## 1 Introduction  
The Kondo lattice model, or s-d(f) exchange model, is widely used to describe the correlation effects in metals (and their compounds) with unfilled d- and f-shells (transition or rare earth metals, for example, CdCr\({}_{2}\)Se\({}_{4}\), HgCr\({}_{2}\)Se\({}_{4}\) or Eu, U chalcogenides, respectively) [1; 2; 3]. The nature of the ground state in these systems is largely determined by the result of competition between two interactions. On the one hand, the s-d(f) exchange coupling between spins of itinerant s- and localized d(f)-electrons, due to Kondo fluctuations, screens the localized spins and tends to form a non-magnetic ground state. In the opposite direction, an indirect exchange (RKKY) interaction takes place between spins of d(f)-electrons, trying to set a long-range magnetic order which is not necessarily ferromagnetic or antiferromagnetic. Ferromagnetic semiconductors (FMS) which find applications in spin electronics and informatics are materials exhibiting both ferromagnetic and semiconductor properties [1; 4; 5]. While traditional electronics are based on the control of charge carriers, FMS allow also control of the quantum spin state, providing an almost complete spin polarization, an important property for spintronics applications [4]. The fundamental element in the theory of the so-called s-f model which is widely accepted as a good description of magnetic 4f- or 5f-systems is the exchange coupling (s-fcoupling) between the conduction band electrons and the lattice of localized magnetic moments [1; 6; 7; 8]. The s-f coupling influences the optical, electrical, and magnetic properties of these materials. As first pointed out by [PERSON] and [PERSON] [9], the physical properties depend very strongly on the sign of the s-f exchange. In the Eu chalcogenides, the electron spin couples ferromagnetically (FM) to the f spin [2], whereas in the uranium chalcogenides, the s-f coupling seems to be antiferromagnetic (AFM) [10]. For \(I>0\), the localized-spin magnetization and the electron-spin polarization increase, whereas for \(I<0\), they decrease with increasing the conduction band width [11]. The temperature and carrier concentration dependence of the conduction electron spin polarization for 4f and 5f compounds was discussed in [12; 13]. [PERSON] and [PERSON] [14] calculated the electron and magnon lifetime as a function of the doping in the low-temperature spin wave approximation for the FM s-f exchange. The spin-phonon interaction (SPI) in FMS has been investigated by many authors. Most works have considered the influence of the SPI on the phonon spectrum (but not on the band gap energy), for example, in EuO and EuS [15; 16]. [PERSON] et al. [15] studied the anharmonic spin-phonon and phonon-phonon interaction effects on optical phonon modes and spin wave excitations in FMS. The band gap is a fundamental quantity which affects various optical, electronic and energy applications of the materials. FMS based on Eu chalcogenides are known for their interesting and different magnetic properties. The band gap variation is from 1.12 eV for EuO, 1.6 eV for EuS to 2.0 for EuTe [1]. Theoretically and experimentally, the influence of the electron-phonon interaction (EPI) on the temperature dependence of the band structure of semiconductors has been studied [17; 18; 19; 20; 21]. Raman spectroscopy is a very powerful tool for material analysis. It is used to perform quality control, failure analysis, sample identification, materials characterization, and, in general, to investigate physical and chemical properties. Therefore, it is important to study the influence of different microscopic interactions, such as the SPI and EPI on the phonon spectra. Raman scattering of Eu and U chalcogenides near the magnetic phase transitions is investigated in [22; 23; 24; 25; 26]. The effect of the EPI on the temperature dependence of the electronic and phonon spectrum in the FMS CdCr\({}_{2}\)S\({}_{4}\) is discussed in [27]. [PERSON] et al.