Vibrational modes in ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$Se and ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$Se

We report on a Raman study of lattice vibrations in the diluted magnetic semiconductors ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Fe}}_{\mathit{x}}$Se (0<x<0.64) and ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Co}}_{\mathit{x}}$Se (0<x<0.10). Both alloys possess a homogeneous crystal phase having the zinc-blende structure. The observed dependence of the first-order optical-phonon spectrum on x follows the mixed-mode behavior and is well described by the modified random-element isodisplacement model. Several physical parameters for the end members, cubic FeSe and CoSe, are deduced from the fits to the composition dependence of the lattice vibrations.