THE LOW-TEMPERATURE BEHAVIOR 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 ana lcime having non-cubic structures, the sample in our experiments remai ned cubic (space group Ia3d), and hence disordered, down to low temper atures. The absence of phase transitions involving ordering of the ori entations of the water molecules, ordering of the sodium positions, or a displacive instability as in leucite and related materials, is disc ussed. We speculate that part of the reason for the absence of orderin g of the water molecules or sodium cations is associated with the Al/S i disorder, which cannot order at low temperatures. We also discuss th e likely distribution of the orientations of the water molecules at lo w temperatures, and propose that the water diads lie close to any of t he crystal diads with the H-H vectors lying close to the triads.