Diffuse neutron scattering studies of proton ordering in ice Ih at different pressures

Complete text of publication follows. A series of diffuse neutron scattering measurements from D2O single crystal ice Ih at different pressures were made on Prisma using the standard He pressuriser at ISIS. The measurements show that there is no clear change to the diffuse scattering pattern for pressures up to 2.8 kbar at 230 K. The pressure of 2.8 kbar is also well above the necessary pressure required for the formation of ice II at this temperature (which is 2.2 kbar). Hence the structure of the crystal appears to be stabilised by the presence of He gas which may occupy the centre cavities of the ice structure. However, when the pressure increases to 3.1 kbar at 120 K, the structure of ice Ih starts to deform, firstly the d-spacing of Bragg reflections in the direction of (hh0) increases, for instance, the (220) peak increases from 2.245 to 2.256 A (instead of a decrease under such high pressure) and the transition is completed at temperature of 180 K (i.e. the 2.245 A component completely disappears). More interestingly, all Bragg peaks were disappeared when the temperature increases to 200 K although the background remained the same, after searching for Bragg reflections without success and from examination of the sample, we believe that the sample transformed to a clathrate structure. The lack of diffraction rings indicates the sample to be single crystalline but in a considerably different orientation. (author)