Tetrahedral Cations Research Articles

A calix[4]arene carceplex with four Rh2 4+ fasteners.

It is shown that a spheroidal carceplex can be assembled by linking two bowl-shaped calix[4]arenes via four dimetal units, (DAniF)(2)Rh(2) (DAniF = N,N'-di-p-anisylformamidinate), with a molecule (diethyl ether) or a cation (tetraethylammonium ion) trapped inside. The tetraethylammonium carceplex, 1b, has been characterized by X-ray crystallography, (1)H NMR, IR, and mass spectrometry. The tetraethylammonium ion fits snugly in the interior of the spheroidal carceplex. A two-fold axis of the tetrahedral cation coincides with the idealized four-fold axis of the cage, and the cation is disordered over two rotational orientations. Only one axial position on each dirhodium unit is occupied by a ligand, CH(3)CN or H(2)O. The carceplex is very stable, and the axial ligands can be exchanged in single crystals without disrupting the crystallinity of the samples. In this way, a red crystal of the complex with all axial positions occupied by acetonitrile can be transformed to a green crystal of the complex with two axial positions having acetonitrile and the other two having water by simply putting the crystal in contact with distilled water. The calix[4]arene used to make the carciplex structure is 25,26,27,28-tetra-n-propoxycalix[4]arene-5,11,17,23-tetracarboxylic acid. By employing 25,26,27,28-tetrapropoxy-5,17-dibromo-calix[4]arene-11,23-dicarboxylic acid, two 1:1 dimetal:calixarene compounds have also been made and characterized: 2, cis-Rh(2)(DAniF)(2)(calix)(CH(3)OH), and 3, cis-Mo(2)(DAniF)(2)(calix). The Rh-Rh distances in 1b are in the range of 2.410(2)-2.428(2) A, that in 2 is 2.4383(4) A, and the Mo-Mo distance in 3 is 2.0931(4) A.

Distribution of noble gases in Chinese tektites: Implication for neon solubility in natural glasses

Abstract— Five indochinites from Hainan Island and the Leizhou Peninsula, China were analyzed for noble gas abundances and isotopic ratios. These splash‐form tektites show vesiculation ranging from 0.4 vol% to 8 vol%, as determined by digital image analysis (software SXM®) on thin section photographs. To study the distribution of noble gases in vesicles and in glass, the gases were extracted by heating and by crushing, respectively, on 2 aliquots of the same sample. The results show that 5 to 53% of the total measured 20Ne resides in vesicles. The calculated concentration of neon dissolved in the glass is higher (0.7‐1 times 10−7 cm3STP/g) than that expected from solubility equilibrium (1 times 10−8 cm3STP/g), assuming solubility data from MORB glasses. The neon concentration of splash‐form tektites, those analyzed in this work and those from other strewn‐fields worldwide, is correlated with the SiO2 content of glass and with the nonbridging oxygen per tetrahedral cation in the melt (hereafter NBO/T ratio), the latter being an index of the free‐volume in the silica network where neon could be dissolved. These correlations suggest that the glass structure of tektite has a larger free‐volume available for dissolving noble gases than MORB glasses.

陽イオン間斥力からみた稜共有酸化物の構造変化‐ガーネットを中心にして

The cation-cation repulsions across the shared edges of polyhedra in garnet structure are the most prominent between tetrahedral and dodecahedral cations, and this effect has a significant influence on cation distribution and symmetry change in garnets. Moreover, the shared edge of SiO6 octahedron in stishovite (SiO2), with rutile structure, is more compressible than the unshared edges; this can be explained by Si-Si repulsion across the shared edge. Thus, the cation-cation repulsions across the shared edges of polyhedra cannot be ignored as a significant factor governing the stability of compounds with edge-sharing structures.

Investigation of Al/Si ordering in tetrahedral phyllosilicate sheets by Monte Carlo simulation

We have investigated by Monte Carlo simulation the Al/Si ordering behavior of the tetrahedral phyllosilicate sheet, with a variety of compositions from Al1Si1 to Al1Si7, using atomic interaction parameters determined for the tetrahedral sheet in muscovite. Three different ordering schemes operate, depending on composition, with relatively Al-poor systems ordering in a muscovite-like (Al:Si = 1:3) pattern and relatively Al-rich compositions ordering in an “ABABAB” or margarite-like (Al:Si = 1:1) pattern, where ABABAB indicates the arrangement of atoms around a hexagonal ring of tetrahedral cation sites. The pattern corresponding to Al:Si = 1:2 occurs in intermediate compositions, but always in conjunction with another ordering pattern, except for one composition close to Al:Si = 1:2. Simulations of the same composition but with different ordering schemes can show different behavior, and this is evidence for metastability fields. The transition temperature for order-disorder T c is strongly dependent on composition, and the dilution effect can be observed at low Al concentrations, with a critical concentration x c between 0.12 and 0.15.

The twinned and disordered crystal structure of tetrakis(triphenylphosphine)silver(I)bis(trifluoroacetato)triphenylstannate(IV)

Abstract The crystal structure of tetrakis(triphenyl-phosphine)silver(I)bis(trifluoroacetato)triphenylstannate(IV), [(C6H5)3P]4Ag (C6H5)3Sn(O2CCF3)2 [R3: a = 16.238(1), c = 52.354(3) Å], consists of discrete four-coordinate te trahedral cations and five-coordinate trans-C3SnO2 trigonal bipramidal anions. The asymmetric unit consists of a third of each of two cations and two anions. The structure is a 0.675(3): 0.325 twin of a 0.783(1): 0.217 disorder of an R3 structure that uses a 2-fold rotation operation of R32 for the twinning and the disorder. The structure is refined on 234 variables and 6276 reflections with I > 3σ(I) to R F = 0.053.

The influence of next nearest neighbours on cation radii in spinels: the example of the Co3O4-CoCr2O4 solid solution

AbstractLattice parameters and crystal structures of the synthetic spinels Co3O4, CoCr2O4, and solid solutions in the binary join Co3O4-CoCr2O4, have been determined by powder X-ray diffraction structural refinements. In all these spinels the cation distribution is completely normal at room temperature, and the tetrahedrally coordinated cation site is occupied only by Co2+. The ionic radius of Co2+(tet) increases from 0.556(3) in Co3O4 to 0.599(4) in CoCr2O4. In the spinel structure, the interatomic distance between the tetrahedral cations and oxygen are geometrically independent of those between the octahedral cations and oxygen; thus the variation in effective ionic radii is ascribed to next-nearest neighbour effects, induced by covalent tendencies in the low-spin Co3+-O bond. The results demonstrate that the assumption of constant ionic radii even within an isomorphic group such as the oxide spinels needs to be made with caution.

The Crystal Structure of Roscoelite-1M

AbstractSingle-crystal X-ray diffraction experiments were carried out on roscoelite crystals from Reppia, Val Graveglia, Italy. Roscoelite [structural formula: XII(Ba0.006K0.994)IV(Si3.150Al0.850) VI(Al0.040Fe0.150Mg0.100Mn0.062V1.696Ti0.003)O10(OH)2] shows a near-perfect three-dimensional stacking order with cell parameters a = 5.292(1), b = 9.131(2), c = 10.206(3) Å, β = 100.98(2)° and space group C2/m, which indicate a 1M polytype. The crystal structure was refined on the basis of Fo2 for 846 unique reflections to R1 = 3.29% calculated using 746 unique observed reflections [|Fo| ⩾ 4σ(Fo)]. The mean tetrahedral cation–oxygen atom distance, <T−O> = 1.641 Å, is close to the mean <T−O> value obtained for dioctahedral true micas from the literature, whereas the octahedral sheet is characterized by a larger cis-octahedral cation–oxygen atom bond distance <M2−O> = 2.020 Å which, together with the mean electron count, is consistent with V occupancy. The presence of V within the octahedral sheet produces the smallest tetrahedral rotation (α = 2.3°), the lowest flattening of the basal oxygen surface (Δz = 0.118 Å) and the narrowest interlayer separation (3.030 Å) in dioctahedral micas.

Vibrational spectra of the tetramethylpnikogenonium ions

The tetramethylpnikogenonium ions (CH 3) 4X + (X = N, P, As, Sb) have been studied by infrared and Raman spectroscopy. Additionally, their structures and vibrational frequencies were ab initio calculated at the HF/6-31+G* and for (CH 3) 4Sb + at the HF/6-31+G*/LANL2DZ level of theory. For the tetrahedral cations an assignment of the vibrational frequencies is discussed on the basis of a comparison of calculated and measured frequencies.

Cyanamidonitrate-cobalt(II) complexes of pyrazole and imidazole ligands.: X-ray structure of [Co(NO 2NCN) 2(pyrazole) 4] and [Co(2-methylimidazole) 4](NO 2NCN) 2

New cyanamidonitrate-cobalt(II) complexes of the type [Co(NO 2NCN) 2L 4], where L=pz (pyrazole), mpz [3(5)-methyl-pyrazole], dmpz (3,5-dimethylpyrazole), inz (indazole, benzopyrazole), Br-pz (4-Br-pyrazole), and [Co(dmpz) 4](NO 2NCN) 2 have been prepared and characterized by IR and electronic spectroscopy. The crystal structure of [Co(NO 2NCN) 2(pz) 4] was determined by X-ray crystallography. The donor set of Co(II) forms an octahedron with four tertiary nitrogens from pyrazole ligands situated in the equatorial plane and two nitrile nitrogens from cyanamidonitrates in the axial positions. The d–d transitions show that a similar molecular structure can be expected in other Co(II) complexes under study, except the [Co(dmpz) 4](NO 2NCN) 2 which contains a tetrahedral complex cation and forms with [Co(NO 2NCN) 2(dmpz) 4] an isomeric pair. The central Co(II) atom in [Co(2-methylimidazole) 4] (NO 2NCN) 2 complex is tetrahedraly coordinated with four tertiary nitrogens from the imidazole ligands and the formed cationic unit is balanced by free cyanamidonitrate anions. According to the electronic and infrared spectra the [Co(dmpz) 4](NO 2NCN) 2 complex possesses an analogous molecular structure.

The influence of octahedral and tetrahedral cation substitution on the structure of smectites and serpentines as observed through infrared spectroscopy

AbstractAnalysis of the near-infrared (NIR) spectral bands of phyllosilicates, together with the mid-infrared bands, enables testing and confirmation of band assignments for the structural OH vibrations. Spectral analyses of selected smectites and serpentine-kaolin minerals are presented here. The results of this study indicate that dioctahedral smectites may contain both in-plane and out-of-plane OH-bending vibrations, as suggested by Farmer (1974). In-plane bands occur near 800 ­ 915 cm–1, while the weaker out-of-plane vibrations occur near 600 ­ 700 cm–1 and are enhanced in dioctahedral smectites when the structure is disrupted by substitutions. Analysis of the OH-stretching vibrations and their NIR overtone bands is also presented for both smectites and serpentine-kaolin minerals. These overtones are more straightforward for serpentines than for kaolinite, and a strong overtone associated with the kaolinite OH-stretching band at 3620 cm–1 is found that supports its assignment as an OH-stretching band.

The influence of bulk composition on the diffusivity of carbon dioxide in Na aluminosilicate melts

The bulk diffusivity of dissolved carbon dioxide (CO 2 and CO 3 2- ) in NaAlSi 3 O 8 + nNa 2 O (n = 0- 6.87 wt%) and in NaAlSi 3 O 8 + nH 2 O (n = 0-2 wt%) melts was investigated at 1523 K and 0.5 GPa using the diffusion couple technique. CO 2 contents of the starting glass pairs varied between 0 and 0.2 wt%. Symmetrical concentration-distance profiles of bulk CO 2 were determined by infrared spectroscopy. An error function was fitted to the profiles to obtain apparent chemical diffusion coefficients of bulk CO 2 . In the investigated compositional range, the diffusivity of bulk CO 2 increases exponentially with Na 2 O and H 2 O content and thus exponentially with the ratio of nonbridging oxygen atoms per tetrahedral cations (NBO/ T). The bulk CO 2 diffusivity increases from logD CO₂ = -11.38 (D CO₂ in m2/s) in NaAlSi 3 O 8 melt to logD CO₂ = -10.92 in NaAlSi 3 O 8 melts containing 6.87 wt% Na 2 O excess, and to logD CO₂ = -10.91 in NaAlSi 3 O 8 melts containing 2 wt% H 2 O. These data imply that either: (1) the diffusivities of the CO 2 species (molecular CO 2 and CO 3 2- ) are very similar, or (2) the speciation of CO 2 in the quenched glasses is very different from the speciation in the melt.

Structure of the (001) surface of γ alumina

Using density functional theory, we have studied the structure and energetics of the (001) face of γ alumina. Our results address several experimental issues: (1) When the face with tetrahedral aluminum is exposed in the bulk-terminated system, the surface reconstructs extensively, leading to exposure of the higher-density layer. When only a few layers are present, this reconstruction may even lead to the collapse of the system into a different structure. (2) We find that the lowest energy is obtained if the vacant spinel sites lie on octahedral positions. We also find that vacancies are less preferred on the surface than in the bulk. (3) Migration to and from the surface of vacant spinel sites, by hopping of Al atoms between octahedral and tetrahedral cation sites has a rather high barrier. This suggests the vacancy distribution may not reach equilibrium if the material is not annealed carefully during preparation.

Structural isomerism in CuSCN coordination polymers

CuSCN reacts with the angular ligand 2,4-bis(4-pyridyl)-1,3,5-triazine (dpt) to afford rare examples of coordination polymer structural isomers including a non-centrosymmetric three-dimensional framework with Cd(SO4) topology constructed from tetrahedral metal cations.

Determination of molar absorptivities for infrared absorption bands of H2O in andesitic glasses

We have determined infrared molar absorptivities for water absorption bands in Fe-bearing and Fe-free andesitic glasses. Water dissolves in andesitic glasses as both hydroxyl groups and molecular water as observed in other silicate glasses. Concentrations of molecular water and hydroxyl species are a strong function of total water content. IR molar absorptivities for Fe-bearing andesite are ε 3570 = 62.32 ± 0.42 L/mol·cm, ε 4500 = 0.79 ± 0.07 L/mol·cm, ε 5200 = 1.07 ± 0.07 L/mol·cm, and ε 1630 = 42.34 ± 2.77 L/mol·cm. Molar absorptivities for Fe-free andesite are 69.21 ± 0.52 L/mol·cm for e3570, 0.89 ± 0.07 L/mol·cm for e4500, 1.46 ± 0.07 L/mol·cm for e5200, and 52.05 ± 2.85 L/mol·cm for ε 1630 . Molar absorptivities show significant compositional dependencies that can be predicted based on tetrahedral cation (Si +4 , Al +3 )/total cation fraction

CO 2 solubility and speciation in intermediate (andesitic) melts: the role of H 2O and composition

We determined total CO 2 solubilities in andesite melts with a range of compositions. Melts were equilibrated with excess C-O(-H) fluid at 1 GPa and 1300°C then quenched to glasses. Samples were analyzed using an electron microprobe for major elements, ion microprobe for C-O-H volatiles, and Fourier transform infrared spectroscopy for molecular H 2O, OH −, molecular CO 2, and CO 3 2−. CO 2 solubility was determined in hydrous andesite glasses and we found that H 2O content has a strong influence on C-O speciation and total CO 2 solubility. In anhydrous andesite melts with ∼60 wt.% SiO 2, total CO 2 solubility is ∼0.3 wt.% at 1300°C and 1 GPa and total CO 2 solubility increases by about 0.06 wt.% per wt.% of total H 2O. As total H 2O increases from ∼0 to ∼3.4 wt.%, molecular CO 2 decreases (from 0.07 ± 0.01 wt.% to ∼0.01 wt.%) and CO 3 2− increases (from 0.24 ± 0.04 wt.% to 0.57 ± 0.09 wt.%). Molecular CO 2 increases as the calculated mole fraction of CO 2 in the fluid increases, showing Henrian behavior. In contrast, CO 3 2− decreases as the calculated mole fraction of CO 2 in the fluid increases, indicating that CO 3 2− solubility is strongly dependent on the availability of reactive oxygens in the melt. These findings have implications for CO 2 degassing. If substantial H 2O is present, total CO 2 solubility is higher and CO 2 will degas at relatively shallow levels compared to a drier melt. Total CO 2 solubility was also examined in andesitic glasses with additional Ca, K, or Mg and low H 2O contents (<1 wt.%). We found that total CO 2 solubility is negatively correlated with (Si + Al) cation mole fraction and positively correlated with cations with large Gibbs free energy of decarbonation or high charge-to-radius ratios (e.g., Ca). Combining our andesite data with data from the literature, we find that molecular CO 2 is more abundant in highly polymerized melts with high ionic porosities (>∼48.3%), and low nonbridging oxygen/tetrahedral oxygen (<∼0.3). Carbonate dominates most silicate melts and is most abundant in depolymerized melts with low ionic porosities, high nonbridging oxygen/tetrahedral oxygen (>∼0.3), and abundant cations with large Gibbs free energy of decarbonation or high charge-to-radius ratio. In natural silicate melt, the oxygens in the carbonate are likely associated with tetrahedral and network-modifying cations (including Ca, H, or H-bonds) or a combinations of those cations.

Effect of vacancy jump rate on the permeability and dielectric properties of Ni 0.65Zn 0.35Cu xFe 2− XO 4

The ferrite system Ni 0.65Zn 0.35Cu x Fe 2− X O 4 with x=0.0, 0.1, 0.2, 0.3, 0.4 was prepared using the ceramic method. Relative permeability, dielectric constant, dielectric loss were studied as a function of vacancy jump rate. The results showed that the relative permeability, dielectric constant and dielectric loss decrease with increasing vacancy jump rate. Four bands are observed in the IR spectra. The high-frequency band ν 1 is assigned to the tetrahedral cations and the low-frequency band ν 2 to the octahedral complexes. The dielectric hysteresis loops were displayed for different Cu concentrations and showed a decrease in the dielectric polarization. This evidence confirmed the effect of vacancy jump rate on the decrease of relative permeability and dielectric constant.

Local equilibrium in polymetamorphic gneiss and the titanium substitution in biotite

Secondary metamorphic biotite grew with sillimanite and garnet at the expense of cordierite during a Pan-African metamorphic event overprinting Grenville age granulite-facies metapelites at the western margin of the Namaqualand Metamorphic Complex, South Africa. Fe-Mg exchange thermometry and average pressure calculations constrain the conditions of the overprinting metamorphism to about 660 °C at 6 kbar. Biotite compositions reflect chemical equilibrium in local domains defined by partly to completely pseudomorphed cordierite grains. They show a continuous range of Ti content from 0.0 to 4.5 wt% TiO 2 depending on the proximity of the domain to a source of Ti such as ilmenite. Thus, they allow an assessment of the Ti substitution that is independent of metamorphic grade variation. Significant correlations between Ti and microprobe analysis total indicate that Ti is accommodated largely through a deprotonation substitution Ti 4 + + 2O 2 - + H 2 → M 2 + + 2OH - (where M 2 + is a divalent octahedral cation) rather than by substitutions involving octahedral vacancies. Ti shows a positive correlation with K, and strong negative correlations with octahedral Al and the sum of divalent cations. Ti content also varies with molar Mg/(Mg + Fe) in biotite (X M g ), because the process of cordierite replacement involves the mass transfer of Fe into cordierite domains. However, the above trends are also shown by a set of analyses with uniform X M g , In detail, the correlations depend on the method of formula recalculation. If the deprotonation substitution is dominant, normalization should not be made to a constant number of oxygen atoms. A statistical treatment of biotite analyses normalized to 14 tetrahedral and octahedral cations per formula unit suggests that the introduction of Ti involves deprotonation of two OH groups per Ti, and that the replacement of some octahedral Al by Ti is balanced by increases in K and/or Si in other sites. An appropriate Ti-biotite end-member may be K 2 [Mg 4 Ti 2 ]Al 2 Si 6 O 2 4 , rather than the vacancy-bearing end-members proposed in other studies. In view of the fact that even Ti-free biotite appears to contain a small proportion of octahedral vacancies, a suitable recalculation scheme for microprobe analyses may involve normalizing to 22 + Ti O atoms (neglecting H 2 O).

Spectroscopic properties of Co2+ in related spinels

We report the absorption and fluorescence spectra of divalent cobalt (Co2+) in a series of related spinels Mg(1−x)CoxAlBOC, where B=2, 4, 6, C=4, 7, 10, and x represents the amount of Co2+ that substitutes for Mg2+ in tetrahedral sites of Td symmetry. The spectra support the assumption that both Mg2+ and Co2+ prefer the tetrahedral cation sites during the growth of these crystalline materials. These observations also support earlier conclusions obtained from crystallographic and thermodynamic studies of the MgO and Al2O3 phase diagram. The observed spectra are analyzed and compared to calculated electronic levels derived from lattice-sum models using crystallographic data determined from the crystals grown in the present studies. Comments are made in relevance to the use of these crystals as saturable absorbers for near infrared Q-switched lasers.

Negative Thermal Expansion

ABSTRACTNegative thermal expansion behavior has been found in many oxides where oxygen or a cation has a coordination number of two. The MO2, AM2O7, A2M3O12, AMO5, and AO3 families, where A is an octahedral cation, M a tetrahedral cation, and the oxygen coordination is two, have been investigated for their thermal expansion properties. Negative thermal expansion has been found in all families except the AO3 family, where very low thermal expansion was found in the case of TaO2F. Open networks are necessary to allow free transverse thermal motion of oxygen, which is the apparent cause negative thermal expansion in these families. This openness leads to two problems. One is that structure collapse transitions tend to occur as the temperature is lowered. There is little or no thermal expansion below this transition. A solution to this problem is to maintain sufficient ionic character in the bonds holding the network together. The other problem is that when the networks become sufficiently open, they tend to hydrate. This hydration destroys the negative thermal expansion of the network.

Mica Crystal Chemistry and the Influence of Pressure, Temperature, and Solid Solution on Atomistic Models

The 2:1 mica layer is composed of two opposing tetrahedral (T) sheets with an octahedral (M) sheet between to form a “TMT” layer (Fig. 1a⇓). The mica structure has a general formula of A M2–3 T4 O10 X2 [in natural micas: A = interlayer cations, usually K, Na, Ca, Ba, and rarely Rb, Cs, NH4, H3O, and Sr; M = octahedral cations, generally Mg, Fe2+, Al, and Fe3+, but other cations such as Li, Ti, V, Cr, Mn, Co, Ni, Cu, and Zn can occur also in mica species; T = tetrahedral cations, generally Si, Al and Fe3+ and rarely B and Be; X = (OH), F, Cl, O, S]. Vacant positions (symbol:□) are also common in the mica structure (Rieder et al. 1998). In the tetrahedral sheet, individual TO4 tetrahedra are linked with neighboring TO4 by sharing three corners each (i.e., the basal oxygen atoms) to form an infinite two-dimensional “hexagonal” mesh pattern (Fig. 1b⇓). The fourth oxygen atom (i.e., the apical oxygen atom) forms a corner of the octahedral coordination unit around the M cations. Thus, each octahedral anion atom-coordination unit is comprised of four apical oxygen atoms (two from the upper and two from the lower tetrahedral sheet) and by two (OH) or F, Cl, O and S anions [the X anions, usually indicated as the OH or O(4) site]. The OH site is at the same level as the apical oxygen but not shared with tetrahedra. In the octahedral sheet, individual octahedra are linked laterally by sharing octahedral edges (Fig. 1c⇓). The smallest structural unit contains three octahedral sites. Structures with all three sites occupied are known as trioctahedral, whereas, if only two octahedra are occupied [usually M(2)] …