Pressure dependent Raman studies in the K^sub 2^Mo^sub 2^O^sub 7^·H^sub 2^O crystal

The pressure dependent Raman scattering in the potassium molybdenum oxide hydrate crystal, K2Mo2O7 . H2O, was measured. The high pressure Raman study showed, that the compound remains in the triclinic structure within the 0.0-3.81 GPa range and undergoes a structural phase transition between 3.81 and 4.13 GPa. This particular phase transition is most likely connected with changes in the Raman spectrum, in which the number of modes associated originally with the stretching vibrations in the MoO5 and MoO6 units is increased. However, the phase at atmospheric pressure shows bands due to the presence of only one equivalent site, while in the high-pressure phase, two bands are associated with the stretching modes. Continuing the pressure evolution up to 17.04 GPa, two further phase transitions occurred in this crystal in the 6.3-8.1 GPa and the 12.3-14.0 GPa range, respectively. The Raman spectra measured at about 17.04 GPa presented a crystal structure, which experienced a pre-amorphization with a total loss of all lattice modes. This particular result is indicative that this material may have undergone a complete amorphization at pressures larger than 17.04 GPa. Then, the reversible character in the triclinic P-1 (Ci1) structure was recovered after releasing the pressure.