The phonon spectrum and photoluminescence properties of (La2O3)0.05(As2S3)0.90(Er2O3)0.05 compound were studied in a broad aspect. The energy difference between the two-component maxima of photoluminescence located at the value of energy Е≈1.4eV was 41 meV, which corresponded to the energy of the most intense transverse optical (TO) phonon determined from the general radiation spectrum in the phonon spectrum region. In other words, the first photoluminescence line was observed simultaneously with the emission of a TO phonon with an energy of 41 meV. By changing the concentration of the erbium atom in different percentages in the infrared region of the spectrum and at the same time doping it with the lanthanum element, which plays the role of a sensitizer, a sharp increase in the intensity of photoluminescence was achieved in the intracentral (4J2/11-4J15/2) transition of the erbium atom. All these results show that the compound is suitable for wide application in various fields of optoelectronics.