Fold Coordination Research Articles

La4B14O27: Ein Lanthan‐ultra‐Oxoborat mit Raumnetzstruktur

AbstractLa4B14O27: A Lanthanum ultra‐Oxoborate with a Framework StructureSingle crystals of La4B14O27 could be synthesized by the reaction of La2O3, LaCl3 and B2O3 with an access of CsCl as fluxing agent in gastightly sealed platinum ampoules within twenty days at 710 °C and appear as colourless, transparent and waterresistant platelets. The new lanthanum oxoborate La4B14O27 (monoclinic, C2/c; a = 1120.84(9), b = 641.98(6), c = 2537.2(2) pm, β = 100.125(8)°; Z = 4) is built of a three‐dimensional boron‐oxygen framework containing seven crystallographically different boron atoms. Four of these B3+ cations are surrounded by four O2− anions tetrahedrally, whereas the other three have only three oxygen neighbours with nearly plane triangular coordination figures. Three of the [BO4]5− tetrahedra form [B3O9]9− rings by cyclic vertex‐condensation, which are further linked via [BO3]3− units to infinite layers. Two of these layers connect via one [B2O7]8− unit of two corner‐shared [BO4]5− tetrahedra to double layers, which themselves build up a three‐dimensional framework together with chains consisting of two [BO4]5− tetrahedra and one [BO3]3− triangle. One of the two crystallographically independent La3+ cations (La1) is surrounded by ten O2− anions and resides within the oxoborate double layers. (La2)3+ shows a (8+2)‐fold coordination of O2− anions and occupies channels along the [110] direction.

Real Time Neutron Diffraction Studies of Apatite Glass Ceramics

Apatite-mullite glass-ceramics have been developed based on SiO2-Al2O3-P2O5-CaOCaF2 glasses. The glasses crystallise to form fluorapatite (FAP) and mullite with an appropriate heat treatment. The crystallisation mechanism has been thought to occur via a prior amorphous phase separation. The aim of this study was to carry out real time neutron diffraction of apatite-mullite glass-ceramics in order to develop an understanding of the crystal growth and crystal dissolution phenomena in the temperature regime between 550 and 1200oC. The results show that during the initial stages of fluorapatite crystallisation pronounced line broadening was observed indicating crystallisation on a nanoscale. Mullite starts to crystallise once there is insufficient charge balancing cations in the glass to maintain aluminium in a four fold coordination state. As a result of this work it is suggested that the mechanism of crystal growth of FAP is thought to involve the dissolution of smaller FAP crystals during the crystal growth temperature followed by re-crystallisation of FAP on the remaining larger crystallites during cooling.

Distinct local environments for Ca along the non-ideal pyrope–grossular solid solution: A new model based on crystallographic and EXAFS analysis

A multi-technique approach (based on electron microprobe analysis, structure refinement, and EXAFS analysis at the Ca K-edge) was used to characterise the local geometry of Ca in synthetic and natural garnet compositions referable to the pyrope–grossular solid solution. Multi-shell fits of the EXAFS data indicate that Ca assumes the standard [4 + 4]-fold coordination (the polyhedral shape being a triangular dodecahedron with Ca1–O = 2.30–2.31(1) and Ca2–O = 2.45–2.46(1) Å) when Ca > 1.50 atoms per formula unit (apfu), but assumes a nearly regular [8]-fold coordination with Ca–O = 2.35–2.36 (1) Å when (Mg, Fe 2+, Mn 2+) > 1.50 apfu. Therefore, in the pyrope-dominant structure the Ca1–O distance lengthens and the Ca2–O distance shortens to converge towards the value observed for the Mg2–O bond in pyrope. This finding is consistent with many distinct structural features observed in solid solution terms with (Mg, Fe 2+, Mn 2+) > 1.50 apfu or Ca > 1.50 apfu, as well as with the anomalous properties of the intermediate terms observed both in the short-range and in the long-range perspective. The presence of two distinct Ca coordinations in the pyrope (almandine, spessartine)-like and in the grossular-like structure, and thus of an isosymmetric transition at the intermediate composition, can help to explain both the strong and asymmetric non-ideality of the solid solution between pyrope (almandine) and grossular, as well as the differences in the ability to incorporate some trace elements (such as REE and actinides) which are commonly used as process-specific indicators. This feature must be taken into account when building theoretical models of the garnet solid solutions, which are at the moment the most promising approach for calculating thermodynamic properties or for interpreting and predicting trace-element behaviour in this crucial mineral phase.

A structural study of the perovskite series Ca 1−xNa xTi 1−xTa xO 3

Compounds in the solid solution series Ca 1− x Na x Ti 1− x Ta x O 3 were synthesized at 1300 °C, followed by annealing at 850 °C or 800 °C with quenching and/or slow cooling to room temperature. Rietveld refinement of their powder X-ray diffraction patterns show that all compounds are single-phase ternary perovskites which adopt the space group Pbnm ( a≈b≈√2 a p; c≈2 a p; Z=4) at ambient conditions. The unit cell parameters and cell volumes of the compounds increase regularly with increasing values of x. The coordination of the A-site cations changes throughout the series from eight for CaTiO 3 to nine for NaTaO 3. Compounds with 0⩽ x ⩽0.4 have A-site cations in eight fold coordination, whereas the coordination of those with 0.4< x<0.9 is ambiguous. Analysis of the crystal chemistry of the compounds shows that the change in coordination at x=0.4 is related to the departure of the B-site cations from the second coordination sphere of the A-site cations, as in compounds with x>0.4 the A– IIO distances become less than the A–B intercation distances. Contemporaneous with these coordination changes, the tilt angles of the BO 6 polyhedra decrease with increasing values of x. This solid solution series is unusual in that these structural and coordination changes occur regardless that Goldschmidt tolerance factors remain essentially constant at approximately 0.89, and observed tolerance factors, assuming eight fold coordination of the A-site cations, range only from 0.91 to 0.93 (0⩽ x⩽0.8).

Structural and Spectroscopic Characterization of Two New Thallium(I)/Thiosaccharinate Complexes

AbstractThe crystal structures of [Tl(tsac)] (1) and [Tl(tsac)(ophen)] (2) (tsac = anion of thiosaccharin; ophen = 1, 10 phenanthroline) have been determined at 116 K by single crystal X‐ray diffractometry. Complex 1 crystallizes in the monoclinic space group P21/a with Z = 4 and complex 2 in the monoclinic space group C2/c with Z = 8.In both complexes TI is coordinated to a thiosaccharinate anion through its sulphur and nitrogen atoms. A distorted eight fold coordination sphere around the cation in complex 1 is completed with two other longer Tl‐S bonds and four Tl···O contacts with five symmetry related neighbouring thiosaccharinate anions. A phenanthroline molecule acting as a bidentate ligand through its nitrogen atoms completes a four‐fold coordination around the metal atom in complex 2. The infrared spectra of both complexes were also recorded and their most important features discussed on the basis of its structural peculiarities.

The Structural Transformation from the Pyrochlore Structure, A2B2O7, to the Fluorite Structure, AO2, Studied by Raman Spectroscopy and Defect Chemistry Modeling

Pyrochlore oxides of the composition Y2Ti2−yZryO7, with y= 0, 0.3, 0.6, and 0.9, were studied using Raman spectroscopy. Doping with Zr+4, a homovalent ion, gradually induces the material to undergo a structural change from the perfect pyrochlore structure to a defect pyrochlore structure and ends at an oxygen deficient fluorite structure with the formula (Y, Ti, Zr)1O1.75. Invoking the bigger Zr+4 ion on the Ti+4 site causes this structural transformation. We have also investigated highly Ti-doped YSZ ceramic samples, which all have an oxygen deficient cubic fluorite structure. These samples have formula units between (Y, Ti, Zr)1O1.675 and (Y, Ti, Zr)1O1.90. It is argued that a band observed at 750 cm−1 in both the Ti-doped pyrochlore samples and in the Ti, Y, Zr fluorite samples is due to oxygen in seven fold coordination around Ti. Local structures are accordingly observed by Raman in these samples. We finally report a Raman spectrum of orthorhombic Y2TiO5. A defect chemistry model has been formulated, which qualitatively describes the observed pyrochlore–fluorite structural transition.

Interaction of phenol with NaX zeolite as studied by 1H MAS NMR, 29Si MAS NMR and 29Si CP MAS NMR spectroscopy

The interaction of phenol with zeolite NaX has been studied by 1H and 29Si MAS and 29Si-CP-MAS NMR spectroscopy. A large displacement of the chemical shift of hydroxyl protons from 1.2 ppm in the crystalline state to about 9 ppm occurs upon H-bonding of phenol to basic oxygen atoms of the zeolite. An enhancement of the 29Si-CP-MAS signal is observed for the phenol loaded NaX zeolite and interpreted in terms of an interaction of the aromatic ring with either Na + cations or oxygen atoms of the 12 member ring of the supercage. A small fraction of framework silicon atoms in four fold aluminum coordination, most likely Si atoms in four member ring positions of SIII or SIII’ sites, is electronically affected by interaction of phenol with Na + ions located in these sites. After thermal treatment to 320°C part of the phenol is deprotonated and rearranges to linear Na +… −OPh complexes.

Structural study of a lead–barium–aluminum phosphate glass by MAS-NMR spectroscopy

The 31 P - and 27 Al -MAS-NMR spectra of a lead–barium–aluminum phosphate glass were measured and analyzed, and the short range structure of the glass are discussed. The 31 P -MAS-NMR spectrum of the glass studied has a single isotropic peak at −4.76 ppm, indicating that orthophosphate species are the dominant P-sites in the glass. The 27 Al -MAS-NMR spectrum of the glass shows that the aluminum ions present in the glass are in 4-, 5-, and 6- fold coordination with oxygen, and that the Al(OP) 4 is the dominant moiety for the glass, suggesting the formation of orthophosphate species in the glass.

Microscopic study of superionic transition.

The mechanism of superionic transition is discussed both phenomenologically and microscopically in the framework of the ``chemical bond peculiarities'' approach which was originally proposed by Phillips. The proximity of p-halogen and d-metal electronic levels is proved to be a critical prerequisite to the formation of a local additional minimum of crystalline potential. The fold coordination number change involved in the transition is regarded as a redistribution of ionic occupancies, rendering this additional minimum absolute. The low-energy excitations are ascribed to the ionic motion between the local and absolute potential minima, with their consequent appearance both at temperatures below and above the phase transition. \textcopyright{} 1996 The American Physical Society.

Auger Si LVV, O KLL and N KLL lineshapes at air‐exposed silicon nitride surfaces: Pattern recognition analysis

AbstractAuger electron spectroscopy measurements have been performed on silicon nitride layers exposed to air. The layers were deposited on Si(100) wafers kept at 300°C or 600°C by excimer laser ablation of a silicon target under various ammonia pressures. The Si LVV, O KLL and N KLL lineshapes of silicon nitride layers and of in situ prepared clean surfaces of standards are compared by applying pattern recognition (PR) methods to obtain bonding information of elements within the analysed volume. The PR results based on Auger Si LVV, O KLL and N KLL spectra compare very well. They are also in agreement with the results of XPS. The chemical states of silicon atoms in all silicon nitride layers depend strongly on the deposition conditions used. For low ammonia pressures, surface regions of silicon nitride layers show a twofold coordination with oxygen to silicon atoms (SiO2), whereas for higher ammonia pressures there is a four fold coordination of silicon to nitrogen atoms (Si3N4). The results are interpreted as follows. The silicon‐rich silicon nitride layers are formed at the lower ammonia pressure where the silicon oxide dominates over silicon nitride in the surface region, whereas for the higher ammonia pressure silicon nitride is prevalent. A new approach of the PR method, so‐called ‘cluster analysis’, shows a close similarity in the chemical states of nitrogen atoms in the nitride layer formed at higher pressure and silicon nitride standard, indicating a threefold coordination of nitrogen to silicon atoms. Subtle differences revealed in the spectral shapes elucidate the strongly correlated behavior of the N KLL lineshapes with the deposition conditions used.

Beryllium—Geochemistry, Mineralogy and Beneficiation

This paper presents a review of the geology, geochemistry and mineralogy of beryllium, and methods of beneficiation of beryl including the safety measures to be adopted while crushing and grinding of beryllium minerals. A typical lithophile element, beryllium forms a characteristic four fold coordination with oxygen forming the [BeO4]6− complex. Geochemically it accumulates in the acid and alkalic magmas during magmatic differentiation process. During crystallization of these two magmas beryllium follows different course; in the case of acid magma it gets concentrated in the residual pegmatitic and hydrothermal stage, while in the case of alkali magma it enters into the lattice of several rock forming and accessory minerals by diadochic capture preventing its concentration in the residual stage. Beryllium enters in the structure of as many as 40 minerals as an essential constituent and another 50 minerals as occasional and minor diadochic constituent. Of the 40 beryllium minerals, 26 are silicates reflect...

Syntheses and structures of AgLnS 2 (LnSmYb) compounds

The ternary compounds AgLnS 2 (LnSmYb) have been synthesized and characterized. It has been found, in accordance with a previous study, that all compounds, except AgYbS 3, are monoclinic with a ≈ b ≈ 7.7 Å, c ≈12 Å and 90° < β ⪕ 91°. AgYbs 2 is a body-centred tetragonal with a′= a √2 . Single crystals of AgLnS 2 were grown for LnGd, Ho and Yb by means of vapour transport. The structures of the monoclinic compounds AgLnS 2 (LnGd, Ho), space group P2 1, were determined using single crystal X-ray data. Silver atoms are situated in strongly distorted S 6-octahedrons with two short, three intermediate and one long AgS distances. Rare-earth metal atoms are situated in slightly distorted S 6-octahedrons with LnS distances in good agreement with literature data for Ln 3+ in six fold coordination by sulphur. The structure is a superstructure of the NaCl-type LnS structure, with half of the cations having been replaced by silver. Contrary to the powder data, the single crystal data showed that AgYbS 2 is cubic. It has the NaCl structure as YbS, but with silver and ytterbium atoms randomly distributed on the cation sites. The disordered phase is metastable at room temperature; the ordered tetragonal phase could be obtained by annealing for 5 days at 1000 K. The magnetic susceptibility measurement for AgGdS 2 showed Curie-Weiss behaviour above T≈35K corresponding to μ eff = 7.38 μ b (in agreement with Gd 3+) and Θ c = −32.0 K.

Classification of superconducting oxide structures search for new compounds

The structures of superconducting oxides are analysed as symmetrical combinations ofu oru′ (Ca, Sr)CuO2,v CaTiO3 andw VH or NaCl structural subunits. The structures with a maximum of 8 subunits can be classified according to the different hole donating elementsu′ (9 structures),w′ (21 structures) or both (20 structures). Two hole donating elements likeu′ CuO andw′ PbO can destroy metallic conductivity by internal redoxreaction between both elements as is outlined for Pb2Sr2-YCu3O9. The unit cell constants and the appropriate size of metal atoms of theoretical structures can be estimated froma=385 pm,c=160u+200v+265w pm. The alternative description of crystal structures by the sequence of coordination numbers of metal atoms might support the search for new compounds. The metal atoms Cu, Ni or Ti, Tl, Ag, Pb etc. with 20–30% reduction of radii at increased valence are favourable for hole donation in 4 or 6′ fold coordination ofu′ orw′ subunits.

STRUCTURAL DIAGNOSTIC OF HIGH TEMPERATURE LIQUID PHASES BY 27Al NMR

Improvements of welding processes by plasma or laser require a good knowledge of the microscopic properties of the liquid associated to the melted area. The development of new high temperature NMR probes allows to reach such a goal. In the case of alumina bearing liquids, the analysis of the 27Al resonance line shows a rapid exchange regime of Al in four, five or six fold coordination together with a dependence of the resonance line versus the liquid composition.

High Resolution Study of CoSi2/Si (111) Interfaces

AbstractThe atomic structure of A- and B-type CoSi2/Si (111) interfaces has been investigated by observations of samples in cross-section using a 400 kV high-resolution electron microscope. The samples were prepared by UHV e–beam evaporation of Co layers followed by annealing at temperatures between 300°C and 500°C. Based upon image simulations for various interface bonding models we have found evidence for 7–fold Co coordination at the A–type CoSi2/Si interfaces and for 7– and 8–fold coordination at the B-type interfaces.

Neutron powder diffraction study of Pb2Sr2YCu3O8, the prototype of a new family of superconductors

Abstract The structure of Pb 2 Sr 2 YCu 3 O 8 has been analyzed with powder neutron diffraction techniques and profile analysis. The results of this study confirm the general structural features determined by X-ray single-crystal methods. The material has pseudo tetragonal symmetry, but is orthorhombic, space group Cmmm with lattice parameters a =5.3933(2), b =5.4311(2), and c =15.7334(6)A. The orthorhombic distortion is caused by the disordering in the ab plane of the oxygen atoms of the PbO layers over the general position of the space group, with x =0.275(5) and y =0.402(5) A. The structure of this compound can be derived from that of YBa 2 Cu 3 O 6 by substituting the Cu layers with blocks of … PbO.Cu.PbO… . Of the two Cu atoms in the asymmetric unit, one has two fold coordination while the other has five fold pyramidal coordination with the apex elongated along the c -axis.

The infrared and Raman spectra of ZrO2-SiO2 glasses prepared by a sol-gel process

Infrared and Raman spectra were measured and interpreted on the basis of structure for various ZrO2-SiO2 glasses that were prepared by a sol-gel process involving zirconium nitrate. Interpretation of the spectra indicates the presence of both Zr-O-Si and Si-O-H linkages depending upon heat treatment conditions, and the evidence of zirconium atoms with eight fold coordination for glasses with high ZrO2 contents. The crystalline products formed during glass devitrification were monitored by vibrational spectra. The formation and detection of tetragonal ZrO2, zircon, monoclinic ZrO2 and α-cristobalite depended upon the conditions of devitrification.

Calorimetric study of high-pressure phase transitions among the CdGeO3 polymorphs (pyroxenoid, garnet, ilmenite, and perovskite structures)

At high pressures, CdGeO3 pyroxenoid transforms to garnet, then to ilmenite, and finally to perovskite. Enthalpies of transition among the four phases were measured by high temperature calorimetry. The entropies of transition and slopes of the boundaries were calculated using the measured enthalpies and free energies calculated from the phase equilibrium data. Pyroxenoid and garnet are very similar energetically. However garnet is a high pressure phase because of its lower entropy and smaller volume. The pyroxenoid-garnet transition has a small positiveP-T slope. Ilmenite is intermediate in enthalpy between garnet and perovskite, but is lower in entropy than both phases. Therefore the garnet-ilmenite transition has a positivedP/dT, while a negativedP/dT is calculated for the ilmenite-perovskite transition. The thermochemical data for the CdGeO3 phases are generally consistent with the observed high pressure phase relations. The high entropy of perovskite relative to ilmenite, observed in several ABO3 comounds including CdGeO3, is related to the structural features of perovskite, in which relatively small divalent cations occupy the large sites of 8–12 fold coordination. The thermochemistry of the CdGeO3 polymorphs shows several similarities to that of the CaGeO3 system.

Structure cristalline du monophosphate AgCoPO 4

AgCoPO 4 is a new type of ABPO 4 framework with a twofold oxygen coordination for silver atoms and a five fold coordination for cobalt atoms. Crystal structure has been solved using both Patterson and direct methods with a final R value of 0.038 for 2097 independent reflexions. The symmetry is triclinic P 1 with a unit cell: a = 9.516(2), b = 5.574(1), c = 6.572(2) A, α = 102°20(10), β = 106°16(10), γ = 80°08(10) and Z = 4.

The atomic geometry of c (2×2) overlayers of sulfur on Ni(100)

Our earlier determination of the structure of Ni(100)−c (2×2)−S overlayers via the analysis of elastic low−energy electron diffraction (ELEED) intensities is reexamined following a −2.5−eV contact−potential correction to the energies of the ELEED intensities measured by Theeten. While this correction does not alter our conclusions that the surface is unreconstructed and that the S atoms occupy sites of four−fold coordination, it does render dS = 1.3 Å a slightly better choice for the vertical height of the sulfur overlayer than the value of 1.7 Å previously reported. A discussion is given of the influence of the model of the electron−ion−core potential on the ELEED lineshapes and the empirical optical potential parameters. For our overlapping atomic potentials, the use of about ten phase−shifts is required for complete convergence of the single−atom scattering factors over the energy range 100 eV?E?200 eV. Estimation of the optical potential for clean Ni suggests that calculations based on a 10−phase−shift model would exhibit empirical real inner potentials 2−5 eV smaller in magnitude than those obtained in our present 4−phase−shift analysis.