Neutron powder diffraction study of Pb(HfxTi1-x)O3 ceramics (0.10 ≤ x ≤ 0.50)

The crystal symmetries of lead hafnate titanate (Pb(HfxTi1-x)O-3, PHT) powders with 0.10 <= x <= 0.50 were investigated by high-resolution neutron powder diffraction. Samples with x <= 0.40 were tetragonal (space group P4mm), while the sample with x = 0.50 contained both monoclinic Cm and rhombohedral (modeled using the R3c space group) phases. The role of the B cations (Hf and Ti) and the oxygen octahedra network, in addition to the displacement of Pb ions from their ideal sites, in promoting the phase transformation between the P4mm and Cm phases was considered. Two types of structural disorder were identified. Diffuse scattering between Bragg reflection peaks was assigned to Pb ion displacement. A second type of structural disorder, revealed by the weak intensities of observed pseudo-cubic 00/ reflections with / even and as 001 reflection peak widths significantly broader than the 100 reflection peaks, was observed. This behavior was attributed to disorder in the arrangement of the O-B-O rows parallel to the c axis. For small values of x, this shift was predominantly along the c axis, whereas shifts perpendicular to the c axis increased with increasing x. These features were modeled using an hkl-dependent line-broadening model. The origin of the W-dependent line broadening was assigned to the microstrain accompanying a spatial-composition variation. Structural models were tested by computing valence sums and spontaneous polarization values.