The thermal properties of Sn2P2S6 single crystals doped with Ge, Te, and Sb have been studied in the neighborhood of the ferroelectric to paraelectric second-order phase transition by means of ac photopyroelectric calorimetry, measuring thermal diffusivity. A detailed study of the critical behavior of the transitions has been undertaken using different theoretical models to study the influence of the dopants and the physical mechanisms activated. Ge strongly favors the stereoactivity of the material, which is revealed in the increase of the critical temperature and the sharpening of the transition; this is due to the strengthening of the sp(2) electronic orbitals hybridization. Sb has a small influence on the stereoactivity while Te virtually none. In all cases, the ferroelectric phases are well described using the phenomenological Landau model, while for the paraelectric ones, a combination of two mechanisms is needed: first-order fluctuations of the order parameter plus the presence of charged defects, ruling out other possible mechanisms. The relative importance of the presence of these defects increases with increasing contents of Ge and Sb, while in the case of Te, there is only a slight increase with respect to the undoped sample.