Crystal structure refinement at high pressures using angle-dispersive powder diffraction techniques

Abstract

The development of area detectors such as the imaging plate (IP) allows the use of angle-dispersive techniques for powder diffraction studies at high pressure. Integration of the 2-d pattern greatly improves the powder averaging and signal-to-noise ratio, making it possible to carry out full Rietveld refinements of crystal structures in high-pressure phases. An IP system is being developed at the Synchrotron Radiation Source (SRS), Daresbury, and the current status of this system -hardware and software -is described. Recent results on La₂CuO₄ and lnSb demonstrate the effectiveness and advantages of the techniques.     

Developments of nano-polycrystalline diamond anvil cells for neutron diffraction experiments

ABSTRACT

A new high pressure cell for neutron diffraction experiments using nano-polycrystalline anvils is presented. The cell design, off-line pressure generation tests and a gas-loading procedure for this cell are described. The performance is illustrated by powder neutron diffraction patterns of ice VII to ～82?GPa. We also demonstrate the feasibility of single crystal neutron diffraction experiments of Fe₃O₄ at ambient conditions using this cell and discuss the current limitation and future developments.     

High pressure cell for neutron diffraction investigations

Abstract

The design of a titanium-zirconium clamped cylinder-piston type pressure cell for neutron diffraction investigations under hydrostatic pressure up to 10 kbars without supports is described. It is the first time that Freon-11 has been used as a hydrostatic pressure transmitting medium. The following results carried out at the room temperature are presented: the discovery of the transition from I to 111 phase in a LiKSO₄ single crystal and the results of the investigation on the influence of pressure upon the structure of the YBaCuO HTSC obtained by means of the powder diffraction method.     

High pressure neutron diffraction using the paris-edinburgh cell: Experimental possibilities and future prospects

Abstract

High pressure neutron diffraction using the Paris-Edinburgh cell has attracted considerable interest ever since it has been shown that full structural data can be obtained at pressures up to 10 GPa. In this paper we will focus on the current state of this technique. Specifically, we report on new experimental possibilities concerning: i) access to “ultrahigh” pressures beyond 20 GPa, ii) experiments at variable temperatures down to 100 K, and iii) experiments on single crystals in inelastic neutron scattering. Current attempts to increase the pressure and temperature range are discussed.     

Diffraction study of actinides under pressure

Abstract

Uranium and thorium have sufficiently low radioactive dose rates to allow their study at synchrotrons and neutron facilities. Correspondingly, numerous compounds of these two actinides have been studied under pressure by synchrotron x-ray diffraction. The maximum pressures reached were on the order of 60-80 GPa, and 300 GPa in one case.

The situation is much more difficult for all other actinides. Their high level of radioactivity has up to now prevented their study at synchrotrons, except in a few special cases. In contrast, all actinide metals available in sufficient quantities, and a large number of compounds of highly radioactive actinides, have been studied in highpressure laboratory facilities.

Recent examples of in situ high pressure x-ray diffraction work will be described.     

Two-dimensional detector software: From real detector to idealised image or two-theta scan

Abstract

Detector systems introduce distortions into acquired data. To obtain accurate angle and intensity information, it is necessary to calibrate, and apply corrections. Intensity non-linearity, spatial distortion, and non-uniformity of intensity response, are the primary considerations.

It is better to account for the distortions within scientific analysis software, but often it is more practical to correct the distortions to produce ‘idealised’ data.

Calibration methods and software have been developed for single crystal diffraction experiments, using both approaches. For powder diffraction experiments the additional task of converting a two-dimensional image to a one-dimensional spectrum is used to allow Rietveld analysis. This task may be combined with distortion correction to produce intensity information and error estimates.

High-pressure experiments can introduce additional complications and place new demands on software. Flexibility is needed to be able to integrate different angular regions separately, and to produce profiles as a function of angle of azimuth. Methods to cope with awkward data are described, and examples of the techniques applied to data from high pressure experiments are presented.     

The kiel/berlin pressure cells for neutron powder diffraction

Abstract

Two piston/cylinder type pressure cells for neutron powder diffraction are presented. They provide a large sample volume and hence allow rapid data collection at moderate flux neutron sources. Structure refinements from the diffraction data are possible. The maximum attainable pressure is above 2 GPa for Ti/Zr zero-scattering pressure cylinders. Both cells may be equipped with a micro-furnace. This allows measurement up to 700 K, simultaneously to the application of pressure. The low temperature setup for the cell-I is presented which will allow experiments down to 1·5K.     

Rietveld analysis of high pressure powder diffraction data

Abstract

X-ray and neutron powder diffraction data obtained from samples contained within high pressure cells are generally of lower quality than data collected from samples at ambient conditions. The far smaller sample size as well as possible contamination of the pattern by the pressure cell means that Rietveld refinement techniques must be adapted to extract the maximum useful information from the data. These problems become paramount as larger structures at high pressure are attempted. Techniques such as “leBail extraction”, “soft restraints” and “rigid body refinement” will be discussed with application to analysis of high pressure neutron powder diffraction data.     

Structural investigation of the zincblende-β tin transition for gasb by in situ X-ray absorption spectroscopy

Abstract

The transition to the β tin structure has been observed by X-ray absorption spectroscopy for GaSb at 7.9 GPa. In the low pressure phase, the bulk modulus has been determined. In the high pressure phase, the results for the Ga-Sb and Ga-Ga distances are in good agreement with previous x-ray diffraction data. The pseudo Debye-Waller factor associated to the Ga-Sb distance is abnormally high, indicating that disorder exists for this bond.     

Equations of state of MgO at high pressure and temperature

Abstract

A synchrotron X-ray diffraction study on MgO has been done at simultaneous high pressure and temperature. The lattice parameter of MgO has been measured up to a static pressure of 6 GPa and a temperature of 1273 K, using a large volume pressure cell and energy-dispersive synchrotron X-ray powder diffraction. The compression was made following six high-temperature isotherms. A Vinet equation of state was used to fit the experimental P-V-T data. The Vinet's model compares very well with the experimental data above the Debye temperature (760 K) and allows the use of MgO as an alternative internal pressure calibrant for experiments at high temperature.     

EDXD studies above 400 GPa (and prospects for obtaining pressures near 1 TPa and doing EDXD studies at such pressures)

Abstract

We have now taken two materials (W, Mo) to pressures greater than the pressure at the center of the earth for the first time, with all pressures based on x-ray diffraction and shock-based equations of state. The present paper describes x-ray diffraction studies of a sample in which pressure equals or exceeds 438 GPa. It is noted that as a result of the process of pressure strengthening, pressures of 1 TPa may be attainable.

Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumenration for HighPressure Crystallography’. Daresbury Laboratory 20-21 July 1991     

Rietveld analysis for energy dispersive x-ray diffraction under high pressure with synchrotron radiation

Abstract

A new program has been developed for the conversion of energy-dispersive x-ray diffraction spectra obtained from powder samples at high pressure in a diamond anvil cell (DAC) into conventional pseudo angle-dispersive data. The program is compatible with a conventional Rietveld program. This allows the determination of the structural parameters of the samples investigated. Results of a synchrotron radiation study of polycrystalline SrFCl in the tetragonal phase at high pressure are presented.

Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for High Pressure Crystallography’, Daresbury Laboratory 20-21 July 1991     

High pressure apparatus for neutron diffraction investigation of strongly compressible substances (H2, D2, Ar)

Abstract

The high pressure apparatus (HPA) up to 25kbar from TiZr alloy is presented. The short review of materials used for constructing the HPA for the neutron diffraction is given. It is shown that D₂ may serve as the exellent calibrant for the quasihidrostatic pressure at low temperatures.     

Experience in the design of free-cylinder pressure transducers to about 1 GPa

Abstract

Free-cylinder pressure transducers for pressure measurements to about 1.0 GPa have been developed; they use at present foil strain gauges bonded on the cylindrical surface in order to measure strains as a function of the applied hydrostatic pressure. All such transducers can be used with thin-or thick-film gauges; distortions of the active cylinder can be measured also by optical techniques. Pressure transducers of three types (different design as to pressure full scale and cylinder materials) are described and their metrological characteristics are given as well.     

Neutron diffraction studies of hydrogen-bonded ices at high pressure

Abstract

The development of the Paris-Edinburgh cell has extended the pressure range for neutron diffraction by a full order of magnitude to 30 GPa. This advance has provided a wealth of new information on the high-pressure behaviour of ices such as water-ice, ammonia, methane and hydrogen sulphide. We have now put in place technique developments to permit loading of water-gas mixtures into the Paris-Edinburgh cell.     

High pressure,high resolution synchrotron x-ray powder diffraction with a position-sensitive detector

Abstract

We report results of high-pressure experiments with a new diamond-anvil cell in a monochromatic, high-resolution x-ray scattering geometry with alinear position-sensitive detector. The experiments make possible the study of factors controlling line widths of diffraction profiles at pressures in the 100 GPa range, and demonstrate the potential for the use of line profile analysis and Rietveld refinement techniques with high-pressure powder diffraction data. Combined data for various materials indicate that relative contributions to linewidths due to particle size, intrinsic material strength, pressure and state of stress in the sample can be resolved. With light rare-gas solids as pressure-transmitting media, measured FWHMs of the order 0.03? 2 θ corresponding to resolution Δd/d of 2.5 × 10^?3 for 2θ～10-15? are reported. Formation of a high pressure phase appears to involve growth of submicron domains, judging from substantially broadened diffraction peaks under quasihydrostatic conditions. Detailed analysis of complex, non-quenchable high-pressure phases will likely require annealing techniques such as thermal cycling at pressure.

Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for High Pressure Crystallography’, Daresbury Laboratory 20-21 July 1991     

Single crystal diffraction with X-rays and neutrons: High quality at high pressure?

Abstract

Single crystal X-ray and neutron diffraction is essential for determining occupancies, positional as well as static and dynamic displacement parameters in crystalline matter by measuring Bragg, satellite or diffuse reflections. In our contribution a new low-temperature high pressure cell for neutron single crystal diffraction will be presented. It is designed to operate from a few K to ambient temperature in “orange” cryostats at pressures up to at least 3 GPa. We will present first neutron diffraction results obtained at E4/HMI Berlin and discuss the quality of the data and the significance of the results. A software package has been written (as a part of the PROMETHEUS system), which deals with the data reduction for both X-ray and neutron high pressure cell single crystal data. Likewise a data collection program has been developed for single crystal data collection on four-circle diffractometers using Merrill-Bassett cells. A series of single crystal experiments on H₂O and D₂O ice VI and KDP (KH₂PO₄) show that results of very high quality can be obtained routinely including even higher order terms in the atomic displacement parameters.     

Structural instability of PbSe/SnSe superlattices under pressure

Abstract

PbSe/SnSe superlattice, phase transition, high pressure, SR x-ray diffraction)

Synchrotron x-ray diffraction experiments have revealed successive phase transitions in epitaxially-grown PbSe/SnSe superlattices. The transition pressures from the low-pressre cubic B1- to the high-pressure orthorhombic B16-type structures are observed to vary systematically depending upon thickness of the PbSe layer. For example, a [PbSe(36A)/SnSe(12A)]₁₉, with the B1 structure in both layers stabilized in its asgrown state, undergoes the [B1/B1]-to-[B1/B16] and [B1/B16]-to-[B16/B16] structural transitions at 1.9 and 3.8GPa, respectively. This result is in contrast to their bulk data that the B1-to-B16 transition takes place at 5.3GPa in PbSe while the B16 phase is stable in SnSe at atmospheric pressure.     

Pressure induced spin-reorientation transition in FeBO3

Abstract

It was found out by means of neutron diffraction that “easy plain - easy axis” spin-reorientation transition takes place in FeBO₃ under quasihydrostatic pressure of approximately 17 kbar at room temperature. This is a pressure analog of the Morin transition in hematite.     

High Pressure X-Ray Absorption and Diffraction Study of InAs

We have performed X-ray absorption (XAS) and diffraction (XRD) measurements at high pressure on samples of powdered InAs, up to 50 and 80 GPa, respectively. In the lower pressure range, our data are consistent with the following structural sequence: Zincblende M NaCl M Cmcm . The first order transition from the semiconducting Zincblende phase to the metallic NaCl phase is clearly seen by the shift in the absorption onset at the As K-edge and the strong modifications of the extended X-ray absorption fine structure (EXAFS) due to the changes in the local structure from a 4-fold to a 6-fold coordinated environment. XAS shows the high pressure phase to be locally site-ordered. The diffraction data, analized by Rietveld fitting, gives a volume discontinuity of j V/V 0 ～0.18 for the first order transition. There is no apparent volume discontinuity associated to the NaCl M Cmcm transition.