Pure and Nb2O5 doped PbZrO3 exhibit the following phases: antiferroelectric, ferroelectric, and paraelectric. By means of measurements of heat transition, dielectric constant in paraelectric phase, determination of electrical field – temperature phase diagrams, we calculate the free energy of each phase. Given that the free energy can be developed in powers of the polarization, we show the coefficients of the ferroelectric phase are slightly modified by the addition of Nb2O5, while those of the antiferroelectric phase vary by a factor greater than two.