Grain size effect and local disorder in polycrystalline relaxors via scanning probe microscopy

Local piezoresponse of individual grains in polycrystalline Pb0.9125La0.0975(Zr0.65Ti0.35)0.976O3 (PLZT 9.75/65/35) relaxor ceramics is studied using the scanning probe microscopy (SPM) technique. The observed piezoelectric contrast consisting of irregular (labyrinth-type) polarization patterns is attributed to the compositional disorder and consequent charge imbalance caused by high La concentration. A measure of this disorder, the polarization correlation length ξ, is directly determined using an autocorrelation analysis function implemented in the SPM software. The analysis of the obtained images shows that ξ taken at the scale ~200 nm varies as a function of the position inside the grain notably decreasing upon approaching the grain boundary. As a result, the average correlation length taken over the entire grain is apparently dependent on the grain size saturating at about 2 µm. The obtained dependences are consistent with both the mechanical stress effect of impinging grains and the composition gradient in the PLZT grain and allows for better understanding of the dielectric properties of disordered ferroelectrics.