  The structural evolution of epitaxial mono-oriented (i.e. with the c-axis
perpendicular to the interface) ferroelectric Pb(Zr0.2,Ti0.8)O3 thin film has
been investigated, using high resolution, temperature dependent, X-ray
diffraction. The full set of lattice parameters was obtained, it allowed to
estimate the variation of the polarization as a function of temperature,
underlying the difference between the polarization-induced tetragonality and
the elastic one. The temperature evolution of the misfit strain has been
calculated and found to be in good agreement with the theoretical
temperature-misfit strain phase diagram
