Structure and hydrogen ordering in ices VI, VII, and VIII by neutron powder diffraction

Abstract

The structures of deuterated ices VI, VII, and VIII have been studied under their conditions of stability by neutron powder diffraction. The mode of ordering of (tetragonal) ice VIII is clearly established, and no evidence is found of partial ordering as the temperature is raised. Ice VII is accurately cubic with D2O molecules disordered around their center of mass; there is no evidence of partial ordering at any temperature. The water molecule geometry is normal in both phases, and the hydrogen-bonded first neighbor in ice VIII is confirmed to be more distant than the first nonbonded neighbor. The transition temperature between the two phases occurs at 263±2 K, some 11 K lower than expected. Hydrogen bond lengths in both phases are equal at the transition. Although the ice VI data are not as good, we can see no evidence of the antiferroelectric ordering proposed by Kamb from work on recovered samples. Our results are consistent with thermodynamic measurements indicating disorder in ice VI at 193 K. We conclude that the orientational ordering behavior of these high pressure ices is simpler than thought previously. For our ice VI results to be consistent with those of Kamb, his antiferroelectric ordering must be an artifact of the recovery process, thus throwing doubt on the validity of structural ordering data obtained on samples recovered to ambient pressure.