Abstract
The tilt boundary that separates the well-known rhombohedral phases R3m and R3c of the Pb_Zr1−xTix_O3 solid solution was investigated by means of time-of-flight neutron powder diffraction, dielectric measurements, and resonance Raman spectroscopy. Careful Rietveld refinements yield significantly better agreement factors using monoclinic models for the long-range structure of PbZr0.6Ti0.4O3 with a Cc space group at low temperature and with a Cm space group at high temperature rather than the widely accepted rhombohedral symmetry. The spontaneous polarization was found to lie along the pseudocubic _112_ direction instead of along the _111_ direction. This polarization direction is shown to be adopted for a wide variety of compositions, temperatures, and pressures and has major implications for the domain structure of these materials. Furthermore, the transition between the untilted and the tilted structures was found to be very diffuse and strongly dependent on the thermal history of the sample.