Search

A family of homeotypic porous lanthanide metal−organic frameworks (MOFs), [Ln(btc)(H2O)]·guest (Nd (1), Sm (2), Eu (3), Gd (4), Tb (5), Ho (6), Er (7), and Yb (8); guest: DMF or H2O) was synthesized. The structures of the as-synthesized compounds are tetragonal and contain 1D channels with accessible lanthanide ions. In situ single crystal X-ray diffraction shows that 1 undergoes a single-crystal to polycrystalline to single-crystal transformation from room temperature to 180 °C. During the release of DMF and water molecules from the channels by evacuation and subsequent heating, the structures of 1 and 7 transformed from tetragonal to monoclinic, and then to tetragonal, while the structure of 8 remained tetragonal. The transformation between the monoclinic and the low temperature tetragonal phases is reversible. The Ln(btc) MOFs are stable to at least 480 °C and are among the most thermally stable MOFs. The Ln(btc) MOFs act as efficient Lewis acid catalysts for the cyanosilylation of aldehydes yielding cyanohydrins in high yields within short reaction times. 1 also catalyzes the cyanosilylation of less reactive substrates, such as ketones at room temperature. The Ln(btc) MOFs could be recycled and reused without loss of their crystallinity and activity.

Mixed alkali earth element containing high nitrogen content oxynitride glasses (Ca1−xAEx)1.2(1)SiO1.9(1)N0.86(6), with AE = Mg, Sr, Ba, x ≤ 0.30 for Mg and x ≤ 0.46 for Sr and Ba, and nominally constant (Ca/AE):Si:O:N ratios were prepared in order to investigate the compositional dependencies of physical properties on alkali earth element composition. The glasses were prepared by melting mixtures of AEH2, CaH2, SiO2 and Si3N4 powders in nitrogen atmosphere at 1600–1700 °C and characterized by X-ray powder diffraction and scanning and transmission electron microscopy. Cation and anion glass compositions were determined by respectively energy dispersive X-ray analysis and combustion analysis. The determined physical properties were density, glass transition temperature, Vickers hardness, and refractive index. The physical properties were found to vary linearly with the degree of substitution of Ca by the AE elements. The density of the glasses increases substantially upon substitution by Sr and Ba, up to 3.99 g/cm3. Glass transition temperatures are found to be higher for Mg and Sr substituted glasses, ca. 900 °C, in comparison with Ba substituted glasses, ca. 850 °C. The hardness increases upon substitution by Mg, up to 12.2 GPa at x = 0.46, and decreases upon substitution by Sr and Ba. The refractive index increases upon substitution by Sr and Ba, up to 1.97 for Ba at x = 0.46, and decreases upon substitution by Mg. The transparency of the glasses was found to increase upon increasing substitution by Mg and completely transparent glasses were obtained for x = 0.24., Part of urn:nbn:se:su:diva-8521

Calcium aluminate is the main constituent in calcium aluminate cements, used in a wide range of applications in construction and mining industries and recently also as biomedical implant. In applications that demand very precise reaction features, such as the biomedical ones, the phase purity is of very high importance. In this paper the formation of CaAl2O4 from CaCO3-Al2O3 powder mixtures has been studied, varying holding times between 1 and 40 h and temperatures between 1300 and 1500 degrees C. Phase formation was studied in samples both quenched from the holding temperatures and in samples slowly cooled. Samples were characterized by X-ray powder diffraction (XRPD), using Guinier-Hagg film data and the Rietveld method, and scanning (SEM) and transmission (TEM) electron microscopy. Samples for TEM with very high site accuracy were produced using focused ion beam microscopy. In addition to CA (CaAl2O4) the samples contained major amounts of CA(2) (CaAl4O7), C(12)A(7) (Ca12Al14O33) and minor amounts of un-reacted A (Al2O3). Trace amounts of C(3)A (Ca3Al2O6) were observed only for samples heated to 1500 degrees C. The amount of the Ca-rich phase C(12)A(7) was found to decrease with time as it reacts with A and, to a less degree, CA(2) to form CA. In agreement with previous studies the amount of CA(2) formed decreases comparatively slowly with time. Its un-reactivity is due to that it is concentrated in isolated porous regions of sizes up to 100 mu m. The formation of the Ca aluminates is found to be in response to local equilibriums within small inhomogeneous regions, with no specific phase acting as an intermediate phase. Samples quenched from 1500 degrees C were found to contain smaller amounts of poorly crystallized phases. A reaction between C and A takes place already at 900 degrees C, forming a meta-stable orthorhombic modification of CA. The orthorhombic unit cell with a = 8.732(2) b = 8.078(2) angstrom, c = root 3 center dot a = 15.124(4) angstrom was verifi

We investigate the local structures of oxynitride La–Si–(Al)–O–N glasses by 29Si and 27Al magic-angle spinning (MAS) solid state Nuclear Magnetic Resonance (NMR). The glasses studied span an unprecedented range of compositions, up to >50 at.% lanthanum and nitrogen out of the cations and anions, respectively, and achievable through a recently introduced glass preparation route. Transmission as well as scanning electron microscopy verified homogeneous samples over length-scales down to 20 nm. As the nitrogen content of the glasses increased, 29Si NMR evidenced a progressive formation of Si–N bonds, with SiO2N2 tetrahedra dominating in the nitrogen-rich glass networks. In the oxygen-rich end of the series, aluminum is predominantly present in tetrahedral coordination as AlO4, whereas the glasses with highest nitrogen contents have a major fraction of AlO3N structural units. Trends in isotropic 29Si and 27Al chemical shifts and 27Al quadrupolar couplings are compared with results of La–Si–Al–O glasses and are discussed in relation to the glass compositions and their proposed structures., Part of urn:nbn:se:su:diva-7169

We report a new open-framework silicogermanate SU-61 containing 26-ring channels with a low framework density. It can be seen as a crystalline analogue to the mesoporous silica MCM-41. The structure is built from the assembly of (Ge,Si)10(O,OH)28 clusters. It is the first time that silicon has been successfully introduced in the Ge10 cluster in significant amounts (21% of the tetrahedral sites). Five- and six-coordinated Ge10 clusters have previously been observed in other germanate compounds leading to either dense or open structures. In SU-61, the seven-coordinated clusters fall onto yet another underlying net, the osf net. SU-61, along with other Ge10 based frameworks, shows the versatility of the germanate system to adopt defined topologies playing on the connectivity of the clusters following the principles of net decoration and scale chemistry.

In a series of Ba-based oxonitrido-silicate S-phases (Ba2AlxSi12-xN16-xO2+x) spanning a compositional range up to x approximate to 3, we examine the incorporation of Al and O by Si-29 and Al-27 magic-angle spinning (MAS) solid state nuclear magnetic resonance (NMR) and Al-27 triple-quantum MAS (3QMAS). The 3QMAS spectra reveal Al-27 signals from two distinct structural environments, assigned to AlN4 or AlN3O tetrahedra, respectively, and with their relative amounts depending on the S-phase substitution parameter x. Si-29 NMR show variable fractions of SiN4 and SiN3O environments. The NMR results accord overall with a structural substitution model for which O enters at one crystallographic position (occupied according to N4-xOx), in conjunction with a random Al for Si substitution at two distinct crystallographic positions. This leads to S-phase frameworks built from SiN4, SiN3O, AlN4 and AlN3O tetrahedra., authorCount :4

In developing elongated a-sialon grains, a nitrogen-rich liquidphase sintering method was introduced by using CaH2 as asintering aid, so as to vary the N/O ratio of the liquid phaseformed in the sintering process while keeping the Si/Al ratiosconstant. With increasing addition the phase contents changedfrom single b-sialon to dual b/a-sialons and to single Ca-a-sialon.At low N/O ratios the microstructures contained mainlyequiaxed b-sialon grains, and at high N/O ratios well facetedelongated Ca-a-sialon grains. The improved toughness(KIC57.8 MPa .m1/2) and hardness (HV10517.5 GPa) propertiescan be attributed to the formation of interlocked microstructures.High-temperature compressive deformation testsindicated that the deformation onset temperature is determinedmainly by the Si/Al and N/O ratios, whereas the deformationrate is affected by the microstructure, i.e., the morphology andamounts of elongated a-sialon grains and residual glass phase,especially for the sialons with low N/O ratios., Part of urn:nbn:se:su:diva-8568

Part of urn:nbn:se:su:diva-6800

A series of crystalline Cd-W oxides. with Gd:W ratios spanning the values 0.1-3.5. has heen prepared and treated with a flow of NH3(g) in the temperature range 700-1100 oC. Besides the previously reported formation of GdWO3N and Gd2WO3N2. the following new results were obtained: (I) cubic Gd0.1WO3 upon ammonolysis forms an (oxy)nitride Gd0.1x W(O. N) with an f.c.c lattice; (ii) the previously described Gd2WO3N2 exhibits a solid solubility range Gd3-xW1+x(O.N)y with, approximately, 0≤x≤1; (iii) ammonolysis of Gd14W4O33 yields an oxynitride, the composition of which varies with the synthesis temperature according to Gd14W4O33-xNy (0≤x≤17±2, 0≤y≤9±2). All strong diffraction lines, including those of the pure oxide (x =y= 0), can be indexed with a body-centred monoclinic unit cell derived from that of fluorite. For Gd14W4O33 the cell parameters are a = 5.3592(11), b = 3.7295(7), c = 3.8636(8), β = 92.170(2)o and V = 77.17 Å3. However, strong superstructure rereflections in the electron diffraction patterns indicate that the true unit cell is considerably larger. A partial structur determination of Gd14W4O33-xNy has been carried out by the Rierveld method. Magnetic susceptibility measurements in the temperature interval 15-300 K show a Curie-Weiss behaviour for Gd14W4O33-xNy with μeff = 8.08±0.06 Bohr magnetons per Gd3+ and θ = −12.4±1.1 K for all compositions.