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The A' B' I' C;' semiconducting compounds attract great attention due to their potential application in photovoltaic /1/ and infrared non-linear optical devices /2/. In this connection it is interesting to study the properties of their solid solutions. The purpose of this note is to study the composition dependence of the band gap of CuInS2xSe2(1 been reported in /3 to 6/. solid solutions, some properties of which haveThe transmission spectra in the range from 6000 to 14000 cm-' were taken using a SP-700 C spectrophotometer as well as a special apparatus for spectra recording with wavelength modulation. All investigations were carried out on single crystal samples obtained by the chemical transport /6/ and two-temperature synthesis /7/. For measurements the specimens were polished and lapped from one side to a thickness of about 100 pm. The absorption coefficient a was calculated using the estimated transmission T and the reflectivity R given in /8/ by the well-known expression a = - R2]1'2]) where d is the sample thickness. The ternary chalcopyrite compounds Abrn%" being direct-gap semiconductors, the band gap E was determined by extrapolation of the linear part of (a -. ad) versus photon energy to the intersection with the photon energy axis. In wavelength-modulated experiments the band gap was determined by the position of the maximum in the dI/dh. spectrum. The measurements made with polarized light showed E to be independent of polarization or, in other words, the absorption processes near the absorption edge are independent of the polarization of irradiation, what is related by us to the absence of tetragonal distortion of the unit cell in the investigated system (for CuInS2 c/a = 2.015 and for CuInSe2 c/a * 2.003). 2 g g 1) Podlesnaya 1.7, 220726 Minsk, USSR.

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