Corner-sharing SiO4 Tetrahedra Research Articles

Experimental and theoretical investigation of the formation of two-dimensional Fe silicate on Pd(111)

A single layer of Fe silicate was grown on Pd(111) and analyzed experimentally and theoretically. Following sequential deposition of SiO and Fe and annealing above 900 K in O2, an incommensurate but well-ordered, low-defect density layer was observed with low-energy electron diffraction and scanning tunneling microscopy (STM). The STM images revealed a moiré pattern due to the lattice mismatch between the relaxed oxide layer and the substrate, while high-resolution images showed a honeycomb structure consistent with a silicate layer with six-membered rings of corner-sharing SiO4 tetrahedra at its surface. Reflection-absorption infrared spectroscopy revealed a single peak at 1050 cm−1 due to Si–O–Fe linkages, while x-ray photoelectron spectroscopy data indicated a Si/Fe ratio of one, that the Fe were all 3+, and that the Si atoms were closest to the surface. Consistent with these experimental observations, first principles theory identified a layer with an overall stoichiometry of Fe2Si2O9 with the six-membered rings of SiO4 tetrahedra at the surface. One of the oxygen atoms appears as a chemisorbed atom on the Pd surface, and, thus, the layer is better described as Fe2Si2O8 atop an oxygen-covered Pd surface. The Fe2Si2O8 is chemically bound to the Pd surface through its oxygen atoms; and the passivation of these bonds by hydrogen was investigated theoretically. Upon hydrogenation, the adsorbed O atom joins the Fe silicate layer and thermodynamic analysis indicates that, at room temperature and H2 pressures below 1 atm, Fe2Si2O9H4 becomes favored. The hydrogenation is accompanied by a substantial increase in the equilibrium distance between the oxide layer and the Pd surface and a drop in the adhesion energy to the surface. Together the results indicate that a highly ordered 2D Fe silicate can be grown on Pd(111) and that subsequent hydrogenation of this layer offers potential to release the 2D material from the growth substrate.

Thermal conversion of the hydrous aluminosilicate LiAlSiO3(OH)2 into γ-eucryptite

Abstract LiAlSiO3(OH)2 is a dense hydrous aluminosilicate which is formed from LiAlSiO4 glass in hydrothermal environments at pressures around 5 GPa. The OH groups are part of the octahedral Al and Li coordination. We studied the dehydration behavior of LiAlSiO3(OH)2 by a combination of TEM and multi-temperature PXRD experiments. Dehydration takes place in the temperature interval 350–400 °C. Above 700 °C LiAlSiO3(OH)2 is converted via a transient and possibly still slightly hydrous phase into γ-eucryptite which is a metastable and rarely observed polymorph of LiAlSiO4. Its monoclinic structure is built from corner-sharing LiO4, AlO4 and SiO4 tetrahedra. The ordered framework of AlO4 and SiO4 tetrahedra is topologically equivalent to that of cristobalite.

Taniajacoite and Strontioruizite, Two New Minerals Isostructural with Ruizite from the N'Chwaning III Mine, Kalahari Manganese Field, South Africa

ABSTRACT Two new mineral species, taniajacoite and strontioruizite, ideally SrCaMn3+2Si4O11(OH)4·2H2O and Sr2Mn3+2Si4O11(OH)4·2H2O, respectively, have been identified from the N'Chwaning III mine, Kalahari manganese field, South Africa. Both minerals occur as brown radiating groups or aggregates of acicular or prismatic crystals, with individual crystals up to 0.15 × 0.04 × 0.02 mm for taniajacoite and 1.3 × 0.2 × 0.2 mm for strontioruizite. Minerals associated with taniajacoite include sugilite, aegirine, pectolite, richterite, potassic-ferri-leakeite, and lipuite, whereas those associated with strontioruizite include sugilite, potassic-magnesio-arfvedsonite, and lipuite. Both taniajacoite and strontioruizite are brown in transmitted light, transparent with very light brown streak and vitreous luster. They are brittle and have a Mohs hardness of 5–5.5; cleavage is good on {010} and no parting or twinning is observed macroscopically. The measured and calculated densities are 3.05(2) and 3.09 g/cm3, respectively, for taniajacoite and 3.20(2) and 3.16 g/cm3 for strontioruizite. Optically, both taniajacoite and strontioruizite are biaxial (–), with α = 1.686(2), β = 1.729(2), γ = 1.746(2) (white light), 2V (meas.) = 63.7(5)°, 2V (calc.) = 62.5° for the former and α = 1.692(2), β = 1.734(2), γ = 1.747(2) (white light), 2V (meas.) = 59.1(5)°, 2V (calc.) = 56.6° for the latter. The calculated compatibility index based on the empirical formula is 0.008 for taniajacoite and 0.015 for strontioruizite. An electron microprobe analysis yielded an empirical formula (based on 17 O apfu) of Sr(Ca0.81Sr0.19)Σ1.00(Mn3+1.90Fe3+0.15Al0.01)Σ2.06Si3.96O11(OH)4·2H2O for taniajacoite and (Sr1.61Ca0.42)Σ2.03(Mn3+1.95Fe3+0.05)Σ2.00Si3.98O11(OH)4·2H2O for strontioruizite. Taniajacoite and strontioruizite are isostructural with ruizite. Strontioruizite, like ruizite, is monoclinic with space group C2 and unit-cell parameters a = 9.1575(4), b = 6.2857(4), c = 12.0431(6) Å, β = 91.744(4)°, and V = 692.90(6) Å3, whereas taniajacoite is triclinic, with space group C1 and a = 9.1386(5), b = 6.2566(3), c = 12.0043(6) Å, α = 90.019(4), β = 91.643(4), γ = 89.900(4)°, and V = 686.08(6) Å3. Their structures are characterized by chains of edge-sharing MnO6 octahedra extended along [010], which are linked together by corner-shared SiO4 tetrahedra in four-membered [Si4O11(OH)2] linear clusters, giving rise to a so-called “hetero-polyhedral framework”. The large cations Sr2+ and Ca2+ occupy the seven-coordinated interstices. Unlike monoclinic ruizite and strontioruizite, taniajacoite with Sr:Ca ≈ 1:1 is triclinic, owing to the ordering of Sr2+ and Ca2+ into two crystallographically distinct sites, indicating an incomplete solid solution between Ca and Sr endmembers. The unit-cell volumes for ruizite, taniajacoite, and strontioruizite appear to vary linearly with the Sr/(Ca + Sr) ratio.

Ultrafast removal of radioactive strontium ions from contaminated water by nanostructured layered sodium vanadosilicate with high adsorption capacity and selectivity

Layered sodium vanadosilicates was synthesized for removing soluble strontium ion (Sr2+) from simulated radioactive wastewater. SEM, EDS and TEM images indicated that layered sodium vanadosilicates had a nanostructure with flake-like shape providing an extremely large surface area. XRD, FTIR and XPS analyses further revealed a framework structure of adsorbent consisting of corner-sharing SiO4 tetrahedra and VO6 octahedra, with sodium ion being the major cation in the synthesized nanostructured layered sodium vanadosilicate. This study for the first time showed ultrafast adsorption of Sr2+ by nanostructured layered sodium vanadosilicates in a time frame of seconds with a high adsorption capacity of 174.3 mg/g estimated from Langmuir isotherm. It was found that about 99.0% of Sr2+ at an initial concentration of 5.00 mg/L and adsorbent dosage of 0.5 g/L could be removed within several seconds. The kinetic analysis further revealed that pseudo-second-order instead of pseudo-first-order kinetics could satisfactorily describe the observed ultrafast removal of Sr2+. In particular, the nanostructure layered sodium vanadosilicates exhibited an excellent affinity to Sr2+ over a wide pH range of 3–11. It was also demonstrated that the working mechanisms of nanostructured layered sodium vanadosilicates for Sr2+ removal mainly included surface electrostatic interaction and ion-exchange with sodium ion. Furthermore, nanostructure layered sodium vanadosilicates had significant advantages for Sr2+ removal compared with other adsorbents. Consequently, it is reasonable to expect that nanostructured layered sodium vanadosilicates synthesized in this study could be considered as a promising adsorbent for ultrafast and high-efficiency removal of radioactive Sr2+ from radioactive wastewaters.

1H, 29Si and 119Sn double and triple resonance NMR spectroscopy of the small-pore framework sodium stannosilicate Na2SnSi3O9·2H2O

The small-pore framework sodium stannosilicate AV-10, chemical composition Na2SnSi3O9·2H2O and known crystallographic structure, was synthesized by hydrothermal crystallization. This stannosilicate is built up of a three-dimensional network of corner-shared SiO4 tetrahedra and SnO6 octahedra. The SnO6 sites are linked to six SiO4 tetrahedra (Sn(6Si)) while each of the two crystallographically different SiO4 units are connected to two SnO6 and SiO4 units (Si(2Si,2Sn)). This material was used as model compound for developing a solid-state MAS NMR strategy aimed on the challenges and possibilities for structural studies, particularly considering the short and medium range order to verify the connectivity of SiO4 and SnO6 of such compounds despite the low natural abundances of 4.68% for 29Si and 8.59% for 119Sn nuclei as a real challenge. 29Si{119Sn} and 119Sn{29Si} REDOR (Rotational-Echo Double-Resonance) NMR measurements after 1H cross-polarization (CP) were carried out. The REDOR curves show a significant change after the “normal” quadratic short time evolution from which both (i) the shortest internuclear 29Si – 119Sn distances (and vice versa) and (ii) the number of corner-sharing SiO4 tetrahedra around the SnO6 octahedra (and vice versa) can be obtained. Based on these data, optimized 29Si{119Sn} and 119Sn{29Si} REPT-HMQC (Recoupled Polarization Transfer-Heteronuclear Multiple-Quantum Correlation, again after 1H CP) experiments were implemented, which directly show those heterogroup connectivity as correlation peaks in a 2D spectrum. This information was also obtained using 2D29Si{119Sn}-J-Coupling NMR experiments. Furthermore, 2D29Si INADEQUATE NMR experiments are also feasible, showing the connectivity of SiO4 tetrahedra. The combination of REDOR, REPT-HMQC, J-Coupling and INADEQUATE experiments yielded a complete analysis of the short and medium range structure of this microporous stannosilicate, in agreement with the previously published structure obtained Ab Initio from powder X-Ray diffraction data (XRD).

Synthetic analogues of natural titanosilicate mesoporous minerals as potential functional materials. Synthesis and application

Phase formation upon hydrothermal crystallization in the Na2O-TiO2-SiO2-H2SO4- H2O system has been studied in depth. The obtained results provided the basis for the development of technology for new types of ion-exchangers with the structure of the mineral ivanyukite - Na4(TiO)4(SiO4)34-6 H2O. It was shown that an excessive amount of Na and Si with respect to Ti (mol) and an alkaline medium up to pH 12 affect the rate of the ivanyukite crystallization. The optimal conditions producing the pure ivanyukite phase with designed structure, chemical composition and morphology have been found. The framework composes corner-sharing SiO4 tetrahedra and TiO6 octahedra linked through bridging oxygen atoms and provide pore system with channels containing H2O molecules and exchangeable Na+cations. The synthetic ivanyukite with wide-pore microporous framework is considered to be perspective cation-exchanger.

Dmisteinbergite, CaAl2Si2O8, a Metastable Polymorph of Anorthite: Crystal-Structure and Raman Spectroscopic Study of the Holotype Specimen

The crystal structure of dmisteinbergite has been determined using crystals from the type locality in Kopeisk city, Chelyabinsk area, Southern Urals, Russia. The mineral is trigonal, with the following structure: P312, a = 5.1123(2), c = 14.7420(7) Å, V = 333.67(3) Å3, R1 = 0.045, for 762 unique observed reflections. The most intense bands of the Raman spectra at 327s, 439s, 892s, and 912s cm −1 correspond to different types of tetrahedral stretching vibrations: Si–O, Al–O, O–Si–O, and O–Al–O. The weak bands at 487w, 503w, and 801w cm−1 can be attributed to the valence and deformation modes of Si–O and Al–O bond vibrations in tetrahedra. The weak bands in the range of 70–200 cm−1 can be attributed to Ca–O bond vibrations or lattice modes. The crystal structure of dmisteinbergite is based upon double layers of six-membered rings of corner-sharing AlO4 and SiO4 tetrahedra. The obtained model shows an ordering of Al and Si over four distinct crystallographic sites with tetrahedral coordination, which is evident from the average <T–O> bond lengths (T = Al, Si), equal to 1.666, 1.713, 1.611, and 1.748 Å for T1, T2, T3, and T4, respectively. One of the oxygen sites (O4) is split, suggesting the existence of two possible conformations of the [Al2Si2O8]2− layers, with different systems of ditrigonal distortions in the adjacent single layers. The observed disorder has a direct influence upon the geometry of the interlayer space and the coordination of the Ca2 site. Whereas the coordination of the Ca1 site is not influenced by the disorder and is trigonal antiprismatic (distorted octahedral), the coordination environment of the Ca2 site includes disordered O atoms and is either trigonal prismatic or trigonal antiprismatic. The observed structural features suggest the possible existence of different varieties of dmisteinbergite that may differ in: (i) degree of disorder of the Al/Si tetrahedral sites, with completely disordered structure having the P63/mcm symmetry; (ii) degree of disorder of the O sites, which may have a direct influence on the coordination features of the Ca2+ cations; (iii) polytypic variations (different stacking sequences and layer shifts). The formation of dmisteinbergite is usually associated with metastable crystallization in both natural and synthetic systems, indicating the kinetic nature of this phase. Information-based complexity calculations indicate that the crystal structures of metastable CaAl2Si2O8 polymorphs dmisteinbergite and svyatoslavite are structurally and topologically simpler than that of their stable counterpart, anorthite, which is in good agreement with Goldsmith’s simplexity principle and similar previous observations.

Radioactive Iodine-129 Capture in Mixed Cation Sodalites: ab initio Modelling

Abstract:Sodalites have been investigated experimentally for the capture and long-term containment of 129I, a significant and hazardous waste product of the nuclear fuel cycle. Sodalites are zeolite-type structures commonly occurring in nature in alkaline igneous rocks and having the prototype formula Na8(AlSiO4)6Cl2. The crystal structure is based around β-cages consisting of corner-sharing SiO4 and AlO4 tetrahedra. In the centre of the β-cage is an anion X. Iodine captured by sodalites sits in the centre of the β-cages as iodide anions. Silver iodide (AgI) plays an important role in the capture and subsequent processing of 129I in the nuclear fuel cycle. Using ab initio density functional theory (DFT) modelling, we investigate the energetics and feasibility of iodine capture and containment in mixed cation sodalites Na8-xAgx(AlSiO4)6I2, and compare the results with experimental observations.

Directing the Structure of Two-Dimensional Silica and Silicates

Density functional theory was used to assess the viability of approaches to controlling the structure of a recently discovered two-dimensional form of SiO2. In accord with prior work, a hexagonal bilayer of mirror image planes of corner-sharing SiO4 tetrahedra in six-membered rings yielded only a slightly higher energy than α-quartz. Structures including four- through eight-membered rings were evaluated and in certain cases found to be as little as 17 meV/Si higher in energy than the hexagonal bilayer. When either biaxial or uniaxial tensile strain was applied, combinations of eight-, six-, and four-membered rings became favored due to the lower density of structures with larger rings. These findings, together with experiments that reveal expansion of silica bilayers to match the lattice of metal substrates, suggest that epitaxial strain may be used to control the bilayer structure. Replacement of Si with Ge and Al as prototypical tetravalent and trivalent dopants was also investigated. Substituting Ge fo...

Dehydration and thermal stability of elpidite: An in-situ single crystal X-ray diffraction study

Elpidite Na2ZrSi6O15·3H2O is a natural zirconosilicate with space group Pbcm. A sample from Mt. Saint Hilaire (Canada) was investigated between 25 and 350 °C under different humidity conditions (RH = 0% and at RH = 100%). The largest structural channels are eight- and ten-rings delimited by corner-sharing SiO4 tetrahedra and ZrO6 octahedra.Under dry conditions, the structure started liberating water at 75 °C. At 100 °C, when it lost one H2O molecule, elpidite underwent a structural change from space group Pbcm to Cmce.The structure continuously released water up to 225 °C until it became anhydrous. From 225 °C to 350 °C no structural changes were observed. Associated with the transition from Pbcm to Cmce symmetry, the unit-cell volume reduced by 2.7%. An additional decrease is observed up to 225 °C when the contraction reached 3.4% of the starting volume. After release of all H2O molecules, increase of temperature led to gradual expansion of the unit cell from 2940.79 (7) Å3 at 225 °C to 2947.4 (5) Å3 at 350 °C.Supplementary ex-situ heating experiments showed that the structure was retained up to 950 °C but after heating at 1200 °C the sample melted.Under humid conditions a dehydration-trend similar to that found under a dry N2-atmosphere was observed but the structural changes occurred at slightly higher temperatures.Immersion of a dehydrated sample in water for 30 days led to only marginal rehydration and the Cmce symmetry was preserved.

Solid-state syntheses and single-crystal characterizations of three tetravalent thorium and uranium silicates

Colorless crystals of ThSiO4 (huttonite) (1) and (Ca0.5Na0.5)2NaThSi8O20 (2) have been synthesized by the solid-state reactions of ThO2, CaSiO3, and Na2WO4 at 1073K. Green crystals of (Ca0.5Na0.5)2NaUSi8O20 (3) have been synthesized by the solid-state reactions of UO2, CaSiO3, and Na2WO4 at 1003K. All three compounds have been characterized by single-crystal X-ray diffraction. Compound 1 adopts a monazite-type three-dimensional condensed structure, which is built from edge- and corner-shared ThO9 polyhedra and SiO4 tetrahedra. Compounds 2 and 3 are isostructural and they crystallize in a steacyite-type structure. The structure consists of discrete pseudocubic [Si8O20]8− polyanions, which are connected by An4+ cations into a three-dimensional framework. Each An atom coordinates to eight monodentate [Si8O20]8− moieties in a square antiprismatic geometry. Na+ and Ca2+ ions reside in the void within the framework. Raman spectra of 1, 2, and 3 were collected on single crystal samples. 1 displays more complex vibrational bands than thorite. Raman spectra of 2 and 3 are analogous with most of vibrational bands located at almost the same regions.

(Ba, Sr)3MgSi2O8 structure change caused by Ba/Sr replacement

Crystal structure of BaxSr3–xMgSi2O8 has been determined by Raman spectroscopy and X-ray diffraction. The solid solution series have glaserite-type layered structures made of corner-sharing SiO4 tetrahedra and MgO6 octahedra. Ba2+ and Sr2+ ions are sandwiched in between the layers. Raman spectroscopy has found that structural symmetry changes at x = 0.5 and 2.5. Structural refinement by the Rietveld method has clarified that the symmetry changes occur among C2 (Z = 4), P${\bar 3}$m1 (Z = 1), and P${\bar 3}$ (Z = 3). They originate in SiO4 tilting caused by size mismatch between alkali–earth cations and their site spaces. For x ≤ 0.5, SiO4 tilting occur every other interlayer space, whereas for x ≥ 2.5, all the SiO4 tilt.

Synthesis and Structure Determination of CaSi1/3B2/3O8/3: A New Calcium Borosilicate

This article reports on the identification, synthesis, and in-situ structure determination of a new crystalline calcium borosilicate compound of composition CaSi(1/3)B(2/3)O(8/3). Synthesis was carried out by complete crystallization on annealing from a corresponding glassy composition in the widely studied CaO-SiO2-B2O3 ternary system. The crystallographic structure was determined ab initio using electron diffraction information and the charge flipping algorithm performed on synchrotron and neutron powder diffraction data collected in situ at high temperature. CaSi(1/3)B(2/3)O(8/3) is found to crystallize in the Pna2(1) (no. 33) orthorhombic space group, with a = 12.1025(4) Å, b = 5.2676(1) Å, c = 3.7132(1) Å, and V = 236.71(1) Å(3) at 650 °C. Solid-state (29)Si and (11)B NMR experiments have confirmed the existence of finite chains along the c axis, formed by corner-sharing SiO4 tetrahedra and BO3 units. Silicon and boron species share a crystallographic site, and the Si/B distribution induces different possible arrangements of the chains which are discussed in light of DFT calculations. At room temperature, the existence of a superstructure, resulting from the ordering within nanoscale domains, was explored by transmission electron microscopy.

Single Crystal X-ray Diffraction Study of a Pure Natrolite Sample

The crystal structure of a pure natrolite sample, Na2(Al2Si3O10)·2H2O, coming from Asheken, Ethiopia, has been analysed by single crystal X-ray diffraction. It crystallizes within the orthorhombic space group Fdd2, with the following cell constants: a = 18.2930(2) A; b = 18.6430(5) A; c = 6.5860(5) A; V = 2246.07(18) A3. The three-dimensional framework of this hydrated aluminosilicate zeolite is made up by chains of corner-sharing SiO4 and AlO4 tetrahedra down c; the chains are held together by sharing the external vertices of tetrahedra; water molecules and Na+ extra-framework cations fill up the resulting cavities, the latter forming irregular NaO6 octahedra. Hydrogen bonds complete the array.

K(UO)Si2O6: A Pentavalent—Uranium Silicate.

AbstractFor Abstract see ChemInform Abstract in Full Text.

K(UO)Si2O6: A Pentavalent−Uranium Silicate

We report the first example of pentavalent-uranium silicate synthesized by a high-temperature, high-pressure hydrothermal method. The 3-D framework consists of four-membered single rings of corner-sharing SiO4 tetrahedra and 1-D UO4/1O2/2 chains. Magnetic susceptibility and XPS were measured to identify the valence state of uranium.

Rb3In(H2O)Si5O13: A Novel Indium Silicate with a CdSO4‐Topological‐Type Structure.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Rb3In(H2O)Si5O13: A Novel Indium Silicate with a CdSO4-Topological-Type Structure

A novel indium silicate, Rb3In(H2O)Si5O13, has been synthesized using a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction. The structure consists of five-membered rings of corner-sharing SiO4 tetrahedra connected via corner sharing to four adjacent five-membered rings to form a 3D silicate framework that belongs to the CdSO4 topological type. The InO6 octahedron shares five of its corners with five different SiO4 tetrahedra to form a 3D frame-work that delimits two types of channels to accommodate the rubidium cations. The sixth corner of InO6 is coordinated H2O. The structure is related to that of the titanium silicate ETS-10, and these are the only two metal silicates that have the CdSO4-topological-type structure. In addition, the crystal of Rb3In(H2O)Si5O13 shows an intense second harmonic generation signal. Crystal data: H2Rb3InSi5O14, monoclinic, space group Cc (No. 9), a = 9.0697(5) A, b = 11.5456(6) A, c = 13.9266(8) A, beta = 102.300(1) degrees, V = 1424.8(1) A3, and Z = 4.

Hydrothermal Synthesis, Crystal Structure, Solid‐State NMR Spectroscopy, and Ionic Conductivity of Na5InSi4O12, a Silicate Containing a Single 12‐Membered Ring.

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Hydrothermal Synthesis, Crystal Structure, Solid-State NMR Spectroscopy, and Ionic Conductivity of Na5InSi4O12, a Silicate Containing a Single 12-Membered Ring

Na5InSi4O12 has been synthesized by a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction and solid-state NMR spectroscopy. It is isotypic to Na5ScSi4O12. The structure consists of 12-membered single rings of corner-sharing SiO4 tetrahedra linked together via corner sharing by single InO6 octahedra to form a 3-D framework that delimits two types of channels along the c-axis. There are six independent sodium sites. The Na atoms within the 12-ring channels are immobile and the Na atoms within 7-ring channels are highly mobile. The 29Si MAS NMR exhibits one Q3 and one Q4 resonance, each of which is split into two peaks because of inequivalent chemical environments caused by partially occupied Na sites in the 7-ring channels. Na ion conductivity measurements on a pressed pellet of the title compound were performed. The Arrhenius plot of dc conductivity shows a change in slope near 220 °C. We attribute this to the activation of the Na site that is situated ...