High-pressure Raman study on two modifications of BaAl2Si2

The new compound BaAl2Si2 exists in two structural modifications at ambient condition; a clathrate-like low-pressure phase alpha-BaAl2Si2 and a layered high-pressure phase beta-BaAl2Si2. Raman scattering of alpha-BaAl2Si2 and beta-BaAl2Si2 has been measured at 1 bar and room temperature, and that of alpha-BaAl2Si2 under high pressures up to 20 GPa. For alpha-BaAl2Si2, 17 Raman vibrational modes were observed at frequency of 40-450 cm(-1), which is almost consistent with the group theoretical analysis. Low-frequency vibrational modes associated with Ba atoms in the polyhedrons were found around 65-80 cm(-1) comparable to the typical clathrate Ba8Si46. The 4 Raman bands predicted by the group theory were found for beta-BaAl2Si2. Pressure dependence of Raman modes for alpha-BaAl2Si2 indicates the existence of phase transformation at about 20 GPa, which was irreversible with decreasing pressure. The decompressed alpha-BaAl2Si2 from 20 GPa to 1 bar shows Raman spectrum close to that of beta-BaAl2Si2 at ambient pressure.