Structure determination at megabar pressureStructure determination at megabar pressure

New possibilities for high-pressure single-crystal XRDNew possibilities for high-pressure single-crystal XRD

Development project and COMPRES sponsored workshopDevelopment project and COMPRES sponsored workshop

Przemek Dera, Robert Downs Ho-kwang Mao, Charles Prewitt

Geophysical Laboratory, Carnegie Institution of Washington andUniversity of Arizona, Tucson

COMPRES Annual Meeting, Lake Tahoe June 2004

Problems with Powder Diffraction DataProblems with Powder Diffraction Data

•Diffraction from pressure mediumDiffraction from pressure medium

•Preferred orientationPreferred orientation

•Grain GrowthGrain Growth

•Peak overlapPeak overlap

•Peak broadeningPeak broadening

•Indexing may be difficultIndexing may be difficult

•Structure solution Structure solution

•Multiple phasesMultiple phases

•Diamond, gasket peaksDiamond, gasket peaks

Most of these problems may be overcome through the Most of these problems may be overcome through the use of a single-crystal approachuse of a single-crystal approach

Dedicated Software Dedicated Software DevelopmentDevelopment

• Information about the absolute values of unit cell parameters is lost, • Peak intensity corrections are complicated.

Advantages of the LAMSA approach

Disadvantages of the LAMSA approach

• With the use of transverse-geometry DAC, a much larger portion of reciprocal space can be explored.• The incident intensity is much higher than in monochromatic experiments.• The data collection time for a full dataset is much shorter (<<1 s).• The sample does not have to be rotated during accumulation of the image, which assures constancy of the illuminated spot.• Images corresponding to different orientations can be collected, and merged together.• Extremely small beam size (0.6 microns at UNICAT, APS ) permits sampling of very small grains w/o recording the pattern of the surrounding material.• LAMSA is so far the only promising method for future crystallography with the application of the free electron laser FEL.

Laue Microdiffraction for Structure Analysis Laue Microdiffraction for Structure Analysis at Ultra High Pressureat Ultra High Pressure

Experimental solutions for the principal LAMSA problem:Experimental solutions for the principal LAMSA problem: Retrieving peak energy informationRetrieving peak energy information

• Monochromatic energy scan (ALS 7.3.3) • gives precise energies• requires sophisticated and expensive x-ray optics • time-consuming• Monochromatic data collection with greatly reduced flux (X17C 50 times)

• Channel-cut filter scan (Gene Ice, UNICAT, APS)• Inverse logic (search for peak disappearance rather, than appearance)• Inexpensive optics• Time consuming• Flux similar to WB• Presence of multiple harmonics prevents peaks from disappearing completely

• Energy-dispersive detector (NSLS X17C)• Sample has to be reoriented for each peak• With Laue pattern collected first gives very quick answers• Covers whole energy range in one accumulation • Inexpensive

• Set of energy cutoff attenuators (APS BioCARS)• Inexpensive• Short time• Not very precise

Applications of LAMSA methodApplications of LAMSA method

• Single-crystal studies beyond a megabar,

• Non-hydrostatic single-crystal studies,

• Single-crystal experiments with laser heating,

• Rheological single-crystal studies,

• Reconstructive phase transitions,

• Time-resolved studies,

• Structure determination of micrograins in synthetic samples.

LAMSA UHP facilitiesLAMSA UHP facilities

ALS, beamline 7.3.3

NSLS, beamline X17C

APS, UNICAT

APS, HPCAT (in development)

APS, GSECARS (in development)

There have been dozens of papers published discussing the crystal structure of Fe2O3 II above about 50 GPa, but no definitive Rietveld refinement has been published that confirms whether it has the orthorhombic perovskite or the Rh2O3 II structure. Furthermore, if Shannon and Prewitt (1970) had not determined the structure of Rh2O3 II using a single crystal, everyone would think Fe2O3 II hasthe orthorhombic perovskite structure.

Crystal Structure of Fe2O3 II

Liu et al. (2003)

Fe2O3 at 22 GPa after laser heating,

Liu et al. (2003)

Orthorhombic?

a=7.305, b=7.850, c=12.877

Planned efforts:Planned efforts:

1. Through involvement of community experts, analyze existing technology and define the best route to successful megabar-pressure SXD experiments

2. Formulate and test optimized and unified methodology for a megabar pressure SXD experiment

3. Educate the community and popularize the idea of such experiments

COMPRES workshop on crystal structure determination COMPRES workshop on crystal structure determination at megabar pressureat megabar pressure

Planned date: November 2004Location: HPCAT, APS Chicago

Agenda:

• Create a list of most important megabar pressure phases with unsolved structures, to be targeted by SXD method

• Review and analyze strengths and weaknesses of existing techniques

• Define the most promising technique of the future

• Perform hands-on SXD experiments at HPCAT beamline with samples brought to the workshop

AcknowledgementsAcknowledgements

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Motivation – Why megabar pressure single-crystal XRDMotivation – Why megabar pressure single-crystal XRD

• Structural interpretation of high-pressure phenomena is a necessary key to understanding their nature, theoretical modeling, and predicting their occurrence in similar systems.

• There are numerous phase transitions in geologically important minerals, identified using powder diffraction, or with spectroscopic methods, for which there are no models of high-pressure phases.

• Powder diffraction HP experiments are becoming routine to perform, even in the megabar pressure range. There is little, besides the size of the beam and incident intensity, that can be improved significantly.

• The detailed structural information is very hard to retrieve from powder diffraction data, due to 1-dimensional character of the data and peak overlapping.

• There is a lot of recent development in structure solution from powder diffraction, but the physical limitations of the method (peak overlap) are not likely to be completely overcome.

• Single-crystal experiment carries much more easily interpretable structural information, but with the standard approach is limited to ~20 GPa.