSiO4 solid solution is 7.5 at % Cu. The unit cell crystal-chemical parameters and coefficients of thermal linear and volume expansion are independent of the Cu2+ concentration. Melting points of Zn2−2xCu2xSiO4 samples in the homogeneity region are also independent of the dopant index x and close to the values found for undoped Zn2SiO4 (1512 ± 1°C). The deformation phase transition in Zn2SiO4 and Zn2−2xCu2xSiO4 is found for the first time being 10 ± 2°C below the melting point. The combination of the obtained crystal-chemical and thermal data allowed us to carry out the triangulation of the diagram of the ZnO−SiO2−CuO system to divide it into five secondary triangles by two-phase equilibria lines.

Формирование границ раздела полупроводник-сера-переходный металл

Выполнены термодинамические расчеты при стандартных условиях химических реакций, протекающих при сульфидной пассивации поверхности элементарных полупроводников (Si, Ge), нитридов галлия, индия, алюминия (GaN, InN, AlN). Осуществление сульфидной пассивации нитрида алюминия, кремния и германия термодинамически вероятнее для неокис-ленной поверхности полупроводников, контактирующей с газовой фазой, содержащей серу или сероводород. Существует термодинамическая вероятность сульфидирования окисленной поверхности германия из газовой фазы. Сульфидирование нитридов галлия и индия самопроизвольно возможно из водных растворов сульфидов щелочных металлов. Сравнение кристаллохимических параметров атомов (ковалентные и атомные радиусы), типов симметрии возможных фазовых соединений на границах раздела полупроводник-сульфид-металл и полупроводник-оксид-металл показало, что для сульфидированной поверхности полупроводника имеется большее структурное соответствие, чем для окисленной поверхности полупроводника, контактирующего с переходным металлом. При замещении атомов кислорода на атомы серы в приграничном слое между переходным металлом и полупроводником наблюдается меньший скачок в величинах электроотрицательностей элементов, образующих границу раздела фаз. Формирование с помощью сульфидной пассивации поверхности полупроводника более совершенной и стабильной (по химическим и физическим свойствам) границы раздела фаз способствует созданию выпрямляющих контактов металл-полупроводник с улучшенными электрофизическими параметрами.DOI 10.14258/izvasu(2018)4-02

Contribution of nanointerfaces to colossal permittivity of doped Ba(Ti,Sn)O3 ceramics

The microstructure, crystal chemical parameters and electrical–physical properties of samples of barium titanate-based dielectric and semiconductor ceramics were investigated in a wide frequency range. The contributions of different nanointerfaces to the permittivity of samples under investigation have been determined.

Ion-conducting glass-ceramics for energy-storage applications

Abstract

Synthesis, structure, and properties of substituted bismuth niobates Bi3Nb1–x Er x O7–δ

Results are reported of a study of the structural and electrical characteristics of substituted bismuth niobates with composition Bi3Nb1–x Er x O7–δ, which are promising oxygen-ion conductors. The homogeneity regions of solid solutions were determined by X-ray phase analysis and electron microscopy with X-ray fluorescence microanalysis, and their crystal-chemical parameters were calculated. The electrical conductivity of sintered samples was examined by impedance spectroscopy.

Equilibrium Fractionation of Non-traditional Isotopes: a Molecular Modeling Perspective

The isotopic compositions of natural materials are determined by their parent reservoirs, on the one hand, and by fractionation mechanisms, on the other hand. Under the right conditions, fractionation represents isotope partitioning at thermodynamic equilibrium. In this case, the isotopic equilibrium constant depends on temperature, and reflects the slight change of free energy between two phases when they contain different isotopes of the same chemical element. The practical foundation of the theory of mass-dependent stable isotope fractionation dates back to the mid-twentieth century, when Bigeleisen and Mayer (1947) and Urey (1947) proposed a formalism that takes advantage of the Teller–Redlich product rule (Redlich 1935) to simplify the estimation of equilibrium isotope fractionations. In this chapter, we first give a brief introduction to this isotope fractionation theory. We see in particular how the various expressions of the fractionation factors are derived from the thermodynamic properties of harmonically vibrating molecules, a surprisingly effective mathematical approximation to real molecular behavior. The central input data of these expressions are vibrational frequencies, but an approximate formula that requires only force constants acting on the element of interest can be applied to many non-traditional isotopic systems, especially at elevated temperatures. This force-constant based approach can be particularly convenient to use in concert with first-principles electronic structure models of vibrating crystal structures and aqueous solutions. Collectively, these expressions allow us to discuss the crystal chemical parameters governing the equilibrium stable isotope fractionation. Since the previous volume of Reviews in Mineralogy and Geochemistry dedicated to non-traditional stable isotopes, the number of first-principles molecular modeling studies applied to geosciences in general and to isotopic fractionation in particular, has significantly increased. After a concise introduction to computational methods based on quantum mechanics, we will focus on the modeling of isotopic properties in liquids, which represents a bigger methodological challenge than …

Equilibrium zinc isotope fractionation in Zn-bearing minerals from first-principles calculations

Isotopic composition analysis contributes significantly to the investigation of the biogeochemical cycle of zinc with its economical, environmental and health implications. Interpretation of isotopic measurements is however hindered by the lack of a set of equilibrium isotopic fractionation factors between Zn-bearing minerals. In this study, equilibrium mass-dependent Zn isotope fractionation factors in Zn-bearing minerals are determined from first-principles calculations within the density functional theory (DFT) scheme. A wide range of minerals belonging to sulfide, carbonate, oxide, silicate, sulfate and arsenate mineral groups are modelled to account for the natural diversity of Zn crystal-chemical environment. Calculated reduced partition function ratios (β-factors) span a range smaller than 2‰ at 22°C. All studied secondary minerals (adamite, gahnite, gunningite, hemimorphite, hydrozincite, zincite) but zinc carbonate (smithsonite) are isotopically heavier than zinc sulfide minerals (sphalerite and wurtzite) from which they could form through supergene processes. Zinc–aluminium spinel and zinc silicate are the isotopically heaviest minerals. The investigation of the crystal-chemical parameters at the origin of differences in isotopic properties shows an excellent linear correlation between lnβ and Zn interatomic force constants. β-factors are also observed to increase when the Zn-first neighbour bond lengths decrease and charges on atoms involved in the bonding increase and vice versa. These findings are in line with the observation of heavy isotope enrichment in systems having the largest bond strength.

Structure and properties of solid solutions based on bismuth niobate Bi3NbO7

Solid solutions Bi3Nb1–y W y O7 ± δ, Bi3Nb1–y V y O7 ± δ, Bi3Nb1–y Fe y O7 ± δ (y = 0.1–0.5; Δy = 0.1), and Bi3–x Y x Nb1–y W y O7 ± δ (x = 0.05, 0.1; y = 0–0.3; Δy = 0.1) have been studied. The homogeneity ranges of the solid solutions and crystal-chemical parameters have been determined by means of X-ray powder diffraction. The electrical conductivity of sintered samples has been studied by impedance spectroscopy. The joint introduction of yttrium and tungsten into the niobium sublattice does not lead to an increase in the conductivity of solid solutions, and the change of the dopant type has no noticeable effect on this conductivity.

Peculiar properties of crystal-chemical stucture of spinels of the system Mg(FexCr2-x)O4 obtained through the hydroxide coprecipitation method and solid state technology

Using the coprecipitation method of metal hydroxides, magnesium ferrite-chromites of the system Mg(FexCr2-x)O4 were synthesized, and a comparative analysis of crystal-chemical parameters of the obtained samples with similar samples, made through the solid state technology was performed. The precipitation method provides high dispersion, lower sintering temperature and more even distribution of components in the sintered ferrite. It was found that in samples, synthesized through the hydroxide coprecipitation method, the value of lattice constant is greater and reversibility degree is lower compared with samples synthesized through the solid state technology. The formation of magnesium ferrite-chromites from coprecipitated hydroxides is completed at temperatures 500-700 ºC lower than with the standard solid state technology using oxides. The dimensions of the crystallites in the samples obtained through the hydroxide coprecipitation method are within 50-130 nm and are smaller than samples, obtained through the solid state technology (200-400 nm). The changes in dispersion and ionicity degree of the chemical bond, depending on the composition and synthesis method were monitored. The obtained results demonstrate the possibility of synthesis through the hydroxide precipitation of ferrites of the system Mg-Fe-Cr and allow to predict their crystal-chemical parameters.

Optical monitoring of the deposition process of ferroelectric films

A method of optical monitoring of deposition of ferroelectric Ba0.8Sr0.2TiO3 films grown by high-frequency reactive plasma-enhanced chemical deposition has been considered. The studies of the plasma in the optical range have demonstrated that the emission spectrum at wavelengths λ > 450 nm is not changed in the range of bias voltages. At λ = 300–400 nm, a correlation takes place between the emission spectra of the film-forming medium obtained at different target-substrate voltages. The mass spectra of the film-forming medium indicate that, at bias voltages U = 350–600 V, the gaseous phase mainly contains ionized particles with the mass numbers of 220–240, which are close to the molar mass of Ba0.8Sr0.2TiO3 ± x . At U > 650 V, the gaseous phase contains ions with mass numbers corresponding to the chemical compositions of the target components BaTiO3, SrTiO3, BaO, and SrO, along with multiatomic particles. It has been shown that a transition layer enriched in the substrate material exists at the interface. It has been found that the monitoring of the film-forming medium and the time factor make it possible to reproducibly grow nanosized films with given crystal-chemical parameters.

Crystal-Chemical Aspects of the Stability of the Ordered Phase B2 in Volume Alloying of TiNi

The phenomenological approach based on analysis the crystal-chemical parameters and their change depending on concentration of the alloying element for definition of concentration intervals of existence of stable phases with ordered structures in the ternary Titanium Nickelide based alloys is described. It was shown that a comparison of the data of change patterns in crystal-chemical parameters phenomenologically predicted and obtained from experimental data, allows to predict some of the conditions of stabilization in the examined phase, the solubility limits of alloying elements and, consequently, its homogeneity region, as well as formation conditions of multiphase systems. The analysis carried out allows us to make a conclusion regarding the issue that is often discussed in the analysis of phase diagrams of binary and multicomponent systems - what is the ratio of the size and electronic contributions to the stability of the phases. It was shown that the contribution of the size factor to the stabilization of B2 and B19(or В19′) phases is predominant and the role of this factor grows with the increase in the amount of the alloying element.