Poul Norby

Delamination and restacking of a layered double hydroxide with nitrate as counter anion

A layered double hydroxide (LDH) with nitrate as the counter anion (LDH–NO3 with Mg/Al = 3) was, for the first time, successfully delaminated in formamide under ultrasonic treatment. Atomic force microscopy (AFM) images showed that a large part of the LDH was delaminated into single and double brucite layers (0.7–1.4 nm in thickness). The nano-sheets had disk-like shapes with a diameter of ca. 20–40 nm. Findings from AFM were in good agreement with the average hydrodynamic diameter determined using dynamic light scattering. Powder X-ray diffraction pattern of LDH dispersed in formamide also confirmed that LDH–NO3 was exfoliated. The dispersions of LDH in formamide were stable and transparent up to a concentration of 40 g L−1. However, formation of transparent gels was observed at concentrations higher than 5 g L−1. Delaminated LDH could be restacked by adding sodium carbonate or ethanol.

The crystal structure of lanthanum manganate(iii), LaMnO3, at room temperature and at 1273 K under N2

Abstract Orthorhombic stoichiometric LaMnO 3 is prepared by heating rhombohedral LaMaO 3+δ under N 2 at 1173 K and cooling to room temperature under N 2 . The thermal transformations of orthorhombic lanthanum manganate(III) were investigated using high-temperature X-ray powder diffraction. Transformations orthorhombic—cubic—rhombohedral were observed. The dependence of the unit cell parameters on temperature was determined. The crystal structures of LaMnO 3 at room temperature (orthorhombic) and at high temperature (rhombohedral at 1273 K in N 2 ) were determined using neutron powder diffraction.

The decomposition of LiAlD4 studied by in-situ X-ray and neutron diffraction

Abstract The decomposition of LiAlD 4 was studied by thermal desorption spectroscopy and neutron and synchrotron X-ray in-situ diffraction. The first two reaction stages (3LiAlD 4 =Li 3 AlD 6 +2Al+3D 2 and Li 3 AlD 6 =3LiD+Al+ 3 2 D 2 ) were confirmed. No other intermediate phases were observed. Both reactions are very heat-rate dependent. The lowest temperature measured for complete reactions were 112 and 127xa0°C, respectively. Based on refinements of the unit-cell dimensions, neither LiAlD 4 nor Li 3 AlD 6 lose any D 2 prior to their thermal decompositions.

Structural and morphological evolution of beta-MnO2 nanorods during hydrothermal synthesis

Beta-MnO(2) nanorods were synthesized via a redox reaction of (NH(4))(2)S(2)O(8) and MnSO(4) under hydrothermal conditions. In situ and ex situ x-ray diffraction and scanning electron microscopy were employed to follow the structural and morphological evolution during growth. It was found that the crystallization of beta-MnO(2) nanorods proceeds through two steps: gamma-MnO(2) nanorods form first via a dissolution-recrystallization process and then transform topologically into beta-MnO(2) with increasing temperature. The phase transformation was associated with a short-range rearrangement of MnO(6) octahedra. Vibrational spectroscopic studies showed that the beta-MnO(2) nanorods had four infrared absorptions at 726, 552, 462 and 418 cm(-1) and four Raman scattering bands at 759 (B(2g)), 662 (A(1g)), 576 (Ramsdellite impurity) and 537 (E(g)) cm(-1), which are in agreement with Mn-O lattice vibrations within a rutile-type MnO(6) octahedral matrix.

Characterization of exfoliated layered double hydroxide (LDH, Mg/Al = 3) nanosheets at high concentrations in formamide

For the first time procedures for the direct characterization of exfoliated nanosheets of LDH (Mg/Al = 3) in suspension are reported. The shape, size and layer thickness of the nanosheets were determined using small angle X-ray and neutron scattering (SAXS/SANS) in combination with dynamic light scattering (DLS) and atomic force microscopy (AFM). Furthermore, by using ultrasonic treatment we were able to delaminate a glycinate containing LDH up to a concentration of 42.5 g L−1, which is ten times higher than the value reported in the literature for the same system. The exfoliated LDH nanosheets were directly characterized in suspension by SAXS/SANS and gave an average layer thickness of 1.4 nm for both tested concentrations (10 and 42.5 g L−1). This result provides strong evidence that the LDH, even at concentrations as high as 42.5 g L−1, is totally delaminated into single or a few stacked brucite layers. AFM images of exfoliated LDH, after dilution and deposition on mica, confirm that exfoliated nanosheets mainly consist of single and double layers, corresponding to thicknesses of 0.6 ± 0.1 and 1.3 ± 0.1 nm, respectively. The diameters of the nanosheets in suspension determined using SAXS and SANS are very similar (between 30 and 40 nm), and are in good accordance with the value observed by AFM (between 10 and 40 nm). Hydrodynamic diameters determined by dynamic light scattering (DLS) were 35 and 60 nm at 10 and 42.5 g L−1, respectively.

The crystal structure of Cr8O21 determined from powder diffraction data: Thermal transformation and magnetic properties of a chromium-chromate-tetrachromate

Abstract Thermal decomposition of CrO3 was utilized to prepare a powder sample of the chromium oxide usually designated Cr3O8. Combined information from powder diffraction data using synchrotron, conventional X-ray, and neutron radiation allowed determination of the structure. The structure is triclinic (a = 5.433(1), b = 6.557(1), c = 12.117(2) A, α = 106.36(1), β = 95.73(1) and γ = 77.96(1)°) and was refined in the space group P 1 . The true composition of the compound is Cr8O21. There are two distinct types of chromium atoms in the structure, which may be designated the oxidation numbers (III) and (VI), respectively. The structure is built from pairs of edge-sharing Cr(III)O6 octahedra linked together by Cr(VI)O4 tetrahedra to form sheets. The sheets are then linked together by tetrachromate groups (Cr(VI)4O13) to form a three-dimensional structure. Thus, the chromium oxide may be described as Cr(III)2(Cr(VI)O4)2(Cr(VI)4O13). The magnetic properties of Cr8O21 were investigated in the temperature range 5 to 300 K. Above 100 K the compound is paramagnetic. Magnetic susceptibility data indicate a transition to antiferromagnetism around 100 K, but only vague indications for additional magnetic reflections were found with neutron powder diffraction.

Thermal transformation of zeolite Li-A(BW): the crystal structure of γ-eucryptite, a polymorph of LiAlSiO4

Abstract Among the thermal transformation products of zeolite Li-A(BW), LiAlSiO4·H2O, a number of polymorphs with composition LiAlSiO4 are observed. The transformation does not involve formation of intermediate amorphous phases. Reversible dehydration of the zeolite is possible to a limited extent; thereafter, the structure collapses into an anhydrous phase. Rehydration to zeolite Li-A(BW) is only possible using prolonged hydrothermal treatment. A structural model for the anhydrous phase related to the ABW type is proposed. At ∼ 650°C γ-eucryptite is formed, and its structure is monoclinic with a = 8.228(2), b = 5.032(1), c = 8.274(1) A , and β = 107.46(1)° , space group Pa. Its crystal structure was determined and refined using combined information from MAS n.m.r. spectroscopy and X-ray and neutron powder diffraction. γ-Eucryptite has a stuffed cristobalite-type structure with the lithium ions having distorted tetrahedral coordination. A mechanism for the transformation from the ABW structure to the cristobalite type is proposed. At 900–1000°C γ-eucryptite transforms into the final high-temperature polymorph β-eucryptite.

Phase Transition of KNO3 Monitored by Synchrotron X-ray Powder Diffraction

The solid-state phase transitions of KNO 3 were studied at atmospheric pressure in the temperature range 303 to 533 K by synchrotron X-ray powder diffraction. The detectors used were (i) a curved position-sensitive detector and (ii) a moving imaging-plate system built for time-, temperature- and wavelength-dependent powder diffraction. On heating, the transition from α-KNO 3 to β-KNO 3 occurs at 401 K. On cooling with a cooling rate of 7 K min -1 , the transition from β-KNO 3 to γ-KNO 3 was observed at 388 K. The phase transition from γ-KNO 3 to α-KNO 3 occurred at temperatures that strongly depended upon the cooling rate. With a high cooling rate of 15 K min -1 from 403 to 303 K, the γ-KNO 3 phase was obtained as a pure phase at 303 K, but it was eventually transformed to α-KNO 3 at this temperature, and the phase transition at 303 K was complete within 15 min. With a slow cooling rate of 0.5 K min -1 from 403 to 303 K, the γ-KNO 3 phase was formed at 391 K and transformed at 370 K to α-KNO 3 . With a cooling rate of 7 K min -1 from 403 to 303 K, the γ-KNO 3 phase transformed to α-KNO 3 in a temperature range between 377 and 353 K. The two phases could exist simultaneously in temperature ranges that were apparently dependent upon the thermal history of the sample. The unit-cell parameters of γ-KNO 3 from 383 K to room temperature are reported.

Protonic titanate derived from CsxTi2−x/2Mgx/2O4 (x = 0.7) with lepidocrocite-type layered structure

A layered titanate CsxTi2−x/2Mgx/2O4 (x = 0.7) with lepidocrocite (γ-FeOOH)-type layered structure was prepared via solid-state reaction. Extraction of both Mg2+ ions in the host layers and interlayer Cs+ ions was achieved during an acid-exchange process, producing a new protonic titanate HxTi2−x/2O4−x/2·H2O. This phase was distinguished from isomorphous related compounds in terms of removable lattice Mg and O atoms. The protonic titanate HxTi2−x/2O4−x/2·H2O showed excellent exfoliation/delamination reactivity upon intercalating organic amine ions as well as the ability to produce single two-dimensional titanate nanosheets with small thickness of about 1 nm. These findings offered a new clue for understanding the physicochemical properties of lattice dopants in lepidocrocite titanates.

Synthesis and structure of lithium cesium and lithium thallium cancrinites

X-ray powder diffraction studies of synthetic cancrinites were undertaken to elucidate the role of lithium ions and large cations (Cs, Tl) in zeolite crystallizations. Li 4.56 Cs 1.50 Al 6 Si 6 O 24 , 4.9 H 2 O (a = b = 12.4328 (12) A, c = 4.9692 (6) A, hexagonal, P6 3 , Z = 1). The structure was refined by the Rietveld diffraction-profile refinement technique. The cesium ions — located in the cancrinite cage only — are coordinated to 12 oxygen atoms (at distances 3.15–3.61 A). In accordance with their position, they are not exchangeable. The lithium ions are four coordinated to oxygen atoms (at distances 1.91 to 2.03 A). Li 2.75 Tl 3.23 Al 5.85 Si 6.13 O 24 , 2.0 H 2 O (a = b = 12.4419 (7) A, c = 4.9884 (4) A, hexagonal, P6 3 , Z = 1. The thallium ions are located on more than one position in the cancrinite cage, and there is also thallium on one position in the channels. This is in accordance with the thallium ions being partially exchangeable in this material. The structures are described and the action of small and large ion radius cations in cancrinite crystallization is discussed.