Hydrostatic pressure effects on Raman spectra and the fundamental absorption edge in 2H-PbI2 crystals

The pressure dependence of the Raman and luminescence spectra of 2H-PbI2 ha s been studied under hydrostatic pressure up to 1.5 GPa at 77 K by using a diamond-anvil cell. A structural phase transition occurs at a pressure of P -PT = 0.56 +/- 0.01 Gpa. From the analysis of the Raman spectra, it is prop osed that the unit cell of the high-pressure phase is a 2 X 2 supercell of that of the low-pressure phase. The peak energies of the free exciton recom bination and donor-acceptor pair bands shift continuously across the phase transition, indicating that relative positions of atoms and the nature of i nteratomic bonding hardly change at the phase transition. The pressure depe ndence of the fundamental absorption edge which is well described by the Ur bach rule has also been examined at 77 and 300 K. At P-PT the steepness con stant sigma (0) in the Urbach tail changes drastically from 1.70 to 0.63. T his result is consistent with the proposed doubling of unit-cell dimensions at P-PT.