Composition dependence of the diffuse scattering in the relaxor (1 - x) Pb (Mg_{1 / 3} Nb_{2 / 3}) O₃-x PbTiO₃ (x = 0 and 0.10)

We have measured the neutron diffuse scattering in the relaxor (1 - x) Pb (Mg_{1 / 3} Nb_{2 / 3}) O₃-x PbTiO₃ single crystals with x = 0 % and 10% around (1 1 0) and (1 0 0) to study the effect of Ti substitution on polar nanoregions (PNR). Both the x = 0 % and 10% samples exhibit weak diffuse scattering extending along [1 1 0] at (1 1 0) above 500 K. In addition, we observed strong diffuse scattering below T = 500 K, which extends along [1 over(1, -) 0] at (1 1 0) and [1 ± 1 0] at (1 0 0) . The correlation length ξ derived from the low-T diffuse scattering indicates that the average size of the PNR is enlarged by the substitution of Ti⁺⁺ for (Mg_{1 / 3} Nb_{2 / 3})⁺⁺. For PMN-10% PT, the high-T diffuse scattering, which extends along the longitudinal direction at 650 K, expands along the transverse direction on cooling. This change can be understood in terms of the soft mode condensation. As the temperature decreases below 500 K, the weak high-T diffuse scattering becomes almost isotropic, and the low-T diffuse scattering extending along the transverse direction becomes dominant.