Flexibility and distortion of the collapsible framework of NAT topology: the crystal structure of NH4-exchanged natrolite

Abstract

Cation exchange of NH 4 for Na in natrolite, Na 2 Al 2 Si 3 O 10 2H 2 O, yields an anhydrous ammonium natrolite (NH 4 ) 2 Al 2 Si 3 O 10 . The symmetry is reduced from space group Fdd2 in Na-natrolite to space group P112 1 in NH 4 -natrolite, but the description of the structure has been given in space group C112 1 for better comparison with the Na-form. The volume of the unit cell of NH 4 -natrolite (a=17.899(2) A, b=18.390(2) A, c=6.529(1) A, γ=90°, V=2149.1(8) A 3 ) is reduced compared with the original Na-natrolite (a=18.293(1) A, b=18.641(1) A, c=6.587(1) A, V=2246.2(2) A 3 ) by more than 4%. The agreement factors are R=4.67% and Rw=2.98% for 9269 F obs . The crystal is twinned on a plane normal to [100] or [010]. The ammonium groups in the NH 4 -form occupy similar positions to those of the water molecules in Na-natrolite. The distances Si-O and Al-O within the framework are strongly influenced by the hydrogen bonds from the NH 4 -groups. Hydrogen bonded oxygen atoms have, on average, larger T-O distances (0.01 A) and smaller T-O-T angles (6.8°) than non-hydrogen bonded oxygen atoms of the framework. The effect of the hydrogen bonds on the T-O bond lengths is stronger than the influence exerted on them by the change in the T-O-T angles. This is the first instance in which changes in the geometry of the framework can be attributed solely to hydrogen bonding effects. The contraction of the framework due to the exchange of NH 4 for Na is caused by tilting and twisting of the chains within themselves and not by a rotation of the chains as a whole around their hinghes between the fibers. The NAT framework of natrolite is collapsile, which means that all angles T-O-T (the hinges between the essentially rigid TO 4 tetrahedra) change upon contraction or expansion in the same sense