X-ray Diffraction Methods Subcommittee Agenda and Minutes

X-ray Diffraction Methods Subcommittee
22 March 2006
ICDD Headquarters 
Chairman - D. Balzar

1. The minutes from the March 2005 were approved.

2. Board of Directors' Liaison Report – Paolo Scardi gave a report from the Board on the motions passed by this subcommittee in March 2005:

1. Antipov moved that a 2-D Detectors Task Group (led by Bob He and Kris Kozaczek) be appointed and charged with asses si ng how much collapsed diffraction patterns are affected by corrections during a conversion from 2 dimensional to 1 dimensional data set. Board acknowledged motion.

2. Antipov moved that the 2-D Detectors Task Group be charged with studying the effects of defocusing on 2 dimensional diffraction patterns. Motion was deferred.

Comments from the Executive Director – Feedback from members and OEM's leave little doubt that the usage of 2D detectors is increa si ng. The staff is excited that a task team has been formed to look at issues surrounding these detectors . Davor Balzar was active at both IUCr and DXC in trying to find both members and non-member experts from the OEM's to help with this task.

We have several editorial needs this task group could address. The first is how to correctly process the 2D data so that it can be matched to the 1D data in the database. However, we would need guidelines for taking the 1D data and doing a 2D si mulation. Finally, it would be nice to develop guidelines that a potential grantee could use to generate 2D data and make it useful as a reference material in the database.

3. Antipov moved that the 2-D Detectors Task Group be charged with studying the effects of defocusing on 2 dimensional diffraction patterns. Motion was deferred.

The discussion that followed emphasized that the Task Group will formulate recommendations and publish algorithms used for data correction in order to make it possible to invert the procedure that is currently transparent to the users of 2D detectors. This will make it possible to calculate 2D patterns from crystallographic information.

Report on Motions 1 & 2:

The Task Group has decided to use NIST SRM1976 alumina plate and run a round robin.

Members: Bob He (Bruker) and Kris Kozaczek (co-Chairs), Martijn Fransen and Jeffrey Nicolich (PANalytical) and Davor Balzar. Participation of Rigaku was solicited twice without success.

It is best to work with certified standards to help assess errors. James Cline will provide samples (new SRM1976a and SRM676a) when available.

Report on Motion 3– Raw Data File Format – Brian Toby

Brian's discussion outlined the importance of having a standard data format and why CIF is the only real choice. CIF files are being published with raw diffraction data, coordinate and unit cell and reflection lists (usually not peak) – all useful to ICDD. Efforts to pair XML and CIF are underway. See Brian's presentation for details.

ICDD should make a contribution to the initial step in preparing a CIF file. That is, raw data collected with an X-ray diffractometer should be able to be exported in a format compatible with CIF. In order to do this, instrument vendors help is required. ICDD does get CIF files and they are not very well documented. The vendors are the right resource to be the bearers of this information.

Action item:

The formerly established Raw Data File Format Task Group, chaired by Peter Zavalij, should specify the minimum information that a pdCIF file should contain concerning conditions of raw data collection. This should then be recommended to the X-ray diffraction equipment vendors.

It was suggested that ICDD consult with the journal office of IUCr and ask that certain information be required, as a part of pdCIF. IUCr will then make that part of the CIF template

Report on the Size/Strain Round Robin – Davor Balzar

Balzar concluded that including size-effect modeling in PDF-4+ is welcome and well-intended effort to describe line broadening in terms of physically recognizable parameters. However, it is recommended that the modeling also includes strain-broadening effects and that an assumed gamma size distribution should preferably be replaced by the lognormal size distribution. Scardi commented that PDF-4+ is used for simulation and not for fitting of real data and thus the present model should still be useful for nanomaterials. Balzar pointed that many nanomaterials include significant strain and that the model still could be misleading. Further details can be found in Davor's presentation.