  Pulsed neutron diffraction investigations have been performed in the
ferroelectric PZT system, Pb(Zr1-xTix)O3, doped with 1%wt. of Nb2O5, as a
function of both temperature and composition. The study has been made in a wide
range of temperatures encompassing the three known phases in Zr-rich PZT:
ferroelectric low temperature (R3c), ferroelectric high temperature (R3m) and
paraelectric (Pm3m). The combination of the temperature and the composition
dependence of the structural parameters allowed the determination of the
special relationship, recently pointed out by Corker et al., between the
octahedral strain (z) and the tilt angle (w) in the ferroelectric low
temperature phase (FL) of PZT. The strain-tilt coupling coefficient has been
found to decrease linearly with Ti content and the composition at which z and w
de-couple to be of x=0.30.
