Antiferrodistortive phase transition in pseudorhombohedral (Pb0.94Sr0.06)(Zr0.550Ti0.450)O3: A combined synchrotron x-ray and neutron powder diffraction study

Abstract

The controversies about the structure of the true ground state of pseudorhombohedral compositions of Pb(Zr${}_{x}$Ti${}_{1\ensuremath{-}x}$)O${}_{3}$ (PZT) are addressed using a 6$%$ Sr${}^{2+}$ substituted sample with $x$ $=$ 0.550. Sound velocity measurements reveal a phase transition at ${T}_{c}$ \ensuremath{\sim} 279 K. The temperature dependence of full width at half maximum of (h00)${}_{\mathrm{pc}}$ peaks and the unit cell volume also show anomalies around 279 K 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 ${T}_{c}$ \ensuremath{\sim} 279 K, confirming the existence of an antiferrodistortive phase transition. The Rietveld analysis of the room-temperature and low-temperature SXRD data below ${T}_{c}$ 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 ${T}_{c}$ corresponds to the $Cc$ space group. Our analysis shows that the structural models for the ground-state phase based on the R3c space group with or without the coexistence of the room-temperature monoclinic phase in the $Cm$ space group can be rejected.