Neutron Powder Diffraction Study of Pb(HfxTi1-x)O3 Ceramics (0.10 ≤ x ≤ 0.50)

The crystal symmetries of lead hafnate titanate (Pb(Hf x Ti 1 - 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 P 4 mm ), while the sample with x = 0.50 contained both monoclinic Cm and rhombohedral (modeled using the R 3 c 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 P 4 mm 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 l reflections with l even and as 00 l reflection peak widths significantly broader than the l 00 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 hkl -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.