EXCITON EFFECTS IN THE REFLECTANCE SPECTRA OF MIXED LEAD HALIDES PBCL2XBR2(1-X)

The reflectance spectra of orthorhombic single crystals of PbCl2xBr2(1-x) (0 less-than-or-equal-to x less-than-or-equal-to 1) are studied in the spectral range between 3.5 and 4.8 eV at 293, 95, and 39 K. The strong peaks observed in these isomorphic solid solutions are attributed to cation excition transitions. The fine structure of these exciton peaks is observed as a function of temperature and solid composition x. The coefficient of temperature dependence varies nearly linearly with composition for PbBr2 (x = 0), PbClBr (x = 0.5), and PbCl2 (x = 1). The variation of the singlet exciton peak with the fractional composition x shows two different regions: one for 0 less-than-or-equal-to x less-than-or-equal-to 0.5 and another one for 0.5 less-than-or-equal-to x less-than-or-equal-to 1.