Proposing a plausible molecular structure for Ice XI: A coupled study using Rietveld refinement and Density Functional Theory

A plausible molecular structure for fully ordered Ice XI is proposed. A range of structures were generated by varying the lattice parameters along the trajectory of the observed Ice Ih/Ice XI phase transition and recording the energy -minimised Density Functional Theory structure (using the Perdew-Wang 1991 functional) for each set of lattice parameters. These proposed structures were then fitted to the neutron data using GSAS-II to extract the goodness -of -fit. It is shown that the Rietveld refinement was insensitive to the OH1 bond length and the HO2H bond angle. The refinement with the lowest value of Rw(2)=6.038% gave OH bond lengths of 0.997(7)lambda, 0.999(2)lambda, 0.997(5)lambda, HOH bond angles of 107.5(degrees) and 106.0(degrees), and OH...O hydrogen bond lengths of 1.77(4),1.75(2) and 1.75(1)lambda. It is therefore apparent that the two crystallographically distinct water molecules have very similar molecular hydrogen bond parameters, which are close to that predicted by previous CASTEP PW91 DFT modelling.