The thickness dependency of the Curie temperature in stress-free Pb Zr0.5Ti0.5 O3 ultrathin films under open-circuit conditions is revealed from the computation of some nontrivial statistical quantities such as fourth-order cumulants , via a first-principles-based technique. For thicknesses above 16 Å, this dependency follows the usual finite-size scaling law with a critical exponent that is consistent with the one associated with the three-dimensional-random-Ising universality class. On the other hand, the Curie temperature-versus-film's thickness curve deviates from this scaling law below 12 Å while being rather well described by an empirical equation down to 8 Å.