Abstract : This work is devoted to the study of low phonon energy crystals doped with rare earth ions for the realisation of diode-pumped solid state laser sources emitting in the middle infrared. For that purpose, pure and (Er3+ou Pr3+) doped single crystals of KPb2Cl5 and Tl3PbX5 (X=Cl, Br) have been elaborated by using the Bridgman-Stockbarger method. These non-hygroscopic and congruentmelting materials have been found to exhibit phase transitions during the cooling process but which do not limit the elaboration of centimeter-size single crystals. The spectroscopic study of the Er3+ doped compounds has been performed both at high and low temperatures. It thus appears that these systems present long fluorescence lifetimes and relatively large gain cross sections favorable for a laser emission around 4.5µm. It has been demonstrated further that the up-conversion processes resulting from excited-state absorptions of the Er3+ ions around the pumping wavelength as well as the energy transfer processes between the Er3+ ions do not lead to significant optical losses for the laser system. The derived parameters then have been used to build a model and simulate the laser operation of the system following diode pumping around 800nm. In the end, the spectroscopic study of the Pr3+ ion in various materials has allowed us to evidence large emission cross sections associated with long fluorescence lifetimes, now favorable to a laser emission around 5µm.