Effect of cationic substitutions on ferroelectric properties of Pb5Ta10O30:: Generation of the relaxor behavior

New compositions derived from Pb5Ta10O30 have been synthetized to study the influence of cationic substitutions [heterovalent (Pb2+ --> La3+ and Pb2+ --> K+) and homovalent (Pb2+ --> Ba2+ and Pb2+ --> Ca2+)] on its ferroelectric properties. The ferroelectric behavior of each composition was evidenced by way of dielectric measurements as a function of both temperature and frequency. Structure determinations and refinements were also performed to determine which crystallographic sites are occupied by the substituted cations, Whatever the nature of the substitution, the relaxer behavior is induced by a weak rate of substitution, Furthermore, except in the case of the calcium-containing composition, no significative difference was found concerning crystallographic data. All the results obtained are compared with those described in the literature and discussed in terms of charge, ionic radius, and steric effect. (C) 2001 Academic Press.