E0042

Neutron Crystallography at NBS/NIST. A. Santoro, NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899 USA.

Neutron diffraction at NBS/NIST started soon after the NBS Reactor became operational in the summer of 1969. The first instruments dedicated to crystallographic work were (i) a low-resolution, single counter powder diffractometer using radiation from a 220 Cu monochromator with variable take-off angle and, (ii) a single-crystal four-circle diffractometer. The development of the Rietveld method of profile analysis soon increased the complexity of the structures soluble with the powder method, decreasing the need for single crystal work and increasing the use of efficient high-resolution diffractometers. At NBS/NIST, the original powder machine was replace in rapid succession first by a five-counter diffractometer of intermediate resolution and then by the present day 32-counter diffractometer, whose resolution can be varied from low to high by changing the horizontal divergences of the collimating system and/or by selecting one of three monochromators, each having a characteristic take-off angle.

The object of the work being done at the present time is to relate crystal structure and physical properties, and it involves complex systems analyzed with multiple techniques, many of which are available at the NIST Center for Neutron Research (ranging from small angle scattering to spin-echo spectroscopy). In addition, theoretical approaches, such as bond valence analysis of structures of intermediate complexity, are being used in attempts to clarify the role of valence and non-bonded interactions in the formation of relevant materials. Results of this analysis applied to perovskite structures will be described.