Single crystals of potassium magnesium chloride sulfate (KMCS) doped with cobalt ions were grown by slow
cooling method. Powder XRD study confirmed the monoclinic structure of the grown crystals. The functional
group vibrations were checked through FTIR spectroscopymeasurements. In optical studies, the absorbance behavior
of the crystals and their optical energy gapwere established by Tauc plot. The refractive index, the extinction
coefficient and other optical constants were calculated for the grown crystals. The normal dispersion of the
refractive index was analyzed according to single oscillator Sellmeier's model. The Urbach's rule was applied to
analyze the localized states density in the forbidden gap