Abstract
The existing controversies about the space group symmetries of Pb(ZrxTi1-x)O3 (PZT) above and below antiferrodistortive (AFD) phase transition temperature (TAFD) in the Zr4+- rich (0.620<x<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 octahedral leading to a superlattice phase of PZT. We have substituted 6% Sr2+ at Pb2+-site to enhance the tilt angle and thereby the intensity of the superlattice peaks. The real and imaginary parts of 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 R3c and R3m space groups below and above TAFD, respectively, with or without a coexisting monoclinic phase in Cm space group, and confirm that the true symmetries are monoclinic in Cc and Cm space groups below and above TAFD, 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.
Highlights
INTRODUCTIONStructural phase transitions in perovskites are broadly classified as ferrodistortive (FD) and antiferrodistortive (AFD)
Structural phase transitions in perovskites are broadly classified as ferrodistortive (FD) and antiferrodistortive (AFD)types depending on whether they are driven by the freezing of a zone center (k = 0) or a zone boundary (k = 0) phonon mode [1,2]
In our previous paper [22], samples were prepared by a semiwet route, and it has been shown that the morphotropic phase boundary (MPB) in PSZT lies in the composition range of 0.520 x 0.535 (Fig. 1 of Ref. [22]) whereas it lies in the range of 0.515 < x < 0.530 for pure PZT prepared by the same method [9,17]
Summary
Structural phase transitions in perovskites are broadly classified as ferrodistortive (FD) and antiferrodistortive (AFD). The ground state of PZT for the entire range of compositions with tetragonal structures, except those very close to the MPB, is decided by the ferrodistortive transition resulting from the freezing of the polar. The ground state of both the strongly piezoelectric and the pyroelectric PZT compositions is decided not by the FD transition but by AFD transition resulting from the freezing of the R4+ mode involving antiphase rotation of the oxygen octahedra. In the technologically important piezoelectric ceramic compositions near the MPB, the Curie transition from the paraelectric cubic to the ferroelectric tetragonal phase resulting from the freezing of one of the components of the is followed by another structural phase transition driven by the freezing of the remaining two components of the. We use the present results in conjunction with our previous results [32] to extend the phase diagram proposed earlier by us in the MPB region [32] to the Zr4+-richer side
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