Cation-cation Repulsion Research Articles

Adsorption isotherms and removal of lead (II) and cadmium (II) from aqueous media using nanobiochar and rice husk

Removal of Cd(II) and Pb(II) from aqueous solutions is a challenging task and the search for novel adsorbents is underway. This study examined the efficiency of nanobiochar (NB) and rice husk (RH) in the adsorption and removal of Cd(II) and Pb(II) from water. The effect of various physicochemical parameters such as initial pH, initial Cd and Pb concentration, adsorbent dosage, and contact time were tested. SEM/EDX images confirmed the adsorption of Pb and Cd with surface physical and chemical changes. The maximum Pb removal was noted at pH 6 using NB (96%) and at pH 8 for RH (90%), and the maximum Cd removal by NB was recorded at 8 pH (91%) and by RH at pH 6 (87%). The decline in adsorption intensity at lower pH suggested protonation of the adsorbent surface causing cation-cation repulsion. Most of the adsorption occurred within the initial 60 min. A continuous gradual increase in the adsorption with time suggested multilayer formation. Of the three isotherms, the Freundlich model fits the present data best, implying an infinite surface coverage and indicating the potential for multilayer adsorption of Pb and Cd on the surfaces of RH and NB adsorbents. In conclusion, this study highlights the promising potential of NB as a cost-effective adsorbent for the removal of Cd and Pb ions from aqueous solutions.

In situ potassium and hydrogen ion exchange into a cubic zirconium silicate microporous material.

The selective separation of ions from aqueous systems, and even in the human body, is a crucial to overall environmental management and health. Nanoporous materials are widely known for their selective removal of cations from aqueous media, and therefore have been targeted for use as a pharmaceutical to treat hyperkalemia. This study investigated the detailed crystallographic molecular mechanisms that control the potassium ion selectivity in the nanoporous cubic zirconium silicate (CZS) related materials. Using time-resolved in situ Raman spectroscopy and time-resolved in situ X-ray diffraction, the selectivity mechanisms were determined to involve a synchronous cation-cation repulsion process that serves to open a favorable coordination bonding environment for potassium, not unlike the ion selectivity filter process found in potassium ion channels in proteins. Enhancement of ion exchange was observed when the CZS material was in a partial protonated state (≈3:1 Na:H), causing an expansion of the unit-cell volume, enlargement of the 7 member-ring window, and distortion of framework polyhedra, which allowed increased accessibility to the cage structures and resulted in rapid irreversible potassium ion exchange.

Structural Transition in (C2H5NH3)3Bi2-xSbxI9:[(Bi/Sb)2I9]3- Dimers to [(Bi/Sb)3I12]3- Trimers to (∞1)[(Bi/Sb)2I93-] 1D Infinite Chains.

Antimony/bismuth-based lead-free hybrid halide defect 2D perovskites have been generating enormous research interest due to their inherent excellent optical properties. Exploration of new phases and understanding of their structural and optoelectronic properties are of paramount importance in the process of developing materials for practical solar cell applications. In this article, we have reported a structural transition from the 0D hexagonal phase containing isolated [M2I9]3- (M = Bi/Sb) units to the 1D orthorhombic phase via a new monoclinic phase with novel isolated trimeric [M3I12]3- units in (C2H5NH3)3Bi2-2xSb2xI9. The hexagonal phase is stable up to 2x = 0.6 in (C2H5NH3)3Bi2-2xSb2xI9. With gradual substitution of Sb, the cation-cation repulsion increases, which destabilizes the [M2I9]3- unit, and hence, the hexagonal phase becomes unstable. At intermediate composition, 2x = 0.8-1.6, a new monoclinic phase (S.G.: C2/m) with the composition (C2H5NH3)2Bi2-2xSb2xI8 is formed, containing isolated [M3I12]3- units. The symmetry reduction resulted in larger distortion, which relaxes the strain and stabilizes the trimeric unit in the intermediate compositions. Finally, at higher Sb compositions (2x = 1.9-2.0), the compounds crystallize in the orthorhombic 1D phase. In all three phases of (C2H5NH3)3Bi2-2xSb2xI9, the cationic ethylammonium units are completely disordered over the whole unit cell. Raman study clearly shows the phase transition in (C2H5NH3)3Bi2-2xSb2xI9 and also the structural distortion in (C2H5NH3)2Bi2-2xSb2xI8. Optical property study shows that all the compounds are of indirect band gap type. Furthermore, PL study shows better emission properties of the 1D orthorhombic Sb compounds as compared to the 0D hexagonal and monoclinic phases.

Synergism of mixed cationic collectors in the flotation of quartz unveiled by AFM, solution chemistry and quantum chemical calculations

The considerable increase in worldwide demand for iron ore requires more studies for efficiency of silicate gangue flotation. New reagents and synergistic mixtures have been pursued to tackle this technological problem. Although there are several studies of collector mixtures in the literature, very few were dedicated to mixtures of cationic collectors. To fill such gap, the interaction between ethermonoamine (M) and etherdiamine (D) was evaluated using bench flotation tests with a typical Brazilian iron ore, solution chemistry analysis and density functional theory calculations (DFT) for the first time in literature. The combined use of these collectors in the proportion 3 D: 1 M, w/w, at pH 10.5, provided the best recovery and selectivity, showing synergism. The iron recovery was 83.9% with a rougher concentrate presenting an iron and silica content of 64.0% and 4.6%, respectively. Differences in the pKa values of primary and secondary amino groups point to minimization of electrostatic repulsion between adsorbed collectors. Interaction free energy values indicate that the complexes M(0)-D(+) (−8.29 kcal/mol), D(0)-D(+) (−7.74 kcal/mol) and M(+)-D(0) (−8.66 kcal/mol) are the most thermodynamically favored and, therefore, confirm the experimental results and explain the nature of the favorability, which is related to the reorganization of the collectors in order to minimize ether-ether and cation-cation repulsion and maximize ether-amine or amine–amine neighbors, especially when the amino group is protonated.

Two new members in the quaternary Cs-Ag-As-S family with different arrangements of Ag-S and As-S asymmetric building units: syntheses, structures, and theoretical studies.

New functional materials of As-based multinary chalcogenidometalates have received special attention due to their different oxidation states and flexible building blocks. In this work, two new quaternary thioarsenates(iii), namely Cs2Ag2As2S5 (I) and Cs3AgAs4S8 (II), have been obtained by a simple surfactant-thermal technique. Their structures were determined on the basis of single-crystal X-ray diffraction; compound I belongs to the triclinic space group P1[combining macron] (no. 2) [a = 7.49(2) Å, b = 9.30(2) Å, c = 9.94(2) Å, α = 63.28(3)°, β = 82.11(3)°, γ = 87.78(3)°, V = 612.4(2) Å3 and Z = 2], whereas compound II crystallizes in the monoclinic space group C2/c (no. 12) [a = 27.76(6) Å, b = 6.98(2) Å, c = 19.68(4) Å, β = 109.85(3)°, V = 3586.7(2) Å3 and Z = 8]. Both structures are shown to feature two-dimensional (2D) layers with different arrangements of Ag-S and As-S asymmetric building units (ABUs). In compound I, the layer is constructed from vertex-sharing dimeric [As2S5] units linked by tetrameric [Ag4S8] moieties. The layered structure in compound II is formed by 21 helical chains based on [As4S9] groups which are further interconnected by tetrahedral [AgS4] chains. The analysis of the quaternary X-Ag-As-S (X = cations) system shows that various combination ways of Ag-S and As-S ABUs in these sulfides can be attributed to the cation-cation repulsion and stereochemically active lone-pair effect. The UV-vis/NIR absorption spectrum indicates that the optical band energy (Eg) of compounds I and II is 2.48 eV and 2.24 eV, respectively. Moreover, theoretical calculations indicate that the As-S and Ag-S bonding states determine the optical properties of the title compounds. This work not only enriches the structural chemistry of thioarsenates, but also opens up a new avenue for exploring novel multinary chalcogenidometalates.

BaLiTe2O5X (X = Cl, Br): mixed alkali/alkaline-earth metal tellurite halides with [Te2O5]∞ chains.

Isostructural mixed alkali/alkaline-earth metal tellurite halides BaLiTe2O5X (X = Cl, Br) have been successfully synthesized by a mild hydrothermal method, which possess the [Te2O5]∞ chains formed by the corner-sharing connections of the TeO3 and TeO4 polyhedra. The [Te2O5]∞ chains are bridged via the LiO3 polyhedra to form the [LiTe2O5]∞ layers with 4- and 6-membered rings. The adjacent [LiTe2O5]∞ layers are linked via the BaO9 polyhedra to form the [BaLiTe2O5]∞ double layers, which are further connected via weak Ba-X-Li interactions to generate a 3D framework. Analysis of different lithium tellurites shows that the different arrangements of the Te-O and Li-O units in the lithium tellurites can be attributed to the cation-cation repulsion and lone-pair effect. Both compounds display good thermal stability, wide optical bandgaps (4.25 and 4.13 eV), and large birefringence. First-principles calculations indicate that the TeOn groups and X- determine the optical properties of BaLiTe2O5X (X = Cl, Br).

Identification of a pH sensor in Influenza hemagglutinin using X-ray crystallography

Hemagglutnin (HA) mediates entry of influenza virus through a series of conformational changes triggered by the low pH of the endosome. The residue or combination of residues acting as pH sensors has not yet been fully elucidated. In this work, we assay pH effects on the structure of H5 HA by soaking HA crystallized at pH 6.5 in a series of buffers with lower pH, mimicking the conditions of the endosome. We find that HA1-H38, which is conserved in Group 1 HA, undergoes a striking change in side chain conformation, which we attribute to its protonation and cation-cation repulsion with conserved HA1-H18. This work suggests that x-ray crystallography can be applied for studying small-scale pH-induced conformational changes providing valuable information on the location of pH sensors in HA. Importantly, the observed change in HA1-H38 conformation is further evidence that the pH-induced conformational changes of HA are the result of a series of protonation events to conserved and non-conserved pH sensors.

Chemical and structural variability in cubic spinel oxides.

The empirical relations between cubic spinel oxides of different compositions were investigated using data from 349 refined crystal structures. The results show that the spinel structure is able to tolerate many constituents (at least 36) by enlarging and decreasing the tetrahedra and octahedra. This is reflected in a large variation in tetrahedral and octahedral bond distances. The oxygen positional parameter (u) may be regarded as a measure of the distortion of the spinel structure from cubic close packing or of the angular distortion of the octahedron. The distortion can best be explained in terms of ionic potential (IP), which merges the size and charge properties of an ion. Sterically induced distortion depends on ion size, whereas electrostatically induced distortion is caused by cation-cation repulsion across faces of tetrahedra and shared edges of octahedra. The strong correlations between the u parameter and the IP at the T and M sites are consistent with the main role played by the both charge and size. Large distortions (u ≫ 0.27) result in oxygen-oxygen distances of the octahedron shorter than 2.50 Å, which would lead to structural instability because of increased non-bonded repulsion forces between the oxygen atoms.

Effect of zeolitic imidazole framework (ZIFs) shells of core-shell microspheres on adsorption of Roselle red dye from water

Due to the importance of food safety, natural edible pigments attract more and more attention. Because of the application of adsorption technology in the extraction and purification of natural edible pigments, it is necessary to develop a highly efficient adsorbent. In this paper, we prepared a series of core shell spherical adsorbents (CSS-ZIFs) having different types of zeolitic imidazolate frameworks (ZIF-67, ZIF-7, ZIF-8 and ZIF-9) nanoparticles as the shell on the resin microsphere as the core. The XRD、BET and SEM characterizations proved the core-shell structure of the CSS-ZIFs, and the adsorption performance of the CSS-ZIFs were further studied for removing a natural edible pigment (Roselle red) from water. The CSS-7 (ZIF-7 as the shell) showed the best removal rates of 95.3–97.7% mainly due to the strong π-π attraction and weak cation-cation repulsion between ZIFs and Roselle red molecules. This study demonstrates that novel CSS-ZIFs can be used as a kind of feasible adsorbents for Roselle red extraction and separation, and further applied to the adsorption separation of other natural active substances.

Protonated nucleobases are not fully ionized in their chloride salt crystals and form metastable base pairs further stabilized by the surrounding anions.

This paper presents experimental charge-density studies of cytosinium chloride, adeninium chloride hemihydrate and guaninium dichloride crystals based on ultra-high-resolution X-ray diffraction data and extensive theoretical calculations. The results confirm that the cohesive energies of the studied systems are dominated by contributions from intermolecular electrostatic interactions, as expected for ionic crystals. Electrostatic interaction energies (Ees) usually constitute 95% of the total interaction energy. The Ees energies in this study were several times larger in absolute value when compared, for example, with dimers of neutral nucleobases. However, they were not as large as some theoretical calculations have predicted. This was because the molecules appeared not to be fully ionized in the studied crystals. Apart from charge transfer from chlorine to the protonated nucleobases, small but visible charge redistribution within the nucleobase cations was observed. Some dimers of singly protonated bases in the studied crystals, namely a cytosinium-cytosinium trans sugar/sugar edge pair and an adeninium-adeninium trans Hoogsteen/Hoogsteen edge pair, exhibited attractive interactions (negative values of Ees) or unusually low repulsion despite identical molecular charges. The pairs are metastable as a result of strong hydrogen bonding between bases which overcompensates the overall cation-cation repulsion, the latter being weakened due to charge transfer and molecular charge-density polarization.

Interaction between the exchanged Mn2+ and Yb3+ ions confined in zeolite-Y and their luminescence behaviours

Luminescent zeolites exchanged with two distinct and interacted emissive ions are vital but less-studied for the potential applications in white light emitting diodes, solar cells, optical codes, biomedicine and so on. Typical transition metal ion Mn2+ and lanthanide ion Yb3+ are adopted as a case study via their characteristic transitions and the interaction between them. The option is considered with that the former with d-d transition has a large gap between the first excited state 4T1 and the ground state 6A1 (normally >17,000 cm−1) while the latter with f-f transition has no metastable excited state above 10,000 cm−1, which requires the vicinity of these two ions for energy transfer. The results of various characterizations, including BET measurement, photoluminescence spectroscopy, solid-state NMR, and X-ray absorption spectroscopy, etc., show that Yb3+ would preferably enter into the zeolite-Y pores and introduction of Mn2+ would cause aggregation of each other. Herein, cation-cation repulsion may play a significant role for the high valence of Mn2+ and Yb3+ when exchanging the original cations with +1 valence. Energy transfer phenomena between Mn2+ and Yb3+ occur only at elevated contents in the confined pores of zeolite. The research would benefit the design of zeolite composite opto-functional materials.

High-pressure synthesis, structure, phase relation of polar LiNbO3-type ZnTiO3

Ferroelectricity, piezoelectricity, pyroelectricity, and second-order nonlinear optical behavior are technologically important. Because these properties are attributable to the noncentrosymmetric (NCS) structure[1], the search for materials exhibiting such characteristics must begin with a search of NCS materials. Among them, LiNbO3-type (LN-type) compounds with a chemical formula of ABX3 exhibit NSC structures with hexagonal polar space group R3c whose BX6 octahedra three-dimensionally share their corners the same as perovskite-type compounds[2]. In LN-type compounds, the A cation - B cation repulsion directs the spontaneous polarization along c-axis(Fig. 1). Therefore, we might find attractive functional properties by the selection of constituent ions based on their having a naturally occurring polar LN-type structure attributable to the cation-cation repulsion. With the ideas mentioned above, we have investigated the high-pressure synthesis and characterization of novel LN-type oxides such as ZnSnO3, PbNiO3, CdPbO3 as well as known MnMO3 (M = Ti, Sn). Recently, we have successfully synthesized a polar LN-type titanate ZnTiO3 (LN-ZTO) under high pressure and high temperature [3]. The first principles calculation indicates that LN-ZTO is a meta-stable phase obtained by the transformation in the decompression process from the perovskite-type phase, which is stable at high pressure and high temperature. The Rietveld structural refinement reveals that LN-ZTO exhibits greater intra-distortion of the TiO6 in LN-ZTO than that of the SnO6 in LN-type ZnSnO3 (LN-ZSO). The estimated spontaneous polarization are greater than those of LN-ZSO, which is attributed to the great displacement of Ti along c-axis and the greater Born effective charge of Ti (+6.1) than that of Sn (+4.1). Furthermore, the spontaneous polarization of LN-ZTO is greater than that of LiNbO3, indicating that LN-ZTO, like LiNbO3, is a candidate ferroelectric material with high performance. The second harmonic generation (SHG) response of LN-ZTO is 24 times greater than that of LN-ZSO. The findings indicate that the intra-octahedral distortion, spontaneous polarization, and SHG response are caused by the stabilization of the polar LN-type structure and reinforced by the second-order Jahn-Teller effect attributable to the orbital interaction between oxygen ions and d0 ions such as Ti4+. We also discuss the relationship between the intra-distortion of BO6 and polarity in several LN-type oxides.

Cation ordering over short-range and long-range scales in the MgAl2O4-CuAl2O4 series

A multi-analytical approach using electron microprobe analysis, X-ray structural refinement, and optical absorption spectroscopy was applied to characterize short-range and long-range structures of synthetic spinel single crystals along the MgAl2O4-CuAl2O4 solid-solution series. Site populations, derived from the results of site-scattering refinement and stereochemical analysis, show that the tetrahedrally coordinated site (T) is mainly populated by Mg and Cu2+, while the octahedrally coordinated site (M) is dominated by Al. Crystals also show a significant degree of inversion, i.e., occurrence of Al at T counterbalanced by occurrence of divalent cations at M, which increases slightly from 0.24 to 0.29 for the highest Cu2+ contents. Short-range information derived from optical spectra suggests that the local TCu2+-O distances remain constant at increasing Cu2+ content, whereas local MCu2+-O distances are ca. 0.02 Å shorter in Cu-poor MgAl2O4 spinels as compared to MCu2+-O distances in end-member CuAl2O4. The observed splitting of an absorption band, at ca. 7000 cm-1 , caused by electron transitions in TCu2+ as well as the anomalous broadness of an absorption band, at ca. 13 500 cm-1, caused by electron transitions in MCu2+ indicates the occurrence of local Jahn-Teller distortions at T and M. Long-range information, however, shows no violation of Fd3m symmetry. From refinements of our single-crystal XRD data we could for the first time derive for a cubic spinel phase a MCu2+-O distance of 2.080 Å and a TCu2+-O of 1.960 Å. The very limited variations in the unit-cell parameter a from 8.079 to 8.087 Å are mainly related to the disordering of Al. Because of the very similar size of Cu2+ and Mg at the T and M sites, the spinel structure responds to the Cu2+ → Mg substitution by increasing cation disordering in such a manner that mean M-O distances remain constant and the mean T-O distances decrease slightly. This results in increasing length of shared octahedral edges and thereby increase of the octahedral cation-cation repulsion. In line with other studies, the importance of steric factors for controlling the cation distributions in the spinel structure is demonstrated to be valid also in the MgAl2O4-CuAl2O4 solid-solution series.

Zn-O tetrahedral bond length variations in normal spinel oxides

Six synthetic single crystals of spinel phases with different compositions along the ZnAl 2 O 4 -ZnCr 2 O 4 solid solution were structurally and chemically characterized by X-ray diffraction and electron microprobe techniques. As predicted, unit-cell parameters and octahedral bond lengths (M-O) increase with increasing replacement of Al 3+ by Cr 3+ . Despite the constant occupancy of the T site by Zn, also the tetrahedral bond length T Zn-O shows significant variations along this binary. These variations are positively correlated with variations in M-O bond lengths. The present data in conjunction with data from literature provide a basis for quantitative analyses of the variation in T Zn-O in normal spinel structures. A negative correlation between T Zn-O and the ionic potential at M ( M IP ) suggests that increasing M IP is related to a stronger electrostatic cation-cation repulsion across the shared octahedral edge M (O-O) shared of the structure. An observed negative correlation between M IP and M (O-O) shared suggests that a decrease of M (O-O) shared provides a more efficient shielding effect to reduce the octahedral cation interactions. In normal Zn B 2 O 4 spinels (where B = Al 3+ , Cr 3+ , Ga 3+ , V 3+ , Fe 3+ , and Mn 3+ ) cations with a smaller size provides a higher charge density. Increasing charge density at the M site causes shortening of M (O-O) shared , which in turn results in shorter T Zn-O bond length. In general, variations in T Zn-O are required by the structure to better provide an oxygen shielding effect to the octahedral cation-cation repulsion.

Jahnsite-(NaFeMg), a new mineral from the Tip Top mine, Custer County, South Dakota: Description and crystal structure

Jahnsite-(NaFeMg), NaFe 3+ Mg 2 Fe 2 3+ (PO 4 ) 4 (OH) 2 ·8H 2 O, space group P 2/ a , a = 15.0811(16), b = 7.1403(8), c = 9.8299(11) A, β = 110.445(1)°, V = 991.8(2) A 3 , and Z = 2, is a new mineral from the Tip Top mine, Custer County, South Dakota. It occurs as the dominant chemical zone in transparent pseudo-orthorhombic twinned prisms up to 1 mm long with shallow wedge-like terminations, both as isolated twinned crystals and in subparallel to divergent intergrowths. The crystals are yellow with orange-red bands just below the terminations. The mineral has a white streak, vitreous luster, Mohs hardness of 4, and good {001} cleavage. The measured density is 2.58(2) g/cm 3 , and the calculated density is 2.608 g/cm 3 . It is optically biaxial (–), with α = 1.632(1), β = 1.669(1), γ = 1.671(1) (589 nm); 2 V (meas.) = 25(5)°, 2 V (calc.) = 26°; dispersion: r > v , very strong; orientation: Y = b , Z ^ a = +28° (in β obtuse); pleochroism: X = colorless, Y and Z = beige; Y = Z > X . Electron microprobe analyses provided: Na 2 O 2.82, CaO 0.34, MnO 0.32, MgO 10.27, Fe 2 O 3 27.35, P 2 O 5 35.93, H 2 O 21.58 (calc.), total 98.61, which in terms of the general formula for the whiteite-jahnsite group XM1M2 2 M3 2 (PO 4 ) 4 (OH) 2 ·8H 2 O, yields the empirical formula: (Na 0.72 Ca 0.05 Mn 0.04 2+ )(Fe 0.72 3+ Mg 0.01 )Mg 2.00 Fe 2.00 3+ (PO 3.77 OH 0.23 ) 4 (OH) 2 ·8H 2 O. Crystal structure refinement ( R 1 = 0.041) supports these cation site assignments. Typical crystals have as many as five chemical zones from base to termination corresponding to the following jahnsite species: jahnsite-(NaMnMg), jahnsite-(NaFeMg), jahnsite-(CaMnMg), jahnsite-(NaFeMg), jahnsite-(CaMgMg). The characterization of the new mineral jahnsite-(NaFeMg) is based upon the second zone listed, which corresponds to roughly 80% of the crystal volume. Jahnsite-(NaMnMg) and jahnsite-(CaMgMg) also represent new species; however, the very small volume of these zones is insufficient to permit complete characterization of these species. Jahnsite-(NaFeMg) is the first member of the whiteite-jahnsite group in which a monovalent cation (Na + ) predominantly occupies the X site and a trivalent cation (Fe 3+ ) the M1 site. The structure differs from that determined for jahnsite-(CaMnMg) in that the X site is [8]-coordinated and the M1 and P1 polyhedra exhibit greater distortions caused by cation-cation repulsion across their shared edges. The determination of H positions allows the elucidation of a definitive H-bonding scheme.

Patterns in the compositions of oxysalt and sulfosalt minerals, and the paradoxical nature of quartz

Compositions of oxysalt, sulfosalt, seleniosalt, tellurisalt, and fluosalt minerals all follow the same generalizable patterns with regard to hardness or softness of cations, and with regard to ionic potential. For example, the hard F1− anion forms simple fluosalt minerals (fluoborates and fluosilicates) with hard cations. The relatively hard O2− anion forms simple oxysalts (e.g., sulfates and arsenates) largely with hard to intermediate cations. On the other hand, the softer S2− anion forms sulfosalt minerals almost entirely with intermediate to soft cations, and the still softer anions Se2− and Te2− form seleniosalt and tellurisalt minerals with an even more predominately soft suite of cations. Thus, across all these classes of minerals, harder anions generally form minerals with harder cations, and softer anions form minerals with softer cations. The compositions of simple oxysalt minerals of one inter-radical cation vary greatly, but they do not include minerals in which both the inter-radical cation and intra-radical cation (e.g., Ca2+ and S6+ in CaSO4) have high ionic potential. Their compositions thus reach, but do not exceed, a limit on a plot of the ionic potential of cations in these minerals. The same general relationship, wherein no simple minerals contain both inter-radical and intra-radical cations of high ionic potential, also exists among sulfosalts, seleniosalts, tellurisalts, and fluosalts. These limits of ionic potential of cations are greatest in oxysalts and progressively less in fluosalts, sulfosalts, seleniosalts, and tellurisalts. The limits thus decrease, both through the 2− anions and across the hard anions, from more negative ionic potential of the anion to less so, seemingly because anions of less negative ionic potential have lesser capability to shield cations from each other and thereby preclude cation-cation repulsion. Among the simple oxysalts of one inter-radical cation, three minerals near the oxysalt limit form less commonly, or with greater inhibition, than one might expect. For example, thermodynamic calculations indicate that magnesite should precipitate abundantly from seawater, but it does not instead, in low-temperature systems Mg2+ only forms simple carbonate minerals with a cation of lesser ionic potential (Ca2+). Similarly, the chemistry of Earth’s crust suggests that (Al,Fe3+)2(SO4)3 would be a common mineral, but instead analogous hydrous minerals with additional cations of lesser ionic potential are common. Finally, SiO2 minerals crystallize at distinctly low temperatures in magmatic systems, and they either fail to nucleate or precipitate as a hydrous disordered mineral in many low-temperature systems. In all three cases, cation-cation repulsion seemingly inhibits formation of what might otherwise be more common or more readily forming minerals.

Assignment of dielectric loss of K–L zeolite

Dielectric spectra of zeolite L (LTL) with potassium ions were measured to investigate movements of potassium ion in it. K,TMA-L was prepared through an ion-exchange of K9-L with tetramethylammonium (TMA) ion, and changed to K,H-L by gentle calcination in air. Since the TMA ion in K,TMA-L was expected to exclusively occupy site D, the hydrogen ion in K,H-L was considered to be located around site D. K9-xHx-L with 0<or=x<or=1.1 showed an apparent single loss peak, and the magnitude of loss peak decreased with increasing x. Based on the variation of magnitude of loss peak with x, the loss was attributed to movements of K+/D. The apparent single peak contained two loss peaks. The reason why the movements of K+/D brought about two losses was due to jumps among three inequivalent sites containing the site D. The activation energy of movement related to the main loss decreased with an increase in x. The decrease in activation energy was explained by the influence of cation-cation repulsion, namely, the repulsion energy between K+/D and cation/neighbor-D decreases after the replacement of K+/neighbor-D with a hydrogen ion because of the increase in distance between cations, and hence the activation energy of movement of K+/D decreases.

Mixing and ordering behavior in manganocolumbite-ferrocolumbite solid solution: A single-crystal X-ray diffraction study

The structural changes upon cation substitution in natural AB 2 O 6 columbites have been studied by means of single-crystal X-ray diffraction. Most of the structural variations across the MnNb 2 O 6 -FeNb 2 O 6 solid solution in completely ordered samples can be simply understood in terms of ionic radii. The substitution of Fe for the larger Mn cation causes a linear decrease of all unit-cell parameters. Going from manganocolumbite to ferrocolumbite the site A is reduced in volume and becomes less distorted. The oxygen cage around the cation assumes a more regular arrangement since the mismatch between A and B chains decreases. At the same time, the divalent cation moves toward the barycenter of the polyhedron. The B site, which is not involved in the Fe-Mn cation substitution, maintains its geometry unchanged. Ordering of divalent cations at A sites and pentavalent cations at B sites causes linear variations of a and c cell parameters. A non-linear behavior is shown by the b cell parameter that shows a minimum at order parameter Q m ~ 0.7. A discontinuity at this Q m value is also shown by other structural parameters. Cation ordering also causes volume variations of the two octahedral sites as a consequence of the different ionic radii of the various species. Octahedral bond-length distortion parameters show that the B site is in general more distorted than the A site; distortion of the B site increases with ordering due to higher cation-cation repulsion along the B octahedral chain and to the second-order Jahn-Teller (SOJT) effect. Octahedral chains respond to modifications of the polyhedra by folding along the common edge.

High-pressure single-crystal X-ray and powder neutron study of F,OH/OD-chondrodite: Compressibility, structure, and hydrogen bonding

The crystal structure of a natural F-bearing chondrodite [Mg4.64Fe0.28Mn0.014Ti0.023(Si1.01O4)2F1.16OH0.84; a = 4.7328(1), b = 10.2749(2), c = 7.8756(1) Å, a = 109.065(1)∞; space group P21/b (a unique)] from the Tilly Foster mine (Brewster, New York) and the structure of a deuterated synthetic analogue [Mg5(SiO4)2F1.10OD0.90] were refined at high pressure using single-crystal X-ray and time-of-flight neutron powder diffraction data, respectively. The isothermal equation of state was determined from single-crystal high-precision X-ray diffraction measurements of unit-cell parameters as a function of pressure up to 9.63(1) GPa. A third-order Birch-Murnaghan equation-of-state fit to pressurevolume data results in V0 = 362.00(2) Å3, K0,T = 117.0(4) GPa and KT' = 5.6(1). The larger value of K0,T relative to that determined for the OH end-member confirms that F makes the chondrodite structure less compressible. The axial compressibilities behave anisotropically with fitted parameters a0 = 4.73282(9) Å, K(a)0 = 458(3) GPa, and K(a)' = 28.9(9); b0 = 10.2759(3) Å, K(b)0 = 298(1) GPa, and K(b)' = 13.4(4); c0 = 7.8759(2) Å, K(c)0 = 316(2) GPa, and K(c)' = 16.7(5). X-ray structural investigations at 0.0001, 4.952(7), and 9.550(7) GPa in a diamond-anvil cell reveal a complex compression mechanism. The greatest changes with pressure occur in the M2 octahedra, which are located at the flexion points of the octahedral chain. These octahedra have the longest bonds at ambient conditions due to cation-cation repulsion, which displaces the cation from the centroid of the octahedron. The application of pressure overcomes the effect of cation-cation repulsion inducing a strong bond-length compression. The neutron powder data were collected at 0.0001, 1.26(1), 2.89(2), 3.87(1), 5.27(2), and 7.04(2) GPa in the Paris-Edinburgh cell using a new gasket design and a fluid pressure medium. Changes in the O-D···O/F hydrogen bond geometry with pressure can be interpreted in terms of a rotation of the OD vector and a reduction in the degree of static disorder associated with the hydrogen bond acceptor/donor site. These new crystallographic data are discussed in terms of corresponding spectroscopic measurements reported recently in the literature

ガーネット構造における近接陽イオン間斥力の重要性

Crystal chemistry and structure of garnets are reviewed with special attention to the cation-cation repulsion across the shared edges of polyhedra. The cation-cation repulsion plays an important role in the stability of garnet structure, and this effect is the most prominent between tetrahedral and dodecahedral cations. The magnitude of the cation-cation repulsion is closely related to the degree of polyhedral distortion, based on the difference in length between the shared and the unshared edges in polyhedra, due to the structural geometric restriction. Especially, the cation-cation repulsion increases in garnet solid solutions owing to the decrease in the shielding effect when the structural geometric restriction brought by the substitution of the cations with different sizes forcibly stretches the shared edges compared with the unshared ones. This is a factor to destabilize the crystal structure. Both the tendency of cation distribution in Y3Fe5O12 (YIG)-Y3Ga5O12 (YGG) system and the symmetry in Mg3Al2Si3O12-MgSiO3 system change with the compositional variations in these solid solutions. These changes are the good examples of the destabilization of the crystal structures due to the effect of the cation-cation repulsion.