Complex impedance spectroscopy and dielectric relaxation studies of lead-free layered perovskite Bi<inf>4−x</inf>La<inf>x</inf>Ti<inf>3</inf>O<inf>12</inf> ceramics: a ferroelectric to relaxor crossover

Structure, microstructure, complex impedance spectroscopy, and dielectric relaxation studies were carried out on La-substituted Bi4−xLaxTi3O12 lead-free ceramics (x = 0 to 1.5), which were synthesized by solid-state reaction method. Structural and microstructural analysis depicted orthorhombic (O) to tetragonal (T) transformation and decrease of average grain size from 20 to 2 μm with increasing “x.” Complex impedance spectroscopy studies showed contribution of grain boundary, plate boundary, and grain interior of Bi4−xLaxTi3O12 ceramics to its overall electrical properties. The results revealed substitutional site preference of the La3+-ions to the Bi ions of the (Bi2Ti3O12)2−perovskite unit for x < 1 and at the Bi-site of the Bi2O22+ layer for x ≥ 1. The temperature-dependent dielectric studies revealed ferroelectric to relaxor crossover for x = 1.5 implying reduction in interaction within the polar regions. The observed relaxor-like dielectric behavior analysis suggests generation of small-sized polar regions because of random fields generation by diminishing charge compensating role of Bi2O22+layer with high La3+-doping. Random weakly interaction between the small polar regions leads to critical slowing down of polar regions dynamics below 119 K.