Neutron powder diffraction study of orthorhombic and monoclinic defective silicalite

The crystal structure of silicalite (SiO2) with a substantial amount of structural hydroxyl groups [Si(1-x)O(2-4x)(OH)4x, with 0.08<x<0.10] has been refined from neutron powder diffraction data measured using the HRPD instrument at the ISIS pulsed neutron source. Powder data were collected on the as-synthesized orthorhombic sample at 298 K, and on the deuterated monoclinic sample at 100 K. Preferential location of Si-atom vacancies was found on four out of 12 independent T sites in the orthorhombic silicalite [Si(6), Si(7), Si(10) and Si(11)], although the H atoms of the substituting hydroxyl groups could not be located because of the low statistical site occupancy on multiple sites. No significant population of D atoms or of Si vacancies was found in the tetrahedral sites of the monoclinic sample. The detected long-range order of adjacent Si atoms in defective orthorhombic [MFI] structures is compatible with a mechanism of Si vacancy clustering and with the model of hydroxyl nests assumed in the literature on the basis of IR spectroscopic evidence. Crystal data: orthorhombic, Pnma, Z = 8, a = 20.0511 (1), b = 19.8757 (1), c = 13.36823 (9) A, V = 5327.62 (5) A3, Dx = 1.798 g cm(-3), Mr = 721.01; monoclinic, P2(1)/n, Z = 4, a = 19.8352 (2), b = 20.0903 (2), c = 13.3588 (1) A, beta = 90.892 (1), V = 5322.78 (6) A3, Dx = 1.799 g cm(-3), Mr = 1442.02.