Cations In Cavities Research Articles

Revealing the impact of organic spacers and cavity cations on quasi-2D perovskites via computational simulations

Two-dimensional hybrid lead iodide perovskites based on methylammonium (MA) cation and butylammonium (BA) organic spacer—such as {hbox {BA}_{2}hbox {MA}_{n-1}hbox {Pb}_{n}hbox {I}_{3n+1}}—are one of the most explored 2D hybrid perovskites in recent years. Correlating the atomistic profile of these systems with their optoelectronic properties is a challenge for theoretical approaches. Here, we employed first-principles calculations via density functional theory to show how the cation partially canceled dipole moments through the {{hbox {NH}_{3}}^{+}} terminal impact the structural/electronic properties of the {hbox {Pb}_{n}hbox {I}_{3n+1}} sublattices. Even though it is known that at high temperatures, the organic cation assumes a spherical-like configuration due to the rotation of the cations inside the cage, our results discuss the correct relative orientation according to the dipole moments for ab initio simulations at 0 K, correlating well structural and electronic properties with experiments. Based on the combination of relativistic quasiparticle correction and spin-orbit coupling, we found that the MA horizontal-like configuration concerning the inorganic sublattice surface leads to the best relationship between calculated and experimental gap energy throughout n = 1, 2, 3, 4, and 5 number of layers. Conversely, the dipole moments cancellation (as in BA-MA aligned-like configuration) promotes the closing of the gap energies through an electron depletion mechanism. We found that the anisotropy rightarrow isotropy optical absorption conversion (as a bulk convergence) is achieved only for the MA horizontal-like configuration, which suggests that this configuration contribution is the majority in a scenario under temperature effects.

Local Structure and Protons in Non-Stoichiometric Pseudo-Cubic Pollucite Mineral by Multinuclear NMR

The pollucite structure is considered as a candidate ceramic crystalline matrix for the ceramic immobilization and long-term storage of 135Cs and 137Cs fission products, and thus, their structural characteristics have particular importance. However, its local structure has not been fully resolved from reciprocal-space techniques and infrared spectroscopy, and important discrepancies exist in the available literature. Two birefringent and non-stoichiometric pollucite specimens from Tanco pegmatite (Cs0.83Na0.20Al1.13Si2.56O6) and from Mt. Mica pegmatite (Cs0.94Na018Al1.23Si2.78O6), with powder X-ray diffraction patterns fully consistent with the cubic Ia-3d space-group symmetry, and with a very different degree of hydrothermal alteration, were used in this work. High-resolution magic-angle spinning multinuclear magnetic resonance (MAS NMR) spectroscopy, including 29Si, 27Al, 23Na, 133Cs, and 1H spectra at 9.4 T, as well as 1H, 27Al, 27Al{1H} dipolar evolutions and 27Al{29Si} Heteronuclear Multiple Quantum Coherence (HMCQ) spectra at 17.6 T, has been used to investigate the local structure of pollucite and the role of protons. The 29Si spectra suggest a local structure with a disordered Si/Al distribution in only one tetrahedral T site, but with a preference of Si atoms for Q41 (3Si,1Al) and Q42 (2Si,2Al) environments, in comparison with random and Loewenstein distributions, due to charge dispersion effects. However, the 27Al{1H} dipolar evolutions suggest two spectroscopically distinct T sites for Al atoms. The 23Na and 133Cs spectra indicate broad site distributions for these cavity cations. The anisotropic character of the long-range disordered pollucite structure, with a pseudo-cubic symmetry and lack of strict periodicity, can be explained from an incipient displacive transition to lower symmetry. These pollucite specimens are essentially anhydrous minerals despite the 1H and the cross-polarization experiments suggesting that some protons exist in the structure as -OH groups, whereas water molecules were only found in relation to the phyllosilicate impurities from alteration in specimen Tanco and perhaps also as liquid water in fluid inclusions.

Water-Mediated Interactions Enhance Alkaline Earth Cation Chelation in Neighboring Cavities of a Cytosine Quartet in the DNA Quadruplex.

Larger Coulombic repulsion between divalent cations compared to the monovalent counterparts dictates the cation-cation distance in the central ion channel of quadruplexes. In this work, density functional theory and a continuum solvation model were employed to study bond energies of alkaline earth cations in adjacent cavities of the central ion channel. Four crystallized tetramolecular quadruplexes with various geometric constraints and structural motifs available in the Protein Data Bank were examined in order to understand how the cation binding affinities could be increased in aqueous solution. A cytosine quartet sandwiched between guanine quartets has a larger bond energy of the second alkaline earth cation in comparison with guanine and uracil quartets. Four highly conserved hydrogen-bonded water molecules in the center of the cytosine quartet are responsible for a higher electrostatic interaction with the cations in comparison with guanines' carbonyl groups. The reported findings are valuable for the design of synthetic quadruplexes templated with divalent cations for optoelectronic applications.

Mixed-cation driven magnetic interaction of interstitial electrons for ferrimagnetic two-dimensional electride

Magnetism of pure electrons is fundamental for understanding diverse magnetic phenomena in condensed matters but has not been fully investigated in experiments due to the lack of a tractable model system. Such an exotic material necessitates an exclusive magnetic interaction of electrons being devoid of orbital and lattice degrees of freedom. Here, we report the two-dimensional mixed-cation [YGdC]2+∙2e− electride, showing ferrimagnetic nature from the direct exchange interaction of magnetic interstitial electrons in interlayer space. We identify that magnetic interstitial electrons are periodically localized in octahedral and tetrahedral cavities between 2D cationic Y2−xGdx arrays. The mixed configuration of non-magnetic and magnetic cations in cavities induces divergent spin states and interactions of magnetic interstitial electrons, in which their direct exchange interaction overwhelms the interactions with magnetic cations, triggering the ferrimagnetic spin-alignment. This discovery facilitates further exploration of magnetic electrides and nurtures the study of two-dimensional magnetism of layered crystals and electron phases.

Nepheline solid solution compositions: stoichiometry revisited, reviewed, clarified and rationalised

AbstractMolecular formulae used to recalculate nepheline analyses generally have different numbers of oxygens (e.g. NaAlSiO4(Ne), KAlSiO4,(Ks), CaAl2Si2O8(An) and SiO2(Q)). A 32 oxygen cell has 16 T cations and 8 cavity sites, but ideal nepheline stoichiometry is not necessarily followed. Ca end-member □CaCaAl2Si2O4(CaNe) and excess silica end-member □SiSi2O4(Q’) calculation requires inclusion of both vacancy species as cavity cation values. Q’ parameter calculations can involve different assumptions and four parameters are described: Qxs; QSi; Q(Si–Al); and Qcavity; these should have closely similar values for high-quality, stoichiometric analyses.Representative published compositions are recalculated to assess whether authors followed ideal nepheline stoichiometry. Phenocrysts from peralkaline rocks and nephelinites typically exhibit Al deficiencies reflected in negative Δ(Al – cavity cation) parameters (ΔAlcc), negative ‘normative’ corundum (Al2O3,Cn), and anomalously low or negative Qxsparameters; for such rock types Q(Si–Al)provides a better estimate of excess silica contents. A ΔT-site (cation charge) parameter (ΔTcharge), is closely coupled to ΔAlccand end-member NaAlSiO4has a ΔAlcc/ΔTchargeratio of 1.4296; the derivation of this value is controlled by strict stuffed-tridymite, unit-cell constraints. Natural nephelines all contain excess silica with a mean ΔAlcc/ΔTchargeof ~1.134 reflecting their Si/Al ratio being > 1.Nepheline analyses with relatively low Al and Si and high Na (also Ca) contents are common; this might reflect the presence of small amounts (up to ~5%) of cancrinite as an alteration phase or perhaps even in solid solution. The compositions of alteration lamellae of Ca-rich cancrinite in altered nepheline phenocrysts in phonolites from the Marangudzi alkaline complex, Zimbabwe, are used to define diagnostic parameters for recognising such non-stoichiometry. These alteration lamellae formed hydrothermally from Ca-rich and K-poor fluids.An EXCEL file is provided to help researchers to standardise calculation of nepheline end-member molecular proportions.

Minireview of pentatomic cations in sodalite cavities

Centered-tetrahedral cationic clusters are frequently found in the sodalite cavities of zeolites and in the cavities of analogues of the mineral sodalite. Examples are Mn4S6+, Fe4Te6+, Zn4Se6+, Cd4S6+, Cu4Cl7+, Na4Cl3+, Na4I3+, Na54+, In57+, Ga57+, and Sn512+.

Variable stoichiometry in tectosilicates having the leucite/pollucite-type structure with particular emphasis on modelling the interframework cavity cation environment

Unit cell data for synthetic samples of I41/a or Ia-3d AAlSi2O6 (A=K,Rb,Tl,Cs) leucite/pollucites are compared to those for feldspar-stoichiometry leucite analogues (Rb,Tl,Cs)0.75□0.25Al0.75Si2.25O6. I41/a solid solutions between KAlSi2O6 and KAlSi3O8 (orthoclase feldspar, Or) and between K(Al0.85Fe0.15)Si2O6 and Or both show increasing volumes and decreasing c/a with increasing Or. Phase relations are given for KAlSi3O8-RbAlSi3O8; monoclinic feldspars occur at low temperatures and I41/a leucites at higher temperatures; volumes of leucite solid solutions increase with increasing Rb. The cell volume of Rb0.75□0.25Al0.75Si2.25O6 leucite is larger than that of RbAlSi2O6 leucite.Published leucite/pollucite structures belonging to different Space Group symmetries are treated as having the cubic Ia-3d topology with a ‘pseudocell’ edge V1/3 (V = unit cell volume); inequivalent site bond lengths and angles provide mean structural parameters for the cubic ‘pseudocells’. Multivariant linear regression equations using mean A – O distances as the independent variable were obtained using other structural parameters as dependent variables. Only two independent variables (V and mean tetrahedral ionic radius) are required to account for >90% of the variation. These regression equations can be used to predict the sizes of the cavity cation sites in leucite/pollucite-type phases of any stoichiometry.

The quaternary arsenide oxides Ce9Au5–x As8O6 and Pr9Au5–x As8O6

Abstract The quaternary gold arsenide oxides Ce9Au5−x As8O6 and Pr9Au5−x As8O6 were synthesized from the rare earth elements (RE), rare earth oxides, arsenic and gold powder at maximum annealing temperatures of 1173 K. The structures were refined from single crystal X-ray diffractometer data: Pnnm, a=1321.64(6) pm, b=4073.0(3), c=423.96(2), wR2=0.0842, 3106 F 2 values, 160 variables for Ce9Au4.91(4)As8O6 and Pnnm, a=1315.01(4), b=4052.87(8), c=420.68(1) pm, wR2=0.0865, 5313 F 2 values, 160 variables for Pr9Au4.75(1)As8O6. They represent a new structure type and show a further extension of pnictide oxide crystal chemistry. A complex polyanionic gold arsenide network [Au5As8]15− (with some disorder in the gold substructure) is charge compensated with polycationic strands of condensed edge-sharing O@RE 4/4 and O@RE 4/3 tetrahedra ([RE 4O3]2 12+) as well as RE 3+ cations in cavities.

Two-dimensional zeolite-like network in the new caesium copper aluminate Cs2CuAl4O8.

Monoclinic dicaesium copper tetraaluminate, Cs2CuAl4O8, space group P2(1)/c, a = 8.4551 (7), b = 10.012 (1), c = 17.073 (2) Å, β = 101.643 (9)°, Z = 6, was obtained by high-temperature crystallization from a phosphate flux. Its microporous crystal structure presents the first example of double layers built from [AlO4] tetrahedra combined in 4-, 6- and 8-rings, topologically similar to those found in the ATT-type zeolites and isostructural minerals armstrongite, davanite and dalyite. These layers show a rare arrangement of three [AlO4] tetrahedra sharing one oxygen vertex. The aluminate slabs are further linked by chains of edge-sharing [CuO4] square planes to form a mixed anionic three-dimensional framework with Cs(+) cations in channels and cavities. An unusually short Cu···Cs distance of 3.166 Å is ascribed to the strong Jahn-Teller effect of Cu(2+). The magnetic subsystem demonstrates properties of an alternating antiferromagnetic chain with a gap in the spectrum of magnetic excitations.

Structure evolution in "114" oxides CaBaZn₂Ga(2-x)Al(x)O₇ (x = 0, 1, 2) and layered cationic ordering in tetrahedral sites for CaBaZn₂Al₂O₇.

Rietveld refinements were performed on CaBaZn2Ga(2-x)Al(x)O7 (x = 0, 1, 2) to investigate the site preference of cations in tetrahedral cavities. Tri-valent Ga(3+)/Al(3+) preferred the tetrahedral sites (T1-sites) within the triangular layers. Moreover, a layered type cationic ordering was observed in CaBaZn2Al2O7, where all the T1-sites were occupied by Al(3+). This represents the first example of a cationic ordering in "114" compounds. By substituting Al(3+) into CaBaZn2Ga2O7, the major change in the structure was the shrinkage of the T1O4 tetrahedrons, especially the shortening of the T1-O2 bond distance along the c-axis, which led to an anisotropic shrinkage of the unit cell. In the literature, such an anisotropic change of unit cell would induce a structure distortion, as well as a decrease in the symmetry from P6₃mc to Pbn2₁. All the title compounds crystallized in P6₃mc, which is unexpected. A comparison with the selected "114" magnetic oxides help us to confirm that the strong antiferromagnetic interactions of magnetic lattices would also be beneficial for symmetry lowering. Overall, the factors affecting cationic ordering on T1-sites and symmetry lowering are discussed for the title compounds, which may also be applicable to other "114" oxides.

Calorimetric and Structural Studies of Tetrabutylammonium Bromide Ionic Clathrate Hydrates

In the present work, characteristic properties of tetrabutylammonium bromide (TBAB) ionic clathrate hydrates structures were studied by single-crystal X-ray structure analysis. The structures of three different tetragonal TBAB ionic clathrate hydrates that were formed in our experiments were based on the same water lattice of tetragonal structure I (TS-I) differing in the ways of including bromide anions and arranging tetrabutylammonium cations. We demonstrated that (1) Br(-) can be included into the water lattice, replacing two water molecules, (2) the butyl group of the cation can be inserted not only in large T and P cavities but also in small D cavities of the water lattice TS-I, and (3) one of the reasons for polytypism of ionic clathrate hydrates on the basis of TS-I is the occurrence of alternative modes of arrangements of four-compartment cavities in adjacent layers of the water framework. The compositions of three TBAB ionic clathrate hydrates TBAB·38.1H2O, TBAB·32.5H2O, and TBAB·26.4H2O were determined by chemical analysis, and their enthalpies of fusion were measured by differential scanning calorimetry (DSC). From the obtained results, the enthalpies of the TBAB hydrate formation from TBAB and water were calculated thermodynamically.

Evidence for a Thermodynamically Distinct Mg2+ Ion Associated with Formation of an RNA Tertiary Structure

A folding strategy adopted by some RNAs is to chelate cations in pockets or cavities, where the ions neutralize charge from solvent-inaccessible phosphate. Although such buried Mg(2+)-RNA chelates could be responsible for a significant fraction of the Mg(2+)-dependent stabilization free energy of some RNA tertiary structures, direct measurements have not been feasible because of the difficulty of finding conditions under which the free energy of Mg(2+) chelation is uncoupled from RNA folding and from unfavorable interactions with Mg(2+) ions in other environments. In a 58mer rRNA fragment, we have used a high-affinity thermophilic ribosomal protein to trap the RNA in a structure nearly identical to native; Mg(2+)- and protein-stabilized structures differ in the solvent exposure of a single nucleotide located at the chelation site. Under these conditions, titration of a high affinity chelation site takes place in a micromolar range of Mg(2+) concentration, and is partially resolved from the accumulation of Mg(2+) in the ion atmosphere. From these experiments, we estimate the total and site-specific Mg(2+)-RNA interaction free energies over the range of accessed Mg(2+) concentrations. At 0.1 mM Mg(2+) and 60 mM K(+), specific site binding contributes ∼-3 kcal/mol of the total Mg(2+) interaction free energy of ∼-13 kcal/mol from all sources; at higher Mg(2+) concentrations the site-binding contribution becomes a smaller proportion of the total (-4.5 vs -33 kcal/mol). Under approximately physiological ionic conditions, the specific binding site will be saturated but will provide only a fraction of the total free energy of Mg(2+)-RNA interactions.

Calorimetric and Structural Studies of Tetrabutylammonium Chloride Ionic Clathrate Hydrates

Three ionic clathrate hydrates of different water content are formed in the binary system tetrabutylammonium chloride-water. The data on heats of fusion of these hydrates measured by DSC are presented. Single-crystal and powder X-ray diffraction studies of ionic clathrate hydrates of tetrabutylammonium chloride have been carried out. Structural justification of the occurrence of three different ionic clathrate hydrates derived from tetragonal structure-I in the (C(4)H(9))(4)NCl-H(2)O system has been found for the first time. A novel mode of hydrophilic inclusion of the halide anion with displacement of two hydrogen-bonded host water molecules has been revealed. Structural data obtained are indicative of the possibility of location of tetrabutylammonium cations in combined cavities formed with participation of D-voids.

ChemInform Abstract: Characterization of Surface Active Sites of Catalysts with High- Resolution Solid-State NMR

Abstract Results obtained in studies of the surface active sites of heterogeneous catalysts by use of high-resolution solid-state NMR are presented. The 1 H NMR MAS (magic angle spinning) data on chemical shifts of surface OH groups and their interaction with the supported catalytically active complexes are discussed. The relation between the 1 H chemical shift values of OH groups, on the one hand, and characteristics of their acid strength, such as the proton affinity PA OH of the acid residue formed on heterolytic dissociation of the OH groups, on the other hand, is presented. Changes in the intensity of 1 H NMR lines for OH groups of commonly used supports (Al 2 O 3 , TiO 2 , SiO 2 etc.) on interaction with supported oxides suggest the selective interaction of surface OH groups with supported complexes. For the solid heteropolyacid H 3 PW 12 O 40 (HPA) supported on SiO 2 , the binding of the HPA molecule with the SiO 2 surface is most probably multicentred and results in the formation of supported species with different proton structures. Studies of 1 H NMR MAS spectra of zirconium hydrides supported on SiO 2 have revealed the formation of four types of zirconium hydride complex. Solid-state high-resolution 1 H NMR spectra of H 2 S adsorbed on Al 2 O 3 modified with KOH, K 2 CO 3 and HF and on zeolites NaX, NaY and H-ZSM indicate dissociative adsorption of H 2 S which occurs on the coordinatively unsaturated aluminium sites and Na + cations in zeolite cavities. The 1 H and 13 C NMR spectra indicate that adsorbed CH 3 OH reacts more readily with SH − species than with H 2 S adsorbed in the molecular form. Also presented are results of studies on Lewis acid sites in Al 2 O 3 , and zeolites Y and ZSM-5 using the NMR ( 1 H, 13 C and 15 N) of weak adsorbed bases. The formation of Lewis acid sites in HY zeolites during their hydrothermal treatment has been studied by 15 N NMR of adsorbed N 2 O. The strongest Lewis sites are the isolated or weakly associated extralattice Al atoms. The 1 H, 13 C and 15 N NMR chemical shifts of CH 4 , N 2 and N 2 O molecules, as well as 1 H and 15 N relaxation rates of CH 4 and N 2 O adsorbed on a series of HY and H-ZSM zeolites, SiO 2 and Al 2 O 3 , containing controlled amounts of iron impurities, show that unlike SiO 2 , γ-Al 2 O 3 and HY zeolites, the H-ZSM-5 zeolites have strong electron-accepting sites formed by Fe 3+ and probably Al 3+ cations. The possible structure of these sites and the geometry of complexes formed during N 2 O and CH 4 adsorption are discussed. In the final section the results obtained with 109 Ag NMR of supported silver catalysts are presented. The potential of 109 Ag NMR for studying catalysts of this type is discussed.

Synthesis and structural characterization of copper–molybdenum–sulfur and copper–tungsten–sulfur cluster complexes ( n-Bu 4N)[OMS 3Cu 3Br 2L 2] with heterocyclic thiones as ligands

Reaction of (NH 4) 2[WOS 3] or (NH 4) 2[MoO 2S 2], n-Bu 4NBr, CuCl and Imt in acetone solvent afforded air- and moisture-stable clusters ( n-Bu 4N)[MOS 3Cu 3(Imt) 2Br 2] (M = W or Mo, Imt = imidazolidine-2-thione). These compounds have been characterized by IR, UV–Vis, 1H and 13C NMR spectra and single-crystal X-ray diffraction. They are crystallographically isostructural, with Imt and [MOS 3] 2− acting as monodentate and bidentate S-donor ligands, respectively. In the anions, the three Cu atoms have different coordination environments, one being distorted tetrahedral with Imt and Br as terminal ligands, the other two being approximately trigonal planar with Imt as a terminal ligand for one and Br as a terminal ligand for the other. W and Mo have approximately tetrahedral coordination geometry, with a terminal O and three triply-bridging S atoms. The NH groups of the Imt ligands serve as donors in intramolecular N–H⋯Br and in intermolecular N–H⋯Br and N–H⋯O hydrogen bonds, linking the anions to form layers with the cations in cavities.

Homopiperazinium bis(dihydrogenarsenate)

The crystal packing in the title compound, C5H14N22+·2H2AsO4−, involves anion-to-anion O—H⋯O hydrogen bonds, resulting in double chains of dihydrogenarsenate tetra­hedra. The double chains are crosslinked by further O—H⋯O bonds to result in a three-dimensional framework that accommodates the organic cations in large cavities. The cations inter­act with the framework by way of N—H⋯O bonds.

Raman Studies of the FE-AFE Phase Transition in Ceramic Li0.12Na0.88Ta0.2Nb0.8O3 Solid Solution

The ferroelectric to antiferroelectric phase transition at 350°C in ceramic Li0.12Na0.88-Ta0.2Nb0.8O3 solid solution is studied by Raman spectroscopy. As the temperature of the solid solution sample is raised, considerable broadening of translational vibration bands of cations in the octahedral and cubo-octahedral cavities and of vibration bands of the oxygen carcass is observed. Intensity of Raman band corresponding to bridge valent vibrations (BVV) of oxygens decreases to zero. The ferroelectricity of the solid solution is shown to be lost mainly because of the growing anharmonicity of vibrations of the cavity cations.

Compressibility to 7 GPa at 298 K of the protonated octahedral framework mineral burtite, CaSn(OH)6

AbstractThe equation of state of synthetic deuterated burtite, CaSn(OD)6, has been determined to 7.25 GPa at 298 K by synchrotron X-ray powder diffraction. Fitting to a third-order Birch-Murnaghan equation of state gives K0 = 44.7(9) GPa and K0′ = 5.3(4). A second-order fit gives K0 = 47.4(4) GPa. Within experimental error the two fits are indistinguishable over the pressure range studied. The decrease in the a parameter with pressure is smooth and no phase transitions were observed. Burtite is much more compressible (by a factor of three or four) than CaSnO3 and CdSnO3 perovskites, indicating that the absence of a cavity cation has a major effect upon the compressibility of the octahedral framework. Burtite is also markedly more compressible than the closely-related mineral stottite FeGe(OH)6 (K0 = 78 GPa). Their different compressibilities correlate with the relative compressibilities of stannate and germanate perovskites. Although different octahedral compressions are likely to be the primary reason for the different compressibilities of burtite and stottite, we also consider the possible secondary role of hydrogen-bonding topology in affecting the compressibilities of protonated octahedral frameworks. Burtite and stottite have different hydrogen-bonding topologies due to their different octahedral-tilt system. Burtite, space group Pn and tilt system a+a+a+, has a hydrogen-bonded network of linked four-membered rings of O-H…O linkages, whereas stottite, space group P42/n and tilt system a+a+c−, has <100> O-H…O crankshafts and isolated four-membered rings. These different hydrogen-bonded configurations lead to different bracing of the empty cavity sites by the O-H…O linkages and very different hydrogen-bonding connectivities in these two minerals that may also enhance the difference between their compressibilities.

Study of the state of nickel in ion-exchanged NiNaY zeolite by IR spectroscopy using molecular hydrogen adsorbed at low temperatures

The state of nickel cations in NiNaY zeolite subjected to thermal vacuum and oxidative-reductive treatment was analyzed using diffuse reflectance IR spectra of molecular hydrogen, CO, and hydroxy groups. In a sample dehydrated at 570 K, nickel forms oxide particles, which are presumably located in big cavities of a zeolite. Heating of the sample at 670 K results in decomposition of the oxide with the participation of hydroxy groups followed by the formation of stabilized Ni2+ cations in large cavities of the zeolite. Calcination of NiNaY zeolite in a hydrogen atmosphere at 730 K leads to Ni2+ cation reduction to the metal state and to the formation of acidic hydroxy groups. Nevertheless, Ni2+ cations can be regenerated by reoxidation of the reduced sample with oxygen at 670 K and further evacuation at 870 K. Thermal destruction of NiO in the reoxidized sample also proceeds on acidic hydroxy groups, but the temperature of this process is higher than the temperature of oxide decomposition in the initial NiNaY zeolite. The IR spectra of molecular hydrogen complexes with Ni2+ cations were observed for the first time. A considerable decrease in the H-H stretching vibration frequency for the coordinated molecules (up to 210 cm’1) can be explained by electron density redistribution between a hydrogen molecule and the cation in the complex

Leucite-pollucite structure-type variability and the structure of a synthetic end-member calcium wairakite (CaAl2Si4O12·2H2O)

AbstractThe structure of a synthetic end-member wairakite (CaAl2Si4O12·2H2O) has been determined using Rietveld analysis of high-resolution, synchrotron X-ray powder diffraction data, and 29Si and 27Al magic angle spinning nuclear magnetic resonance spectroscopy. The framework in the synthetic sample is more disordered than that in natural wairakite. Ca is distributed over the cavity cation sites M2, M12A, M12B in the approximate proportions 0.8:0.1:0.1, respectively, with M11 being vacant. 29Si MAS NMR data are consistent with about 80% of the Si occupying tetrahedral T11 and T12 sites linked to two Al atoms [Q4(2Al) silicons]. Tetrahedral and cavity cation site disorder are coupled so that Al mainly occupies T2 sites, with Ca in M12A and M12B being balanced by Al in T12A and T12B; T11A and T11B sites appear to only contain Si, in agreement with the M11 site being vacant. The crystal chemistries of the wide range of stoichiometries which crystallize with the leucite/pollucite structure-type are also reviewed, with particular attention being paid to the tetrahedral ordering configurations present in these phases, and the implications to crystallographic phase transitions.