Raman scattering under hydrostatic pressures in layered BiI3 and SbI3 crystals

Raman scattering from BiI3 and SbI3 crystals has been studied at atmospheri c and hydrostatic pressures up to 2 GPa. From the polarization characterist ics, mode assignments were made for both crystals. The Raman shifts monoton ically increased with pressure for all modes in Bi13. Although both crystal s belong to the same space symmetry group, the pressure coefficients in SbI 3 were significantly different from those in Bib. For a few modes, the coef ficients gradually changed near P-1 = (0.30 +/- 0.1) GPa, some of which wer e negative; also, near P-2 = (0.9 0.1) GPa, the pressure coefficients of al l modes changed. Above P-2, all modes in SbI3 had positive pressure coeffic ients as they have in Bib. Furthermore, deformation-type stacking faults ap pear in SbI3 only above P-2, although it is known that these faults can for m at all pressures in Bib. By comparing this unusual behavior of the Raman lines in SbI3 around P-2 with the results in BiI3 and comparing the two lat tice structures, we suggest that the type of bonding in SbI3 changes to bec ome more ionic and hence more similar to the bonding in Bib. This change in structure is supported by a change in the slope of the absorption-edge pho ton energy versus pressure.