Space group symmetries of the phases of (Pb_(0.94)Sr_(0.06))(Zr_xTi_(1-x))O_3 across the antiferrodistortive phase transition in the composition range 0.620 approx＜ x approx＜ 0.940

摘要

The existing controversies about the space group symmetries of Pb(Zr_xTi_(1-x))O_3 (PZT) above and below the antiferrodistortive (AFD) phase-transition temperature (T_(AFD)) in the Zr~(4+)-rich (0.620 approx＜ x approx＜ 0.940) compositions are addressed using the results of dielectric, synchrotron x-ray powder diffraction (SXRPD) and neutron powder diffraction (NPD) studies. These compositions undergo an AFD phase transition above room temperature due to tilting of oxygen octahedra leading to a superlattice phase of PZT. We have substituted 6% Sr~(2+) at a Pb~(2+) site to enhance the tilt angle and thereby the intensity of the superlattice peaks. The real and imaginary parts of the complex dielectric permittivity have been used to locate the paraelectric to ferroelectric and ferroelectric to AFD phase transitions. Rietveld analysis of SXRPD and NPD profiles unambiguously reject the rhombohedral phases in the R3c and R3m space groups below and above T_(AFD), respectively, with or without a coexisting monoclinic phase in the Cm space group and confirm that the true symmetries are monoclinic in the Cc and Cm space groups below and above T_(AFD) respectively. Based on these and previous findings a phase diagram of PSZT for 0.40 ≤ x ≤ 0.90 showing stability fields of monoclinic Cc and monoclinic Cm, tetragonal P4mm, and cubic Pm3m phases has also been presented.