Optical phonons in CdGa2S4x Se4(1 − x) alloys

Abstract

The Raman spectra of CdGa2S4xSe4(1 − x) alloys (x = 0.1, 0.2, … 0.9) are studied. Both the singlemode and double-mode behavior of optical phonons are observed in CdGa2S4xSe4(1 − x) alloys. The observed optical mode at 138 cm−1 is independent of the composition. It seems likely that this mode is the “breathing mode” and is caused by atomic motion in the anion sublattice relative to vacancies. It is shown that the high-frequency modes of symmetry B1(LO, TO) and B2(LO, TO) are caused by the in-phase motion of atoms in the anion sublattice along the tetragonal axis c relative to trivalent Ga atoms. The doubly degenerate symmetry modes E1(LO, TO) and E2(LO, TO) are caused by the in-phase motion of atoms in the anion sublattice relative to trivalent Ga atoms of the cation sublattice in the xy plane (dipole 2Ga–4C), where C is S, Se. The optical symmetry modes B3(LO, TO) and B4(LO, TO) are associated with the motion of atoms in the anion sublattice relative to Cd atoms along the tetragonal axis c. The doubly degenerate modes E3(LO, TO) and E4(LO, TO) are associated with the motion of atoms in the anion sublattice relative to Cd atoms (dipole Cd–4C). The low-frequency modes B5(LO, TO) and E5(LO, TO) are the analogs of acoustic phonons at the edge of the Brillouin zone of sphalerite.