Crystal structure and order parameters in the phase transition of antiferroelectric PbZrO3

X-ray and neutron diffraction and dielectric measurements were performed for the antiferroelectric phase of PbZrO3. The antiferroelectric Sigma(3)(TO) and the R-25 superlattice-reflection intensities, and the pseudo-tetragonal lattice distortion of the perovskite sublattice showed the same temperature dependence below room temperature, showing a saturation below about 60 K. Above room temperature, however, they showed rather different temperature dependences. These temperature dependences can be well described by the free energy based on a group theoretical method, which includes a quantum effect. The atomic shifts do not necessarily conform to a simple concept of order parameter in soft mode condensation. However the antiferroelectric phase transition can be understood by the phenomenological theory for coupled order parameters if applied over the whole temperature region.