  The controversies about the structure of the true ground state of
pseudorhombohedral compositions of Pb(ZrxTi1-x)O3 (PZT) are addressed using a
6% Sr2+ substituted sample with x=0.550. Sound velocity measurements reveal a
phase transition at Tc~279K. The temperature dependence of FWHM of (h00)pc
peaks and the unit cell volume also show anomalies around 279K even though
there is no indication of any change of space group in the synchrotron X-ray
powder diffraction (SXRD) patterns. The neutron powder diffraction patterns
reveal appearance of superlattice peaks below Tc~279K confirming the existence
of an antiferrodistortive phase transition. The Rietveld analysis of the room
temperature and low temperature SXRD data below Tc shows that the structure
corresponds to single monoclinic phase in the Cm space group while the analysis
of neutron powder diffraction data reveals that the structure of the ground
state phase below Tc corresponds to the Cc space group. Our analysis shows that
the structural models for the ground state phase based on R3c space group with
or without the coexistence of the room temperature monoclinic phase in the Cm
space group can be rejected.
