In situ high-pressure x-ray diffraction study of H2O ice VII.

Ice VII was examined over the entire range of its pressure stability by a suite of x-ray diffraction techniques in order to understand a number of unexplained characteristics of its high-pressure behavior. Axial and radial polycrystalline (diamond anvil cell) x-ray diffraction measurements reveal a splitting of diffraction lines accompanied by changes in sample texture and elastic anisotropy. In situ laser heating of polycrystalline samples resulted in the sharpening of diffraction peaks due to release of nonhydrostatic stresses but did not remove the splitting. Radial diffraction measurements indicate changes in strength of the material at this pressure. Taken together, these observations provide evidence for a transition in ice VII near 14 GPa involving changes in the character of the proton order/disorder. The results are consistent with previous reports of changes in phase boundaries and equation of state at this pressure. The transition can be interpreted as ferroelastic with the appearance of spontaneous strain that vanishes at the hydrogen bond symmetrization transition near 60 GPa.