The low-temperature behaviour of analcime .1. High-resolution neutron powder diffraction

The structure of a synthetic sample of analcime has been determined as a function of temperature between 30-300 K by high-resolution neutron powder diffraction. Although there are some reports of samples of analcime having non-cubic structures, the sample in our experiments remained cubic (space group Ia3d), and hence disordered, down to low temperatures. The absence of phase transitions involving ordering of the orientations of the water molecules, ordering of the sodium positions, or a displacive instability as in leucite and related materials, is discussed. We speculate that part of the reason for the absence of ordering of the water molecules or sodium cations is associated with the Al/Si disorder, which cannot order at low temperatures. We also discuss the likely distribution of the orientations of the water molecules at low temperatures, and propose that the water diads lie close to any of the crystal diads with the H-H vectors lying close to the triads.