The influence of the pressure of the argon-oxygen gas mixture on the composition of Pb(Zr,Ti)O3 (PZT) films deposited by RF magnetron sputtering on the silicon substrate with a platinum layer has been studied. It was demonstrated that the change in the self-polarization vector is unambiguously associated with the variations of the concentration of lead in PZT films in relation to stoichiometric composition. It was found that at the high gas pressure (8 Pa) the entire volume of the thin PZT is crystallized in the perovskite phase, and with decreasing pressure to 2 Pa (and, consequently, with a decrease in lead content) the fraction of the perovskite phase in the film volume significantly decreases. For explanation of the reorientation of the polarization vector we have proposed the model which is based on mechanochemical phenomenon of the diffusion of lead atoms under the mechanical stresses caused by a distinction in the thermal expansion coefficients of the film and the substrate.