Pressure dependence of elastic constants and optical phonon frequencies in Se and Te. A study of the homological relationship between the vibrational modes in these materials

Abstract

AbstractThe hydrostatic pressure coefficients for the elastic constants of Se and Te are shown to be large and positive in contrast to the negative mode Grüneisen parameters reported earlier for the optical vibrations. These results can be understood on the basis of the idea that the hydrostatic pressure promotes transfer of electrons from intrachain bonding orbitals to interchain bonding states. This strengthens the interchain “elastic” force constants while simultaneously weakening the intrachain “optic” force constants. The great similarity in the Se and Te lattices suggests that their vibrational spectra can be related by homology. Of the three lattice parameters describing each crystal, the intrachain bond angle is nearly identical and relatively pressure insensitive in the two lattices. Therefore, by a two‐step transformation in which the c/a‐ratio is first made equal by applying hydrostatic pressure, and secondly by properly scaling the mass and the unit cell, the two lattices can be made structurally identical. With this transformation and the available pressure data all zone centre phonons studied in the two materials could be related by a universal scaling factor for the “mode” force constants. Rough estimates for the higher‐order pressure coefficients can also be made.