ICDD Technical Committee Minutes

Electron Diffraction Subcommittee Minutes

Meeting: Portland, Oregon August 5, 1999
During the annual meeting: Microscopy and Microanalysis 1999

1. Introduction and review of the minutes of the meeting in April.

2. Task Group for the enhancement of the involvement of electron diffractionists with the work of the ICDD. Review of mission statement and invitation to participate.

3. Database needs for the electron diffraction community.

12 people were in attendance, a attendance sign-up sheet is on file at ICDD Headquarters.

1. Alwyn Eades described the ICDD and its work for the benefit of those who were not familiar with it. The work of the Electron Diffraction Sub-Committee was described. The plan to operate a task force to try and bring electron diffractionists closer to the ICDD was also outlined.

2. There was substantial discussion of the proposal to hold a joint meeting of MAS and the Denver X-ray Conference™. The problems appear to be substantial even if they are largely problems of inertia. MAS is committed to hold joint meetings with MSA for several years to come and the sites of MSA meetings are fixed for five or six years out. MAS holds meetings independently of MSA only rarely. There would be no reason, in principle, why the DXC should not be held in conjunction with a joint MSA/MAS meeting – except that this would probably mean moving the Denver meeting outside Colorado. The view of those present with some familiarity of the situation was that this is not likely to be accepted. The idea of a joint meeting was attractive to those at the meeting but there seemed no expectation that there would be a way through the fog.

3. There was a strong reiteration of a number if issues regarding the PDF (and other databases) which have been expressed by the Subcommittee at previous meetings. The issues were raised by attendees, new to the Subcommittee. This implies that these issues are thought to be important vary widely in the electron diffraction community, not just to the regular members.

The issues raised include:

  • The need to provide a way to identify a "standard" pattern from the set of entries corresponding to the same structure. Electron diffraction is (in most cases) not so precise that there is any point in seeing entries which are duplicated (because of the inclusion of outdated entries, or because of such small differences as temperature or pressure changes, or data from minerals from different sources).
  • The need to provide a means to give degenerate intensities separately. The canonical example is the coincidence (for a cubic crystal in powder patterns) of the 511 and 333 reflections.
  • It would be a great help to most of us, if there were a single database which included both atomic coordinate information and structure factor/intensity/d-spacing data.

4. A new topic was also raised in this context. There are structures where the atomic positions are not known and where the x-ray powder intensities are not known. Nonetheless, the unit cell is known and the phase therefore has a distinct identity. As far as those at the meeting were aware, such phases would not be included in any of the present databases. But they should be.

Alwyn Eades - jae5@lehigh.edu