A neutron diffraction study of the zeolite edingtonite

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Abstract
A neutron diffraction study at 294 K of a single crystal of edingtonite (Ba2Al4Si6O20 ⋅ 7H2O; a 9.537(3) b 9.651(2) c 6.509(2) Å; P21212) utilized 1876 diffraction intensities from the Brookhaven National Laboratory high‐flux beam reactor. The agreement factor R(F2)=0.055 for conventional anisotropic refinement was reduced to 0.045 for a Gram–Charlier expansion up to fourth order for the thermal factors of the water atoms. The Si–O and Al–O distances correlate inversely with the Si–O–Al angle as in scolecite. There is no indication of substitutional disorder. The barium atom is coordinated to three pairs of framework oxygens (2.89, 2.96, and 3.04 Å) and two pairs of water oxygens (2.79 and 2.79 Å). Two framework oxygens have weak hydrogen bonds to both water molecules [O(4)–OW(1) 2.87, −OW(2) 2.96; O(5) −OW(1) 3.02, −OW(2) 3.02 Å] and the other three framework oxygens are each bonded to a Ba atom. The OW–H ⋅⋅⋅ O angles (163.5°, 165.1°, 173.9°, and 178.0°) are fairly close to 180°, the H ⋅⋅⋅ O distances are long (1.91, 2.02, 2.09, and 2.10 Å) and the observed uncorrected OW–H distances range from 0.928(6) to 0.959(4) Å. Only seven out of the eight water positions are occupied [W(1) 84% occupancy; W(2) 91%]. The average rms displacement of each hydrogen (0.32, 0.29, 0.27, and 0.24 Å) correlates approximately with the hydrogen bond length (2.09, 2.10, 2.02, and 1.91 Å). Third‐ and fourth‐order tensor components in the displacements of the water molecules may result from anharmonic or curvilinear vibrations; however, the effect of the static displacements of the center‐of‐motion from interaction with unoccupied water sites may also be important.

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