A systematic Raman study has been carried out on single crystals of the heavy fermion URu 2Si 2 compound at low temperatures (down to ∼ 6 K ) and high hydrostatic pressures (up to ∼ 5 GPa ). At ambient pressures and below 20 K, small modifications in the Si A 1 g mode have been observed. With increasing pressure, this mode shifts to higher energies at a high increment rate ( > 6 cm - 1 / GPa ) at room temperature, while the pressure dependence of its linewidth shows an anomalous behavior decreasing suddenly above ∼ 2 GPa . Hydrostatic pressure measurements at ∼ 2.4 GPa and low temperatures have revealed a broad weak peak at ∼ 370 cm - 1 , which is absent at ambient pressures and disappears above ∼ 200 K . These effects can be related with a pressure-dependent phase separation.
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