Jens-Erik Jørgensen

Versatile in situ powder X-ray diffraction cells for solid–gas investigations

Two multipurpose sample cells of quartz (SiO2) or sapphire (Al2O3) capillaries, developed for the study of solid–gas reactions in dosing or flow mode, are presented. They allow fast change of pressure up to 100 or 300 bar (1 bar = 100 000 Pa) and can also handle solid–liquid–gas studies.

Structures and Phase Transitions of Na2SO4

Sodium sulfate, Na2SO4, has been reported to exist in five polymorphous forms labelled I–V. Structure analysis has been reported on three of the polymorphs and thermal analysis on all of them. The system has been reinvestigated by differential scanning calorimetry (DSC) and by X-ray powder diffraction at several temperatures in order to examine the Na2SO4 system with the same batch of material in the same laboratory by two independent methods. The X-ray results confirm the existence of the crystalline phases labelled I, II, III and V but not IV. The DSC results do not entirely rule out the existence of five phases. The structure of phase II has been determined and the structures of all phases have been refined by the Rietveld method.

Sol-gel synthesis and characterization of barium titanate powders

A simple aqueous sol-gel route has been refined to prepare yttrium-doped barium titanate powders. Thermal decomposition of the fabricated gel was studied by means of DTA/TGA measurements. The precursor gel was intermediate calcined at 600 °C, and the final synthesis products were characterized by IR spectroscopy, X-ray powder diffraction and scanning electron microscopy. The barium titanate obtained from the calcination of the gel at 1300 °C was a tetragonal phase, and showed a positive temperature coefficient of resistivity (PTCR) effect which depends on sample cooling rate during the final heat treatment.

In situ X‐ray diffraction environments for high‐pressure reactions

New sample environments and techniques specifically designed for in situ powder X-ray diffraction studies up to 1000 bar (1 bar = 105 Pa) gas pressure are reported and discussed. The cells can be utilized for multiple purposes in a range of research fields. Specifically, investigations of gas–solid reactions and sample handling under inert conditions are undertaken here. Sample containers allowing the introduction of gas from one or both ends are considered, enabling the possibility of flow-through studies. Various containment materials are evaluated, e.g. capillaries of single-crystal sapphire (Al2O3), quartz glass (SiO2), stainless steel (S316) and glassy carbon (Sigradur K), and burst pressures are calculated and tested for the different tube materials. In these studies, high hydrogen pressure is generated with a metal hydride hydrogen compressor mounted in a closed system, which allows reuse of the hydrogen gas. The advantages and design considerations of the in situ cells are discussed and their usage is illustrated by a case study.

Phase transitions in ReO3 studied by high-pressure X-ray diffraction

ReO3 has been studied at pressures up to 52 GPa by X-ray powder diffraction. The previously observed cubic Im3¯ high-pressure phase was shown to transform to a monoclinic MnF3-related phase at about 3 GPa. All patterns recorded above 12 GPa could be indexed on rhombohedral cells. The compressibility was observed to decrease abruptly at 38 GPa. It is therefore proposed that the oxygen ions are hexagonally close packed above this pressure, giving rise to two rhombohedral phases labelled I and II. The zero-pressure bulk moduli Bo of the observed phases were determined and the rhombohedral phase II was found to have an extremely large value of 617 (10) GPa. It was found that ReO3 transforms back to the Pm3¯m phase found at ambient pressure.

High-pressure X-ray diffraction study of bulk- and nanocrystalline GaN

Bulk- and nanocrystalline GaN have been studied by high-pressure energy-dispersive X-ray diffraction. Pressure-induced structural phase transitions from the wurtzite to the NaCl phase were observed in both materials. The transition pressure was found to be 40 GPa for the bulk-crystalline GaN, while the wurtzite phase was retained up to 60 GPa in the case of nanocrystalline GaN. The bulk moduli for the wurtzite phases were determined to be 187 (7) and 319 (10) GPa for the bulk- and nanocrystalline phases, respectively, while the respective NaCl phases were found to have very similar bulk moduli [208 (28) and 206 (44) GPa].

High-pressure neutron powder diffraction study of the Im phase of ReO3

The structure of ReO 3 has been studied over the pressure range from 1.27 to 8.01 GPa by neutron powder time-of-flight diffraction as well as by X-ray diffraction up to 41 GPa using Fluorinert as the pressure-transmitting medium. The cubic Im3 phase was found to be stable in the pressure range 0.5 to 13.2 GPa and to be highly compressible with a zero-pressure bulk modulus of B o = 43 (1) GPa. The neutron data were used in least-squares refinements, showing that the ReO 6 octahedra remain almost undistorted, while the Re-O-Re bond angle decreases from 166.5 (1) to 146.4 (3)° within the investigated pressure range of 1.27 to 8.01 GPa. The compression mechanism is also described in terms of sphere-packing models of the oxygen anions.

Diffusion of Cu+ in β-phase CuI

Abstract Measurements of ionic diffusion of Cu + in solid CuI in the β-phase is carried out with a non-destructive radioactive tracer technique, utilizing coincidence counting of the annihilation gammas from the positron decay of 64 Cu. The diffusion coefficient and the activation energy for diffusion are evaluated. The experimental results show distinct diffusion character in the β-phase which differs from those in the γ-and α-phases. The β-phase diffusion properties together with the previous results for γ-and α-phases will provide valuable guidance for MD calculations, in which the diffusion coefficients and activation energies have been overestimated and the γ-β phase transition does not appear. The ionic conductivity of CuI estimated from tracer diffusion results and the Nernst-Einstein relation are compared with values from electrochemical methods. In all three phases the conductivities obtained from electrochemical methods are much lower than those calculated from the measured tracer diffusion coefficients.

Effect of strontium substitution on superconductivity in YBa 2 Cu 4 O 8

Superconducting powders with the composition Y(Ba 1 mx Sr x ) 2 Cu 4 O 8 ( x = 0.00-0.50) have been prepared by an aqueous sol-gel method. The effects of strontium substitution on the properties of compounds were studied by resistivity measurements, X-ray powder diffraction, thermogravimetric analysis, electron microscopy and elemental analysis. The data indicate that single-phase Y(Ba 1 mx Sr x ) 2 Cu 4 O 8 superconducting samples were obtained. They also show that doping with strontium has a strong effect on the superconducting properties of the YBa 2 Cu 4 O 8 phase. The critical temperature is enhanced from 78 K (for a non-substituted sample) to 88 K (for Y(Ba 1 mx Sr x ) 2 Cu 4 O 8 ). Elemental analysis data clearly indicate that small amounts of strontium enter the copper sites in the Y(Ba 1 mx Sr x ) 2 Cu 4 O 8 compounds. A point-defect chemistry approach, which explains the enhancement of T c by substituting strontium for barium, is presented.

Tracer diffusion study of Br- mobility in α-CuBr

Abstract The tracer diffusion method is used to observe the anion mobility in CuBr, well known to show large mobility for the cations. The anion Br - is found to diffuse with a diffusion coefficient of 1×10 −7 cm 2 /s in the bcc α-phase, comparable to that of Cu + in the γ-phase copper halides. This is several order of magnitudes higher than the ionic conductivity of ordinary ionic solids and represents the first quantitative information on the diffusion of the relatively immobile anion specie.