Structured diffuse scattering and the fundamental 1-d dipolar unit in PLZT (Pb_1-yLa_y)_1-α(Zr_1-xTi_x)_1-βO₃ (7.5/65/35 and 7.0/60/40) transparent ferroelectric ceramics

The observation via electron diffraction of relatively sharp, G±{111}* sheets of diffuse intensity arising from the large amplitude excitation of inherently polar, transverse optical modes of distortion in [(Pb_1-yLa_y)_1-α□_α][(Zr_1-xTi_x)_1-β□_β]O₃ (PLZT), 7.5/65/35 and 7.0/60/40, samples close to the morphotropic phase boundary in this system shows that the fundamental dipolar units in these materials correspond to highly anisotropic 〈111〉 chain dipoles formed from off-centre Pb/La and coupled Ti/Zr displacements. The correlation length along the chain of these 1-d dipoles can, in principle, be determined from the width of the observed {111}* diffuse sheets in reciprocal space and is estimated to be at least 2-3 nm. The primary role of the dopant La ions appears to be to set up random local strain fields preventing the condensation of long wavelength homogeneous strain distortions of the unit cell thereby suppressing transverse correlations of the fundamental 〈111〉 chain dipoles and the development of macro-, or even nano-scale, ordered ferroelectric domain state/s in the absence of an applied external electric field.