Rhumb. -Les phonons optiques (k -0) de CuInS2, AgInS2 et CuFeS, ont ete analyses par des mesures d'absorption et de reflexion dans la region de 1 000 jusqu'a 50 cm-1. La determination des frequences des phonons optiques transversaux et longitudinaux a et6 effectuk B I'aide dqCquations Kramers-Kronig ainsi que de modtles classiques d'oscillateurs. Les rksultats sont cornparks aux spectres connus de CuAIS" CuGaS2 et AgGaS2. Une comparaison systematique des frequences des modes montre quc les

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... antcs de force des differents coniposes 1-111-S2 sont presque egales. Les differences des frequences des modes correspondants paraissent dependre principalement des differentes masses des cations. Abstract. -The optical phonons at k = 0 of CuInS" AgInS2 and CuFeS?. were analysed by infrared reflection and absorption measurements in the region from 1 000 to 50 cm-1. To determine the transverse and longitudinal optical phonon frequencies Kranlers-Kronig integrations as well as classical oscillator models were used. The results are compared with the known phonon spectra of CuAIS2, CuGaS, and AgGaSz. It is shown by a systematic comparison of the mode frequencies that the force constants of the different I-111-S2 compounds are almost equal. The frequency difference between corresponding modes appear to be mainly due to the different cation masses.