The bound coherent neutron scattering lengths of the oxygen isotopes.

The technique of neutron interferometry was used to measure the bound coherent neutron scattering length b(coh) of the oxygen isotopes (17)O and (18)O. From the measured difference in optical path between two water samples, either H(2)(17)O or H(2)(18)O versus H(2)(nat)O, where nat denotes the natural isotopic composition, we obtain b(coh,(17)O) = 5.867(4) fm and b(coh,(18)O) = 6.009(5) fm, based on the accurately known value of b(coh,(nat)O) = 5.805(4) fm which is equal to b(coh,(16)O) within the experimental uncertainty. Our results for b(coh,(17)O) and b(coh,(18)O) differ appreciably from the standard tabulated values of 5.6(5) fm and 5.84(7) fm, respectively. In particular, our measured scattering-length contrast of 0.204(3) fm between (18)O and (nat)O is nearly a factor of 6 greater than the tabulated value, which renders feasible neutron diffraction experiments using (18)O isotope substitution and thereby offers new possibilities for measuring the partial structure factors of oxygen-containing compounds, such as water.

Isotope effects in water as investigated by neutron diffraction and path integral molecular dynamics.

The structures of heavy and light water at 300 K were investigated by using a joint approach in which the method of neutron diffraction with oxygen isotope substitution was complemented by path integral molecular dynamics simulations. The diffraction results, which give intra-molecular O-D and O-H bond distances of 0.985(5) and 0.990(5) Å, were found to be in best agreement with those obtained by using the flexible anharmonic TTM3-F water model. Both techniques show a difference of  ≃ 0.5% between the O-D and O-H intra-molecular bond lengths, and the results support a competing quantum effects model for water in which its structural and dynamical properties are governed by an offset between intra-molecular and inter-molecular quantum contributions. Further consideration of the O-O correlations is needed in order to improve agreement with experiment.

Oxygen as a site specific probe of the structure of water and oxide materials.

The method of oxygen isotope substitution in neutron diffraction is introduced as a site specific structural probe. It is employed to measure the structure of light versus heavy water, thus circumventing the assumption of isomorphism between H and D as used in more traditional neutron diffraction methods. The intramolecular and intermolecular O-H and O-D pair correlations are in excellent agreement with path integral molecular dynamics simulations, both techniques showing a difference of ≃0.5% between the O-H and O-D intramolecular bond distances. The results support the validity of a competing quantum effects model for water in which its structural and dynamical properties are governed by an offset between intramolecular and intermolecular quantum contributions.