Optimisation of functional properties in lead-free BiFeO3–BaTiO3 ceramics through La3+ substitution strategy

The paper is concerned with the structure and functional properties of bismuth ferrite–barium titanate, 0.75BiFeO3–0.25BaTiO3, solid solutions. Such materials are attracting attention due to their potential applications in high temperature piezoelectric transducers for use in demanding environments in process monitoring, for example. The article focuses on the mechanism of incorporation of the minor dopant lanthanum oxide, either in the form of isovalent or donor-type substitution. It is shown that the development of chemical heterogeneity in the form of core–shell grain microstructures, linked to donor-type substitution, plays a key role in controlling the functional behaviour. The use of an air-quenching procedure results in dramatic improvements in the ferroelectric properties, accompanied by a transformation of the shell regions from a pseudo-cubic, Pm3m, to rhombohedral, R3c, structure. These observations are interpreted in terms of a novel mechanism involving nanoscale phase separation in the ‘shell’ regions during slow cooling, which impedes the development of ferroelectric ordering and leads to the formation of a nano-polar relaxor ferroelectric state. The work highlights the importance of immiscibility in bismuth-based ferroelectric perovskite solid solutions and illustrates how their ferroelectric, piezoelectric, dielectric energy storage, ferromagnetic and magneto-electric properties can be tuned by consideration of the substitution mechanism and control of thermal processing parameters.