Single crystal neutron diffraction study of the natural zeolite barrerite in its ND₄-exchanged form

Barrerite single crystals from Kuiu Island (Alaska) were treated in a ND₄Cl aqueous solution to obtain the ND₄-exchanged form. The crystal structure of the exchanged barrerite (Na_0.85K_0.24Mg_0.38Ca_1.30Ba_0.02(ND₄)_10.52 Al_15.01Si_56.99O₁₄₄ ·41D₂O, a = 13.601 Å, b = 18.232 Å, c = 17.810 Å, space group Fmmm) was studied by single crystal neutron diffraction on data collected at 20 K at the D19 line at ILL (Grenoble). A comparison with a previously studied NH₄-exchanged barrerite showed that the framework was identical in the two refinements, and that a substantial agreement occurred also in the extraframework content in the residual cations, in D₂O molecules and in ND₄+ ions. Surprisingly, no evidence was found of D atoms either around nitrogen or around the oxygens of water molecules. This result could be interpreted as a consequence of the partial occupancy of the extraframework sites and the large distances of these sites from framework oxygens, with consequent weak hydrogen bonds, which make a disorder in the orientation of D₂O and ND₄ groups highly probable.