Neutron-diffraction structure of a low-water grossular at 20 K

The crystal structure of a low-water (0.16 wt% OH) grossular has been refined at 20 K by single-crystal time-of-flight neutron-diffraction methods. Comparisons with the room-temperature structure indicate that between 298 and 20 K, the silicate tetrahedron undergoes a small rigid-body rotation that results in an increase in the y parameter of oxygen. The mechanism of thermal expansion is significantly different from that at high temperature where the tetrahedra do not rotate and changes in the structure reflect only unit-cell expansion.