Optical phonons in CdGa$_2$S$_{4x}$Se$_{4(1-x)}$ alloys

The Raman spectra of CdGa$_2$S$_{4x}$Se$_{4(1-x)}$ alloys ($x$ = 0.1, 0.2, … 0.9) are studied. Both the singlemode and double-mode behavior of optical phonons are observed in CdGa$_2$S$_{4x}$Se$_{4(1-x)}$ 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 $B_1$(LO, TO) and $B_2$(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 $E_1$(LO, TO) and $E_2$(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 $B_3$(LO, TO) and $B_4$(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 $E_3$(LO, TO) and $E_4$(LO, TO) are associated with the motion of atoms in the anion sublattice relative to Cd atoms (dipole Cd–4C). The low-frequency modes $B_5$(LO, TO) and $E_5$(LO, TO) are the analogs of acoustic phonons at the edge of the Brillouin zone of sphalerite.