We have studied the pressure-induced phase transition and chemical decomposition of hydrogen peroxide and its mixtures with water to 50 GPa, using confocal micro-Raman and synchrotron x-ray diffractions. The x-ray results indicate that pure hydrogen peroxide crystallizes into a tetragonal structure (P _(41) 21 2), the same structure previously found in 82.7 H_2 O_2 at high pressures and in pure H_2 O_2 at low temperatures. The tetragonal phase (H_2 O_2 -I) is stable to 15 GPa, above which transforms into an orthorhombic structure (H_2 O_2 -II) over a relatively large pressure range between 13 and 18 GPa. Inferring from the splitting of the s(O-O) stretching mode, the phase I-to-II transition pressure decreases in diluted H_2 O_2 to around 7 GPa for the 41.7 H_2 O_2 and 3 GPa for the 9.5. Above 18 GPa H_2 O_2 -II gradually decomposes to a mixture of H _2 O and O_2, which completes at around 40 GPa for pure and 45 GPa for the 9.5 H_2 O_2. Upon pressure unloading, H _2 O_2 also decomposes to H_2 O and O_2 mixtures across the melts, occurring at 2.5 GPa for pure and 1.5 GPa for the 9.5 mixture. At H_2 O_2 concentrations below 20, decomposed mixtures form Oxygen hydrate clathrates at around 0.8 GPa-just after H_2 O melts. The compression data of pure H_2 O_2 and the stability data of the mixtures seem to indicate that the high-pressure decomposition is likely due to the pressure-induced densification, whereas the low-pressure decomposition is related to the heterogeneous nucleation process associated with H_2 O_2 melting.
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