  Polarized Raman scattering spectra of superconducting K$_x$Fe$_{2-y}$Se$_2$
and nonsuperconducting K$_{0.8}$Fe$_{1.8}$Co$_{0.2}$Se$_2$ single crystals were
measured in a temperature range from 10 K up to 300 K. Two Raman active modes
from the $I4/mmm$ phase and seven from the $I4/m$ phase are observed in
frequency range from 150 to 325 cm$^{-1}$ in both compounds, suggesting that
K$_{0.8}$Fe$_{1.8}$Co$_{0.2}$Se$_2$ single crystal also has two-phase nature.
Temperature dependence of Raman mode energy is analyzed in terms of lattice
thermal expansion and phonon-phonon interaction. Temperature dependence of
Raman mode linewidth is dominated by temperature-induced anharmonic effects. It
is shown that change of Raman mode energy with temperature is dominantly driven
by thermal expansion of the crystal lattice. Abrupt change of the A$_{1g}$ mode
energy near $T_C$ was observed in K$_x$Fe$_{2-y}$Se$_2$, whereas it is absent
in non-superconducting K$_{0.8}$Fe$_{1.8}$Co$_{0.2}$Se$_2$. Phonon energy
hardening at low temperatures in the superconducting sample is a consequence of
superconductivity-induced redistribution of the electronic states below
critical temperature.
