Tetrahedral Cations Research Articles

Cation distributions and magnetism of Al-substituted CoFe2O4 - NiFe2O4 solid solutions synthesized by sol-gel auto-combustion method

Aluminium substituted cobalt-nickel ferrite nanoparticles were synthesized by citrate gel auto-combustion method followed by annealing at 1000 °C for 1 h in air. Scanning electron micrographs of all the samples show crystalline particles of irregular morphology with a small variation in particle sizes (~ 110–160 nm). From the analysis of the X-ray diffraction results we observed that the unit cell parameter decreases linearly with increase in aluminium concentration due to the smaller ionic radius of the Al3+ ions substituting the other cations such as Co2+, Ni2+ and Fe3+ ions in the compounds. The room temperature Mössbauer spectra of the samples show Zeeman split sextet patterns corresponding to the tetrahedral (Th) and octahedral (Oh) interstitial iron (Fe3+) cations. The observed magnetic hyperfine field (Bhf) decreases with increase in Al-concentration due to the distribution of diamagnetic Al3+ in the environment of 57Fe probe atoms. The saturation magnetization measured by Vibrating Sample Magnetometer (VSM) shows a similar trend like that of Bhf. The distributions of the cations obtained from the Rietveld refinement and Mössbauer spectroscopy results indicate an increase in Fe3+(Th)/Fe3+(Oh) occupancy-ratio on increasing Al3+ concentration, and Ni2+ cations prefer the octahedral site, whereas Co2+ and Al3+ ions redistribute themselves in tetrahedral and octahedral sites, in the ratio 2:3.

Synthesis and Structure of Tetraphenylphosphonium Carboxylates

Hydrates of tetraphenylphosphonium carboxylates were synthesized by the reaction of equimolar amounts of pentaphenylphosphorus with 2-methoxybenzoic, 2-nitrobenzoic, and maleic acids in benzene. The product of the pentaphenylphosphorus reaction with tetrafluorophthalic acid (mole ratio 2: 1) is bis- (tetraphenylphosphonium) tetrafluorophthalate hydrate. According to the X-ray analysis data, crystals of tetraphenylphosphonim carboxylates are formed by tetraphenylphosphonim tetrahedral cations and single- or double-charged carboxylate anions. Structural organization of the crystals is determined by C–H···O weak hydrogen bonds formed with the participation of carboxylate groups and water molecules or manifold C–H···F interactions between cations and anions.

Counterion-Induced Asymmetric Control in Ring-Opening of Azetidiniums: Facile Access to Chiral Amines.

Counterion-induced stereocontrol is a powerful tool in organic synthesis. However, such enantiocontrol on tetrahedral ammonium cations remains challenging. Described here is the first example of using chiral anion phase-transfer catalysis to achieve intermolecular ring-opening of azetidiniums with excellent enantioselectivity (up to 97 % ee). Precise control over the formation and reaction of the chiral ion pair as well as inhibition of the background reaction by the biphasic system is key to the success of the reaction.

Counterion‐Induced Asymmetric Control in Ring‐Opening of Azetidiniums: Facile Access to Chiral Amines

AbstractCounterion‐induced stereocontrol is a powerful tool in organic synthesis. However, such enantiocontrol on tetrahedral ammonium cations remains challenging. Described here is the first example of using chiral anion phase‐transfer catalysis to achieve intermolecular ring‐opening of azetidiniums with excellent enantioselectivity (up to 97 % ee). Precise control over the formation and reaction of the chiral ion pair as well as inhibition of the background reaction by the biphasic system is key to the success of the reaction.

Metal-loaded pollucite-like aluminophosphates: dissymmetrisation of crystal structures and physical properties

Two aluminophosphate analogues of the mineral pollucite with the general formula Cs2(M,Al)3P3O12 (where M = Cu or Mn) have been synthesized by high-temperature flux and structurally characterized using the single-crystal X-ray diffraction. Both samples crystallize in cubic I4132 space group, Z = 8, with a = 13.5911(5) and a = 13.8544(7) for Cu- and Mn-loaded phases, respectively. Their framework structures are based on the ANA-type topology and exhibit the partial ordering of the metal (M/Al) and phosphorus (P) cations over the tetrahedral sites. The regular changes in cell dimensions and volumes in the row Cs2(Cu,Al)3P3O12→Cs2(Mn,Al)3P3O12 obviously correspond to increasing radii of the transition metal. The crystal chemical analysis of both pollucite-like phases show correlations between the difference in the radii size of tetrahedral cations and the degree of distortion of flexible ANA-type framework due to decreasing of the intertetrahedral angles (T–O–T). Magnetic susceptibility measurements indicate that both compounds are paramagnets in the temperature range of 2–300 K.

Structure characterization, photoluminescence and dielectric properties of a new hybrid compound containing chlorate anions of zincate (II)

A new hybrid compound, bis (2-aminophenylenamonium) tetrachlorozincate (II), was synthesized and formulated as (C6H9N2)2ZnCl4. Its crystal structure was solved by single crystal X-ray diffraction reveling that compound crystallizes in the monoclinic system, space group C2/c (N°: 15) with cell parameters a = 7.4957(4) Å, b = 25.6837(15) Å, c = 9.4041(5) Å, β = 94.35(0)°, V = 1805.23(45) Å3. Their atomic arrangement can be described as an alternation of inorganic and organic layers, [ZnCl4]2- tetrahedral anions and 2-aminophenylenamonium cations. The cohesion of the atomic arrangement is ensured by hydrogen bonds (strong NH⋯N and weak NH⋯Cl) and π-π stacking interactions between identical antiparallel organic moieties. In optical transmission and photoluminescence measurements, this material exhibit two absorption bands (253 and 316 nm) and a strong emission line (390 nm), while the thermal analysis disclosed a phase transition at 420–445 K previously to the sample decomposition at 476 K. Finally, electrical measurements were performed to discuss the phase-transition mechanism.

New strategy for designing promising mid-infrared nonlinear optical materials: narrowing the band gap for large nonlinear optical efficiencies and reducing the thermal effect for a high laser-induced damage threshold.

To circumvent the incompatibility between large nonlinear optical (NLO) efficiencies and high laser-induced damage thresholds (LIDTs) in mid-infrared NLO materials, a new strategy for designing materials with both excellent properties is proposed. This strategy involves narrowing the band gap for large NLO efficiencies and reducing the thermal effect for a high LIDT. To support these proposals, a series of isostructural chalcogenides with various tetrahedral center cations, Na2Ga2MQ6 (M = Ge, Sn; Q = S, Se), were synthesized and studied in detail. Compared with the benchmark AGS, these chalcogenides exhibit significantly narrower band gaps (1.56-1.73 eV, AGS: 2.62 eV) and high NLO efficiencies (1.6-3.9 times that of AGS at 1910 nm), and also outstanding LIDTs of 8.5-13.3 × those of AGS for potential high-power applications, which are contrary to the conventional band gap view but can be attributed to their small thermal expansion anisotropy, surmounting the NLO-LIDT incompatibility. These results shed light on the search for practical IR NLO materials with excellent performance not restricted by NLO-LIDT incompatibility.

A re-investigation of ardealite from the type locality, the “dry” Cioclovina Cave (Şureanu Mountains, Romania)

Material from the type locality, the “dry” Cioclovina Cave, Sureanu Mountains, Romania, was re-examined in order to update the descriptive mineralogy of ardealite, a rare hydrated calcium acid phosphate sulfate. Ardealite from Cioclovina has a S/P ratio ranging between 1/0.87 and 1/0.98. Although S remains the main tetrahedral cation in the structure, P is consistently present at concentrations between 19.10 and 20.45 wt% P 2 O 5 (0.928–0.992 P atom per formula unit). Concerning the other cations, the mineral shows a very restricted range of composition, without Fe and with very low Mn, Mg, Na and K contents. The indices of refraction are α = 1.530(2), β = 1.537(2) and γ (calculated for 2 V γ = 86°) = 1.543. The measured density [ D m = 2.335(3)–2.342(5) g/cm 3 ] agrees well with the calculated values [ D x = 2.317–2.350 g/cm 3 ]. The average unit-cell parameters refined from 31 sets of X-ray powder diffraction data are a = 5.719(5), b = 31.012(28), c = 6.249(7) A and β = 117.21(6)°. Thermally assisted X-ray diffraction analyses confirm that water is lost in three steps; the loss of molecular water is a two-step process and is complete below 250 °C. The first thermal breakdown products are brushite and bassanite. The band multiplicity on the IR-absorption spectrum (3 ν 3 + 1 ν 1 + 3 ν 4 + 2 ν 2 ) suggests that the protonated phosphate and sulfate groups have C s point symmetry. The mineral derives from the reaction between calcium carbonate from the moonmilk flows or the cave floor and phosphoric solutions derived from bat guano, with or without hydroxylapatite as a precursor, at pH values up to 5.5.

Estimation of thermal conductivity of silicate melts using three-dimensional thermal resistor network model

The thermal conductivity of silicate melts was evaluated using a thermal resistor network model, which consisted of a diamond lattice. Based on the assumption that the mean free path of phonons is shorter than the interatomic distance, the thermal conductivity of the silicate melts was calculated through a Monte-Carlo simulation based on the non-bridging oxygens to tetrahedral cations (NBO/T) ratio and the ratio of the conductance of silicon with NBOs (CSi-NBO) to that of silicon with bonding oxygens (CSi-BO). For NBO/T<1, the simulated thermal conductivity of the silicate melts was in good agreement with that reported in the literature. Thus, the proposed method and the three-dimensional thermal resistor network model used are suitable for estimating the thermal conductivity of silicate melts.

Investigation of sulfide capacity of aluminosilicate slag based on ionic structure considerations

AbstractThe present study investigated the relationship between sulfide capacity and the average nonbridging oxygens to tetrahedral cations (NBO/T) of CaO‐SiO2‐Al2O3‐MgO slags. The sulfide capacity and NBO/T increase with increasing Al2O3/SiO2 ratio and XCaO due to increasing basicity. The inflected dependence of sulfide capacity and NBO/T observed at a eutectic composition. Thermodynamic investigations revealed that the inflected dependence is caused by the effects of mixing enthalpy and entropy components in eutectic composition. Quantitative analysis of sulfide capacity and NBO/T revealed a proportional linear relationship between the two variables. Based on these experimental results, the corresponding structure‐property relationships concerning sulfide capacity, viscosity, and stability are discussed in detail.

Accounting for the species-dependence of the 3500 cm−1H2Otinfrared molar absorptivity coefficient: Implications for hydrated volcanic glasses

Fourier transform infrared (FTIR) spectroscopy can be used to determine the concentration and speciation of dissolved water in silicate glasses if the molar absorptivity coefficients (e) are known. Samples that are thin and/or water-poor typically require the use of the mid-IR 3500 cm–1 total water (H2Ot) and 1630 cm–1 molecular water (H2Om) absorbance bands, from which hydroxyl water (OH) must be determined by difference; however, accurate determination of H2Ot and OH is complicated because e3500 varies with water speciation, which is not usually known a priori. We derive an equation that uses end-member e3500 values to find accurate H2Ot and OH concentrations from the 3500 cm–1 absorbance for samples where only the H2Om concentration need be known (e.g., from the 1630 cm–1 band). We validate this new species-dependent e3500 method against published data sets and new analyses of glass standards. We use published data to calculate new end-member e3500 values of e3500OH = 79 ± 11 and e3500H2Om = 49 ± 6 L/mol∙cm for Fe-bearing andesite and e3500OH = 76 ± 12 and e3500H2Om = 62 ± 7 L/mol∙cm for Fe-free andesite. These supplement existing end-member values for rhyolite and albite compositions. We demonstrate that accounting for the species-dependence of e3500 is especially important for hydrated samples, which contain excess H2Om, and that accurate measurement of OH concentration, in conjunction with published speciation models, enables reconstruction of original pre-hydration water contents. Although previous studies of hydrous silicate glasses have suggested that values of e decrease with decreasing tetrahedral cation fraction of the glass, this trend is not seen in the four sets of end-member e3500 values presented here. We expect that future FTIR studies that derive end-member e3500 values for additional compositions will therefore not only enable the species-dependent e3500 method to be applied more widely, but will also offer additional insights into the relationship between values of e and glass composition.

Canted antiferromagnetism in KNi3[PO3(F,OH)]2[PO2(OH)2]F2 with a stair-case Kagomé lattice

A new nickel phosphate KNi3[PO3(F,OH)]2[PO2(OH)2]F2 has been synthesized using a modified hydrothermal method. Structural characterizations show that it adopts a 3D framework structure with 2D layers of Ni octahedra in a stair-case Kagomé lattice. The Ni2 octahedron at the inversion center shares two trans-faces with Ni1 octahedra to form a linear trimer (Ni3O8F6) as the basic structural unit. The Ni-trimers are linked between themselves by sharing F-corners and to [PO3(F,OH)] tetrahedral groups by sharing O-corners to form 2D stair-case Kagomé layers, which are parallel to the (100) plane and are stacked along the a-axis. Successive Kagomé layers are combined together by [PO2(OH)2] tetrahedral groups and interstice cations K+. Magnetic measurements reveal that KNi3[PO3(F,OH)]2[PO2(OH)2]F2 exhibits a canted antiferromagnetic ordering with a ferromagnetic component at low temperatures.

Revisiting the nontronite Mössbauer spectra

The distribution of ferric iron (Fe 3+ ) between the octahedral and tetrahedral sheets of smectites is still an active problem due to the difficulty of identifying and quantifying the tetrahedral ferric iron ( [4] Fe 3+ ). Mossbauer spectroscopy has often been used to address this problem, with the spectra being fitted by a sum of doublets, but the empirical attribution of each doublet has failed to yield a uniform interpretation of the spectra of natural reference Fe 3+ -rich smectites, especially with regard to [4] Fe 3+ , because little consensus exists as to the [4] Fe 3+ content of natural samples. In an effort to resolve this problem, the current study was undertaken using a series of synthetic nontronites [Si 4–x [4] Fe x 3+ ] [6] Fe 2 3+ O 10 (OH) 2 Na x with x ranging from 0.51 to 1.3. Mossbauer spectra were obtained at 298, 77, and 4 K. Statistically acceptable deconvolutions of the Mossbauer spectra at 298 and 77 K were used to develop a model of the distribution of tetrahedral substitutions, taking into account: (1) the [4] Fe 3+ content; (2) the three possible tetrahedral cationic environments around [6] Fe 3+ , i.e., [4Si]-(3 [6] Fe 3+ ), [3Si [4] Fe 3+ ]-(3 [6] Fe 3+ ), and [2Si 2 [4] Fe 3+ ]-(3 [6] Fe 3+ ); and (3) the local environment around a [4] Fe 3+ , i.e., [3Si]-(2 [6] Fe 3+ ) respecting Lowenstein’s Rule. This approach allowed the range of Mossbauer parameters for [6] Fe 3+ and [4] Fe 3+ to be determined and then applied to spectra of natural Fe 3+ -rich smectites. Results revealed the necessity of taking into account the distribution of tetrahedral cations ( [4] R 3+ ) around [6] Fe 3+ cations to deconvolute the Mossbauer spectra, and also highlighted the influence of sample crystallinity on Mossbauer parameters.

Clinopyroxene-melt element partitioning during interaction between trachybasaltic magma and siliceous crust: Clues from quartzite enclaves at Mt. Etna volcano

A peculiar characteristic of the paroxysmal sequence that occurred on March 16, 2013 at the New South East Crater of Mt. Etna volcano (eastern Sicily, Italy) was the eruption of siliceous crustal xenoliths representative of the sedimentary basement beneath the volcanic edifice. These xenoliths are quartzites that occur as subspherical bombs enclosed in a thin trachybasaltic lava envelope. At the quartzite-magma interface a reaction corona develops due to the interaction between the Etnean trachybasaltic magma and the partially melted quartzite. Three distinct domains are observed: (i) the trachybasaltic lava itself (Zone 1), including Al-rich clinopyroxene phenocrysts dispersed in a matrix glass, (ii) the hybrid melt (Zone 2), developing at the quartzite-magma interface and feeding the growth of newly-formed Al-poor clinopyroxenes, and (iii) the partially melted quartzite (Zone 3), producing abundant siliceous melt. These features makes it possible to quantify the effect of magma contamination by siliceous crust in terms of clinopyroxene-melt element partitioning. Major and trace element partition coefficients have been calculated using the compositions of clinopyroxene rims and glasses next to the crystal surface. Zone 1 and Zone 2 partition coefficients correspond to, respectively, the chemical analyses of Al-rich phenocrysts and matrix glasses, and the chemical analyses of newly-formed Al-poor crystals and hybrid glasses. For clinopyroxenes from both the hybrid layer and the lava flow expected relationships are observed between the partition coefficient, the valence of the element, and the ionic radius. However, with respect to Zone 1 partition coefficients, values of Zone 2 partition coefficients show a net decrease for transition metals (TE), high-field strength elements (HFSE) and rare earth elements including yttrium (REE+Y), and an increase for large ion lithophile elements (LILE). This variation is associated with coupled substitutions on the M1, M2 and T sites of the type M1(Al, Fe3+)+TAl=M2(Mg, Fe2+)+TSi. The different incorporation of trace elements into clinopyroxenes of hybrid origin is controlled by cation substitution reactions reflecting local charge-balance requirements. According to the lattice strain theory, simultaneous cation exchanges across the M1, M2, and T sites have profound effects on REE+Y and HFSE partitioning. Conversely, both temperature and melt composition have only a minor effect when the thermal path of magma is restricted to ~70°C and the value of non-bridging oxygens per tetrahedral cations (NBO/T) shifts moderately from 0.31 to 0.43. As a consequence, Zone 2 partition coefficients for REE+Y and HFSE diverge significantly from those derived for Zone 1, accounting for limited cation incorporation into the newly-formed clinopyroxenes at the quartzite-magma interface.

Structure-Selective Cation Exchange in the Synthesis of Zincblende MnS and CoS Nanocrystals.

The ability to selectively form one crystal structure among several options in a polymorphic system is an important goal in solid-state synthesis. Nanocrystal cation exchange, which proceeds rapidly under mild conditions, can retain key structural features and yield otherwise inaccessible phases, but the extent to which crystal structure can be retained and therefore selectively targeted during such reactions has been limited. Here, we show that nanocrystals of digenite Cu2-x S transform to zincblende MnS and CoS upon cation exchange. Zincblende MnS and CoS, which are metastable in bulk, retain both the tetrahedral cation coordination and cubic close packed anion sublattice of digenite Cu2-x S. Comparison with wurtzite MnS and CoS, which have been accessed previously through analogous cation exchange of roxbyite Cu2-x S, demonstrates the selective formation of the related zincblende vs. wurtzite polymorphs by cation exchange of structurally distinct templates.

Structure‐Selective Cation Exchange in the Synthesis of Zincblende MnS and CoS Nanocrystals

AbstractThe ability to selectively form one crystal structure among several options in a polymorphic system is an important goal in solid‐state synthesis. Nanocrystal cation exchange, which proceeds rapidly under mild conditions, can retain key structural features and yield otherwise inaccessible phases, but the extent to which crystal structure can be retained and therefore selectively targeted during such reactions has been limited. Here, we show that nanocrystals of digenite Cu2−xS transform to zincblende MnS and CoS upon cation exchange. Zincblende MnS and CoS, which are metastable in bulk, retain both the tetrahedral cation coordination and cubic close packed anion sublattice of digenite Cu2−xS. Comparison with wurtzite MnS and CoS, which have been accessed previously through analogous cation exchange of roxbyite Cu2−xS, demonstrates the selective formation of the related zincblende vs. wurtzite polymorphs by cation exchange of structurally distinct templates.

Modified Anion Packing of Na2B12H12 in Close to Room Temperature Superionic Conductors.

Three different types of anion packing, i.e., hexagonal close packed (hcp), cubic close packed (ccp), and body centered cubic (bcc), are investigated experimentally and with ab initio calculations in the model system Na2B12H12. Solvent free and water assisted mechanical grinding provide polycrystalline samples for temperature-dependent synchrotron radiation X-ray powder diffraction and electrochemical impedance spectroscopy. It is shown that among the common close packed lattices, the hcp anionic backbone creates very favorable conditions for three-dimensional ionic conduction pathways, comprised of O-O, T-T, and T-O-T (O for octahedral, T for tetrahedral) cation hops. The hcp lattice is stable with respect to ccp and bcc lattices only at higher volumes per formula unit, which is achieved either by cationic substitution with larger cations or partial substitution of hydrogen by iodine on the closo-anion. It is found that the partial cationic substitution of sodium with lithium, potassium, or cesium does not lead to enhanced conductivity due to the obstruction of the conduction pathway by the larger cation located on the octahedral site. Substitution on the closo-anion itself shows remarkable positive effects, the ionic conductivity of Na2B12H12-xIx reaching values of close to 10-1 S cm-1 at a rather low temperature of 360 K. While the absolute value of σ is comparable to that of NaCB11H12, the temperature at which it is attained is approximately 20 K lower. The activation energy of 140 meV is determined from the Arrhenius relation and among the lowest ever reported for a Na-conducting solid.

Crystal structure of tetramethylammonium hexafluoridoniobate(V) and hexafluoridotantalate(V)

Crystal structures of newly synthesized tetramethylammonium hexafluoridoniobate(V) and hexafluoridotantalate(V) (CH3)4N[МF6] (M=Nb, Ta) have been determined; they crystallize in the tetragonal crystal system, sp. gr. P4/nmm. Crystal structures of isostructural compounds (CH3)4N[МF6] (M=Nb, Ta) are formed by virtually regular tetrahedral tetramethylammonium cations (CH3)4N+ (NMe4, TMA) and octahedral complex anions [МF6]– (M=Nb, Ta), fluorine atoms of the equatorial plane are statistically disordered over two positions. Ionic interactions and weak hydrogen bonds C–H•••F join the cations and the complex anions in a 3D assembly.

Tetra(para-Tolyl)antimony aroxides (4-MeC6H4)4SbOAr (Ar = C6H3Cl2-2,6, C6H3(NO2)2-2,4, and C6H2(NO2)3-2,4,6): Syntheses and structures

Tetra(para-tolyl)antimony aroxides, [(4-MeC6H4)4SbOC6H3Cl2-2,6] • 1/2TolH (IА, IВ), (4-MeC6H4)4SbOC6H3(NO2)2-2,4 (II), and (4-MeC6H4)4SbOC6H2(NO2)3-2,4,6 (III), are synthesized by the reactions of penta-para-tolylantimony with 2,6-dichlorophenol, 2,4-dinitrophenol, and 2,4,6-trinitrophenol, respectively, in toluene. The structures of the synthesized compounds are determined by X-ray diffraction analysis (CIF files CCDC 1050584 (I), 1433797 (II), and 999305 (III)). The Sb atoms in compounds IА, IВ, and II have a distorted trigonal bipyramidal coordination with the aroxy groups in the axial positions (axial angles CSbO are 178.01(6)°, 177.74(7)°, and 174.42(11)° and Sb–O angles are 2.244(1), 2.230(2), and 2.507(3) A). In crystal III, the CSbC angles in the tetrahedral cation [(4-MeC6H4)4Sb]+ are 103.6(2)°–116.22(2)°. A weak interaction is observed between the cation and picrate anion [OC6H2(NO2)3-2,4,6]–(Sb···O distance is 3.472(3) A).

Catalytic properties and nature of active centers of ferrospheres in oxidative coupling of methane

Ferrospheres with the Fe2O3 content in the range from 76 to 97wt% were applied as catalysts for the oxidative coupling of methane (OCM). To identify their phase composition and distribution of iron sites, the ferrospheres were characterized by X-ray powder diffraction and Mössbauer spectroscopy before and after the OCM reaction. Magnetite-based ferrite spinel, hematite and aluminosilicate glasses were established to be the main phases. The ferrospinel of all ferrospheres partially oxidized to hematite after the OCM reaction. It was established that the yield of C2-hydrocarbons sharply increased at the ferrospheres with Fe2O3 content higher than 89wt% The spinel phase of these ferrospheres includes Fevv3+(B) sites with a Ca2+ tetrahedral cation and an octahedral cation vacancy among the nearest neighbors. A linear correlation between the yield of C2-hydrocarbons (ethane and ethylene) and the content of such sites was established, thus indicating that their electrophilic oxygen species participate in selective CH4 conversion to C2H6.