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 o f the exchanged barrerite (Na0.85K0.24Mg0.38Ca1.30Ba0.02(ND4)(10.52) Al15.0 1Si56.99O144. 41D(2)O, a = 13.601 Angstrom, b = 18.232 Angstrom, c = 17.810 Angstrom, space group Fmmm) was studied by single crystal neutron diffract ion on data collected at 20 K at the D19 line at ILL (Grenoble). A comparis on with a previously studied NH4-exchanged barrerite showed that the framew ork 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 resu lt could be interpreted as a consequence of the partial occupancy of the ex traframework sites and the large distances of these sites from framework ox ygens, with consequent weak hydrogen bonds, which make a disorder in the or ientation of D2O and ND4 groups highly probable.