In situ Raman spectra of transformations of H2O as functions of pressure and temperature have been measured starting from high-density amorphous ice (HDA). Changes above Tx, the crystallization temperature of HDA, were observed. The spectra provide evidence for an abrupt, first-order-like, structural change that appears to be distinct from those associated with the transformation between low-density amorphous ice (LDA) and HDA. In separate experiments, in situ Raman spectra of ice XII transformed from HDA have been measured at various P-T regions, in order to improve the understanding of the stability limits of ice XII. The spectra of ices VI and XII differ in shape, but the vibrational frequencies are very close in the same P-T regimes. A metastable phase of ice found to form within the stability field of ice VI appears to be distinct from ice XII.

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