Results on disordered materials from the GEneral Materials diffractometer, GEM, at ISIS

The GEneral Materials diffractometer, GEM, at the ISIS Facility pulsed neutron source is the most advanced materials neutron diffractometer in the world. A full description of GEM is given from the point of view of structural studies of disordered materials. The key component for these studies is the highly stable detector array, which covers a very wide range in scattering angles and a very large solid angle, leading to a wide dynamic range and a high count rate. The high real-space resolution of GEM, due to a high maximum momentum transfer of 55 Å−1, is illustrated by data on bioactive phosphate glasses, where bonds from phosphorus to bridging and non-bridging oxygens (which differ in length by approximately 0.12 Å) are clearly resolved. The low momentum transfer ability of GEM is demonstrated by a measurement on amorphous Si77D23, which yields reliable data down to 0.1 Å−1. The first successful isotopic substitution experiment on sol-gel materials has been performed on GEM in a study of amorphous titanium silicates, showing the suitability of the instrument for both isotopic substitution, and for studies of complex materials, even in the presence of hydrogen. Studies of the structures of disordered group 11 cyanide crystals show the power of GEM to reveal the nature of disorder in crystalline systems as well as glassy materials. The crystal structure of high temperature CuCN has been solved for the first time. The disorder in AuCN, AgCN and high temperature CuCN arises from random displacements of the linear -M-(CN)- chains relative to each other, and the one-dimensional negative thermal expansion is due to long wavelength cooperative motions of the chains.