We used neutron diffraction with an incident wavelength of 0.5024 A and both ~6Li/~(nat)Li isotopic substitution to study the structure of interlayer water in Li-Wyoming montmorillonite hydrates, with water contents corresponding to two-layer hydrates. The hydrogen isotope first-order difference gives the weighted sum of partial structure factors and radial distribution functions related to the intra- and intermolecular structure around interlayer water hydrogens. The results show unequivocally that the interlayer water adopts a liquid-type structure and that the clay surface does not prevent the interlayer water from adopting a highly hydrogen-bonded configuration. The lithium isotope first-order differences for D_2O and H_2O hydrates, and for the dry clay, are complicated by the presence of residual Bragg peaks. We discuss the possible origins of these features and the prospects for the further application of this technique to the study of hydrated clays.
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