On the structural formula of smectites: a review and new data on the influence of exchangeable cations

The understanding of the structural formula of smectite minerals is basic to predicting their physicochemical properties, which depend on the location of the cation substitutions within their 2:1 layer. This implies knowing the correct distribution and structural positions of the cations, which allows assigning the source of the layer charge of the tetrahedral or octahedral sheet, determining the total number of octahedral cations and, consequently, knowing the type of smectite. However, sometimes the structural formula obtained is not accurate. A key reason for the complexity of obtaining the correct structural formula is the presence of different exchangeable cations, especially Mg. Most smectites, to some extent, contain Mg2+ that can be on both octahedral and interlayer positions. This indeterminacy can lead to errors when constructing the structural formula. To estimate the correct position of the Mg2+ ions, that is their distribution over the octahedral and interlayer positions, it is necessary to substitute the interlayer Mg2+ and work with samples saturated with a known cation (homoionic samples). Seven smectites of the dioctahedral and trioctahedral types were homoionized with Ca2+, substituting the natural exchangeable cations. Several differences were found between the formulae obtained for the natural and Ca2+ homoionic samples. Both layer and interlayer charges increased, and the calculated numbers of octahedral cations in the homoionic samples were closer to four and six in the dioctahedral and trioctahedral smectites, respectively, with respect to the values calculated in the non-homoionic samples. This change was not limited to the octahedral sheet and interlayer, because the tetrahedral content also changed. For both dioctahedral and trioctahedral samples, the structural formulae improved considerably after homoionization of the samples, although higher accuracy was obtained the more magnesic and trioctahedral the smectites were. Additionally, the changes in the structural formulae sometimes resulted in changing the classification of the smectite.

Structural Characterization of Octahedral Sheet in Dioctahedral Smectites by Thermal Analysis

The structures of octahedral sheets of dioctahedral phyllosilicates show cis-vacant (cv) and trans-vacant (tv) configurations due to the different distributions of the octahedral cations over cis- and trans-sites. On the basis of the different dehydroxylation temperatures, a thermal analysis is an effective method used to identify the cv and tv configurations of an octahedral sheet in dioctahedral smectites. The proportions of cv and tv configurations were determined by fitting the derivative thermogravimetry (DTG) curves. A wide range of cv and tv proportions were detected in the samples. The dehydroxylation temperatures of samples consisting of cv configuration are about 150 to 200 °C higher than those consisting of tv configurations. The samples were classified as tv varieties when octahedral Fe3+ > 0.46 mol/FU, and the pure tv dioctahedral smectites were found when Fe3+ > 1.8 mol/FU. A clear linear relationship was found between the content of octahedral Fe3+ and Al3+ and the proportion of cv and tv configurations. The substitution of Al3+ by Fe3+ in the octahedral sheets is the main factor for the formation of tv varieties. There was no relationship detected between the layer charge density, octahedral Mg2+ content, and the proportion of tv and cv. The present results indicate that the iron content has a significant effect on the structure of the octahedral sheet.

Molecular Dynamics Study on the Li Diffusion Mechanism and Delithiation Process of Li2MnO3

Li2MnO3 is critical component in the well-studied Li-excess cathode materials xLi2MnO3•(1- x)LiMO2 for achieving high lithium storage capacity. In this article, the diffusion of Li ions in Li2MnO3 is studied using molecular dynamics (MD) simulation with well-behaved empirical force fields obtained by fitting against the crystal structure from experiment and phonons calculated using Density Function Theory (DFT). We have found two possible tetrahedral hopping channels, 0-TM and 1-TM(Mn4+) channel, which are differentiated by the face sharing octahedral cations. Simulation results show that the 0-TM channel is active for Li hopping, while 1-TM(Mn4+) channel is inactive. During the delithiation process, the Li ions in the TM layered are firstly removed, then those in the Li layer. However, the Li ions will be trapped in the tetrahedral 0-TM channels as long as the four face sharing octahedral sites are cleared. Up to x=1.0 for Li2-xMnO3, almost all the Li ions are located at the tetrahedral sites, forming a regular array along a axis. The de-intercalation of tetrahedral Li ions requires a high voltage (>5.2 V vs. Li/Li+), limiting the practical capacities measured in lab. The diffusion of Mn ions into the Li layers is observed in a deeper delithiated structure (x=1.2 for Li2-xMnO3), indicating an initial phase transformation to a spinel-like structure. However, the Mn ions are mainly trapped in the tetrahedral sites in the Li layer, instead of the octahedral sites fin spinel-like structure. A few of Mn ions diffusing into the octahedral sites in Li layers have no face sharing tetrahedral Li ions, revealing a further Li de-intercalation is imperative for the complete phase transformation. Our model is not stable for x≥1.4 in Li2-xMnO3. Other charge compensation mechanism should be considered in this high delithiation stage, eg. oxygen release.

An unexpected rhenium(IV)–rhenium(VII) salt: [Co(NH3)6]3[ReVIIO4][ReIVF6]4·6H2O

The title hydrated salt, tris[hexaamminecobalt(III)] tetraoxidorhenate(VII) tetrakis[hexafluoridorhenate(IV)] hexahydrate, arose unexpectedly due to possible contamination of the K2ReF6 starting material with KReO4. It consists of octahedral [Co(NH3)6]3+ cation (Co1 site symmetry 1), tetrahedral [ReVIIO4]− anions (Re site symmetry 1) and octahedral [ReIVF6]2− anions (Re site symmetries 1and \overline{3}). The [ReF6]2− octahedral anions (mean Re—F = 1.834 Å), [Co(NH3)6]3+ octahedral cations (mean Co—N = 1.962 Å), and the [ReO4]− tetrahedral anion (mean Re—O = 1.719 Å) are slightly distorted. A network of N—H...F hydrogen bonds consolidates the structure. The crystal studied was refined as a two-component twin.

Acid Activated Palygorskite: An Efficient Solid Acid Catalyst for Acetylation Reactions - A Comparative Study

Mild and efficient solid acid catalysts were prepared from natural palygorskite collected from Karnataka and Hyderabad and compared their catalytic activities using the acetylation reaction. Modification technique used for the preparation of the catalysts are acid-activation. During acid-activation exchangeable cations are replaced by H+ ions and a part of octahedral cations are dissolving and thus creating new acid sites in the crystal. The catalytic performances of these catalysts were investigated by using the acetylation reaction. Acetylation reaction was done by using different primary and secondary alcohol. Physicochemical properties were characterized by XRD, SEM, NH3-TPD measurements.

Predicting displacements of octahedral cations in ferroelectric perovskites using machine learning

In ferroelectric perovskites, displacements of cations from the high-symmetry lattice positions in the paraelectric phase break the spatial inversion symmetry. Furthermore, the relative magnitude of ionic displacements correlate strongly with ferroelectric properties such as the Curie temperature. As a result, there is interest in predicting the relative displacements of cations prior to experiments. Here, machine learning is used to predict the average displacement of octahedral cations from its high-symmetry position in ferroelectric perovskites. Published octahedral cation displacements data from density functional theory (DFT) calculations are used to train machine learning models, where each cation is represented by features such as Pauling electronegativity, Martynov–Batsanov electronegativity and the ratio of valence electron number to nominal charge. Average displacements for ten new octahedral cations for which DFT data do not exist are predicted. Predictions are validated by comparing them with new DFT calculations and existing experimental data. The outcome of this work has implications in the design and discovery of novel ferroelectric perovskites.

Experimental Study of Montmorillonite Structure and Transformation of its Properties under the Treatment of Inorganic Acid Solutions

The paper discusses the mechanism of montmorillonite structure alteration and bentonites properties modification (on the example of samples from clay deposit Taganka, Kazakhstan) due to the thermochemical treatment (treatment with inorganic acid solutions at different temperatures, concentrations and reaction times). With the use of the suit of methods certain processes were distinguished: transformation of montmorillonite structure, which appears in the leaching of interlayer and octahedral cations, protonation of the interlayer and OH groups at octahedral sheets. Changes in the structure of the 2:1 layer of montmorillonite and its interlayer result in significant changes in the properties – reduction of cation exchange capacity and an increase of specific surface area. The results of the work showed that bentonite clays retain a significant portion of its adsorption properties even after the long term and intense thermochemical treatment (6M HNO3, 60°C, 108 hours)

Barium oxide as a modifier to stabilize the γ-Al2O3 structure

This research concentrated on the structural stability of γ-alumina (γ-Al2O3) was investigated by a combination of differential thermal analysis, X-ray diffractometry and surface-area measurements. The γ –to– θ and then α phase transitions were observed as an exothermic peak at 1000°C–1400°C in the DTA curves. The role of barium oxide as a modifier to stabilize γ-Al2O3 structure has been investigated. XRD measurements show that after calcination at 1000°C for 2 h, a significant fraction of the pure γ-Al2O3 (BaO-free) transformed to θ-Al2O3 while that the transition phase in alumina samples modified by BaO have been reduced significantly. Barium oxide, eliminate pentacoordinated aluminum ions through coordinative saturation and alter these ions into octahedral cations and effectively suppressed the γ –to– α phase transition in Al2O3, which concluded as improving the thermal stability and porous properties of the experimental samples.

The Chemistry of Extended Oxide Surfaces

In Chapters 4 and 5, we demonstrated that local structures and charge distributions have an enormous impact on the equilibrium constants, trajectories, and kinetics of reactions involving soluble oxide precursors. In this chapter, we highlight those features that make reactions on extended oxide surfaces either similar to or dramatically different from the reactions documented in hydrolysis diagrams for each metal cation (see Chapter 5). We first describe oxide surface structures and then discuss how these structures impact both acid–base and ligand-exchange phenomena. In addition to dense oxides, we also introduce some of the chemistry associated with layered materials. Lamellar materials are important from both a fundamental and technological perspective, because water and ions can readily penetrate such structures and provide conditions under which almost every oxygen anion is at an oxide–water interface (see Chapter 10 and Chapter 11). This chapter focuses on oxides containing octahedral cations. The distinctive chemistry of oxides based on tetrahedral cations, including the clay minerals and the zeolites, are the focus of Part Five. The structures of bulk oxides were introduced in Chapter 2. However, for many oxides, the surface structures that interact with aqueous solutions are substantially different from structures found in the bulk. Here, we introduce the basic principles of oxide surfaces that make them chemically active. As a starting point, consider ideal oxide surfaces containing +2 octahedral cations. Pristine oxide surfaces can be created by cleaving perfect crystals in an ultrahigh-vacuum environment. The creation of new surfaces requires an expenditure of energy corresponding to the cohesive energy of the solid, which in turn represents the energy required to break every bond along a given fracture plane. For MgO, the Mg−O bond energy is 380 kJ/mole. Each surface created contains 1.4.1019 oxygen atoms/m2, or 2.4.10−5 moles of bonds. Because two surfaces are created in the fracture event, the initial interfacial energy of each resulting MgO surface is (1/2)(380 kJ/mole)/(2.4_10−5 mole/m2 )=4560 mJ/m2.