STRUCTURAL CHANGES IN SINGLE CRYSTALS BY TIME-OF-FLIGHT NEUTRON LAUE DIFFRACTION C C Wilson, ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 OQX, UK.

The study of phase transitions can be carried out in two ways using neutron time-of-flight Laue diffraction. The first is the conventional manner in which full data sets are collected under each set of physical conditions, from which full refinements can yield the significant structural changes. The second exploits the ability of the time-of-flight Laue method to access a large volume of reciprocal space in a single measurement. The method assumes the transition can be characterised by the changes in a small subset of reflections, the aim being to reduce the amount of data collected to a single frame [1].

The possibilities for monitoring phase transitions in single crystals using both methods are considered. The possibilities for rapid collection of full data sets will be discussed, emphasising the flexibility of the time-of-flight Laue technique. For the single frame case, methods for choosing the optimal single frame to best reflect the structural change of interest will be examined, along with techniques for refinement in this case.

Examples discussed will include the temperature dependence of proton transfer in benzoic acid [2] and the hydrogen atom ordering in the mineral PbHAsO4 [3].

These measurements have shown that the time-of-flight Laue method yields real potential for `dynamic' measurements of single crystal materials using neutrons.

[1] C C Wilson (1995). J. Appl. Cryst., 28, 7-13.

[2] C C Wilson, N Shankland & A J Florence (1996). Chem. Phys. Letts., submitted.

[3] C C Wilson (1996). J Synchrotron Rad, 3, 20-23.