Neutron Diffraction Study of the Structure of Ice

The crystal structure of ice in which the location of the hydrogen atoms cannot be determined by x-ray means, has been studied by the neutron diffraction technique. The intensities in the neutron diffraction pattern from a powdered sample of heavy (D₂O) ice, maintained at -90^, were measured and compared with the intensities which would result if ice possessed any one of four proposed structures. The oxygen atom positions have already been determined from x-ray measurements and the proposed structures are considered here with respect to the location of the hydrogen (or deuterium) atoms. The structures considered are (a) the Barnes model, in which the hydrogen atoms are midway between oxygen atoms, (b) the Bernal and Fowler model which maintains a molecular grouping of two hydrogen atoms close (0. 96A) to each oxygen atom with a definite repeating orientation, (c) a model consisting of hydrogen molecules rotating about the oxygen atoms as centers, and (d) the Pauling model, suggested on the grounds of a residual entropy at low temperatures, in which hydrogen molecules possess some randomness of orientation under the restriction that one and only one hydrogen atom lies on each of the lines joining one oxygen atom to its neighbor. Structures (a), (b), and (c) are definitely ruled out by the neutron measurements while the Pauling structure agrees well with the measurement.