Axel Nørlund Christensen

In situ studies of zeolite syntheses using powder diffraction methods. Crystallization of “instant zeolite A” powder and synthesis of CoAPO-5.

A series of hydrothermal zeolite synthesis were performed on a powder diffractometer using synchrotron radiation and a position sensitive detector. Direct observation of the induction period(nucleation stage), crystallization and transformation of zeolite 4A(Na-LTA) was possible due to the intense X-ray beam which allows fast data collection. High pressure experiments were performed, allowing observation of hydrothermal synthesis of a cobalt substituted aluminophosphate molecular sieve, CoAPO-5, up to 165°C. The temperature dependence of crystallization rates of CoAPO-5 was studied. At 135°C the crystallization of CoAPO-5 is completed within a few minutes. Rate expressions were fitted to the crystallization curves. A narrowing of the widths of the diffraction peaks was observed during crystallization, signifying crystal growth. A simplified model for the dependence of half width with time was derrived. This is to our knowledge the first time resolved powder diffraction studies of zeolite syntheses using angle dispersive synchrotron powder diffraction.

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.

Real time study of cement and clinker phases hydration

Real time studies of hydration of a Portland cement and of calcium silicate extracted clinker were made. X-Ray diffraction measurements were made using either an INEL powder diffractometer (Zr-filtered Mo-Kα radiation, CPS120 position sensitive detector) or a MAR diffractometer (synchrotron X-radiation, wavelength 0.90371 A). The dry samples were mounted in glass capillaries and hydrated in-situ by the application of internal nitrogen gas pressure of up to about 150 kPa which pressed the water into contact with the dry sample. This allowed the study of early phase transitions in the reaction mixture. The experiments were conducted over the temperature range 25–100 °C. The laboratory based X-ray experimentation provided similar results to those obtained using the synchrotron radiation, although the counting statistics in the synchrotron data were far superior. The use of constant wavelength synchrotron radiation gave well-resolved lines of powder diffraction data over the angular range investigated. This is in contrast to the less well-resolved reflections previously observed in the energy dispersive diffraction mode. The hydration of the Portland cement showed the immediate formation of ettringite (3CaO·Al2O3·3CaSO4·32H2O) upon the addition of water. Two silicate extracted clinker residues were considered: one being predominantly cubic C3A and the other comprising a mixture of orthorhombic C3A and brownmillerite (Ca4Al2Fe2O10, C4AF). The reaction paths of these two residues were different. The hydration of cubic C3A proceeded via C4AH19 to the end product, C3AH6, Ca3Al2(OH)12. The reaction of orthorhombic C3A, C4AF and water proceeded via C2AH8 to the end product C3AH6.

Structure of calcium aluminate decahydrate (CaAl2O4.10D2O) from neutron and X-ray powder diffraction data.

Calcium aluminate decahydrate is hexagonal with the space group P6(3)/m and Z = 6. The compound has been named CaAl(2)O(4).10H(2)O (CAH(10)) for decades and is known as the product obtained by hydration of CaAl(2)O(4) (CA) in the temperature region 273-288 K - one of the main components in high-alumina cements. The lattice constants depend on the water content. Several sample preparations were used in this investigation: one CAH(10), three CAD(10) and one CA(D/H)(10), where the latter is a zero-matrix sample showing no coherent scattering contribution from the D/H atoms in a neutron diffraction powder pattern. The crystal structure including the positions of the H/D atoms was determined from analyses of four neutron diffraction powder patterns by means of the ab initio crystal structure determination program FOX and the FULLPROF crystal structure refinement program. Additionally, eight X-ray powder diffraction patterns (Cu Kalpha(1) and synchrotron X-rays) were used to establish phase purity. The analyses of these combined neutron and X-ray diffraction data clearly show that the previously published positions of the O atoms in the water molecules are in error. Thermogravimetric analysis of the CAD(10) sample preparation used for the neutron diffraction studies gave the composition CaAl(2)(OD)(8)(D(2)O)(2).2.42D(2)O. Neutron and X-ray powder diffraction data gave the structural formula CaAl(2)(OX)(8)(X(2)O)(2).gammaX(2)O (X = D, H and D/H), where the gamma values are sample dependent and lie between 2.3 and 3.3.

Synthesis and characterization of the barium oxalates BaC2O4·0.5H2O, α-BaC2O4 and β-BaC2O4

The synthesis of BaC2O4·0.5H2O and its thermal decomposition to α-BaC2O4 and β-BaC2O4 was investigated. BaC2O4·0.5H2O is precipitated at room temperature from aqueous solutions of barium chloride and ammonium oxalate. The deuterated compound BaC2O4·0.5D2O was made in analogy with D2O as the solvent. The compounds were characterized by X-ray and neutron diffraction analysis. Single-crystal X-ray diffraction of BaC2O4·0.5H2O measured at 120 K gave the triclinic cell a = 8.692 (1), b = 9.216 (1), c = 6.146 (1) A, α = 95.094 (3), β = 95.492 (3), γ = 64.500 (3)°, space group P\bar 1, Z = 4. Two independent Ba atoms are each coordinated to nine O atoms at distances from 2.73 (1) to 2.99 (1) A. One of the two oxalate ions deviates significantly from planarity. The water molecule does form weak hydrogen bonds. In situ X-ray powder diffraction was used to study the thermal decomposition of BaC2O4·0.5H2O and the formation of α-BaC2O4. The X-ray powder pattern of α-BaC2O4 measured at 473 K was indexed on a triclinic cell with a = 5.137 (3), b = 8.764 (6), c = 9.006 (4) A, α = 83.57 (4), β = 98.68 (5), γ = 99.53 (5)°, and the space group P\bar 1 with Z = 4.

Vacancy-induced states on VC0.80 (110) identified using angle-resolved photoelectron spectroscopy

Angle-resolved photoemission measurements have been performed on the (110) face of a VC0.80 single crystal using resonance radiation. A vacancy-induced peak is identified at about 1.7 eV below the Fermi energy. Comparison of the experimental peak dispersions with direct transitions between energy bands of VC1.0 is also made.

Core-level study of WSi2 (110)

Angle resolved core level studies of the Si 2p and W 4f levels have been carried out on the (110) surface of a WSi2 single crystal using synchrotron radiation. Surface shifted components have been revealed both in the Si 2p and W 4f spectra. Investigations were carried out at two different annealing temperatures. The results indicate Si enrichment at the surface, and a larger enrichment after the higher temperature anneals. The reactivity upon initial oxygen exposure was investigated. Strong Si oxidation was observed but chemically shifted W 4f components could also be detected.