  The antiferrodistortive (AFD) phase transition for a pseudotetragonal
composition of Pb(Zr0.530Ti0.470)O3 (PZT) doped with 6% Sr has been
investigated using sound velocity (4 to 320K), high resolution synchrotron
X-ray powder diffraction (100 to 800K) and high resolution as well as high flux
neutron powder diffraction measurements (4K) to settle the existing
controversies about the true ground state of PZT in the morphotropic phase
boundary (MPB) region. The multiplet character of the neutron diffraction
profiles of (3/2 1/2 1/2)pc (pseudocubic or pc indices) and (3/2 3/2 1/2)pc
superlattice peaks, appearing below the AFD transition temperature, rules out
the rhombohedral R3c space group. The true ground state is confirmed to be
monoclinic in the Cc space group in agreement with the predictions of the first
principles calculations and earlier findings for pure PZT in the MPB region. 6%
Sr2+ substitution and the use of high wavelength ({\lambda}=2.44{\AA}) neutrons
have played key role in settling the existing controversies about the true
ground state of PZT in the MPB region.
