  Doped semiconductor crystals of solid solution $Cd_{1-x}Mn_{x}Te:Fe^{2+}$
were grown by the high-pressure Bridgman method covering the range of its
existence as a zinc blende crystal structure. The concentration of
Fe-impurities was approximately 0.001 wt.% in all studied samples. The
correlations between the composition of solid solution crystals of
$Cd_{1-x}Mn_{x}Te:Fe^{2+}$, band gap, lattice period and the maxima positions
of the $Fe^{2+}$ active ion absorption and emission spectra were found. A new
theoretical model based on the principle of additivity for solid solution
semiconductor materials has been used for explaining the long-wavelength
''redshift'' of absorption and luminescence bands in the spectra of
$Cd_{1-x}Mn_{x}Te:Fe^{2+}$ crystals with increasing solid solution
concentration. The obtained results can be used to predict the lasing range for
$Cd_{1-x}Mn_{x}Te:Fe^{2+}$ crystal media (in all possible $Mn$ concentrations),
which is potentially the longest wavelength active material for mid-IR lasing.
