Raman spectroscopic studies of phase E to 19 GPa

Raman spectra of phase E have been measured between 100 cm(-1) and 4000 cm- 1 in a diamond-anvil cell to 19 GPa at 300 K. We observe several broad band s in the region below 1200 cm-1 and three bands in the OH stretching region . All modes shift continuously with increasing pressures to 19 GPa and ther e is no indication for a major change in the crystal structure of phase E. First constraints on hydrogen sites and hydrogen bond strengths are derived from the OH frequencies to-ether with their pressure dependencies and the correlation of OH stretching frequencies with O . . .O bond distances. The 3429 cm-1 band decreases linearly with pressure at -7.8 cm(-1)/GPa and is c learly involved in hydrogen bonding. The corresponding O . . .O distance at ambient conditions is estimated to be around 2.82 Angstrom. The 3617 cm(-1 ) band shows a positive pressure trend allowing only very weak or absent hy drogen bonds. Corresponding O . . .O distances would be greater than 3 Angs trom.