Low-temperature behaviour of ammonium ion in buddingtonite [N(D/H)₄AlSi₃O₈] from neutron powder diffraction

The structural response of buddingtonite [N(D/H)₄AlSi₃O₈] on cooling has been studied by neutron diffraction. Data have been collected from 280 K down to 11 K, and the crystal structure refined using the Rietveld method. Rigid-body constraints were applied to the ammonium ion to explore the structural properties of ammonium in the M-site cavities at low-temperature. Low-temperature saturation is observed for almost all the lattice parameters. From the present in situ low-temperature neutron diffraction studies, there is no strong evidence of orientational order-disorder of the ammonium ions in buddingtonite.