Raman Scattering under Hydrostatic Pressures in Layered BiI3 and SbI3 Crystals

Raman scattering from BiI 3 and SbI 3 crystals has been studied at atmospheric and hydrostatic pressures up to 2 GPa. From the polarization characteristics, mode assignments were made for both crystals. The Raman shifts monotonically increased with pressure for all modes in BiI 3 . Although both crystals belong to the same space symmetry group, the pressure coefficients in SbI 3 were significantly different from those in BiI 3 . For a few modes, the coefficients gradually changed near P 1 = (0.30 ± 0.1) GPa, some of which were negative; also, near P 2 = (0.9 ± 0.1) GPa, the pressure coefficients of all modes changed. Above P 2 , all modes in SbI 3 had positive pressure coefficients as they have in BiI 3 . Furthermore, deformation-type stacking faults appear in SbI 3 only above P 2 , although it is known that these faults can form at all pressures in BiI 3 . By comparing this unusual behavior of the Raman lines in SbI 3 around P 2 with the results in BiI 3 and comparing the two lattice structures, we suggest that the type of bonding in SbI 3 changes to become more ionic and hence more similar to the bonding in BiI 3 . This change in structure is supported by a change in the slope of the absorption-edge photon energy versus pressure.