Effects of crystal chemistry and local random fields on relaxor and piezoelectric behavior of lead-oxide perovskites

Random electric fields caused by the disordered distribution of heterovalent cations over the equivalent crystallographic positions are often considered to be the main reason of the relaxor behavior and the associated giant electromechanical response. Some models relate the development of the relaxor state to the crystal chemical properties of the constituent ions. In this work the functions of random fields and ferroactive cations are compared in perovskite solid solutions in the desirable for technological applications composition range of morphotropic phase boundary. The crystal structure, dielectric and ferroelectric properties, piezoelectric effect and electrostriction are investigated in technologically important ceramic solid solutions Pb z Ba 1− z (Mg 1/3 Nb 2/3 ) m (Zn 1/3 Nb 2/3 ) y (Ni 1/3 Nb 2/3 ) n Ti x O 3 with substitutions in A (Pb 2+ by Ba 2+ )- and B (Ti 4+ by [B 2+ 1/3 Nb 5+ 2/3 ] 4+ ) perovskite sublattices. Two composition sections of the system are designed; in both of them the concentration of ferroactive ions, Pb 2+ and Ti 4+ , respectively, varies within the same range of 15%, but the strength of quenched random electric fields changes significantly only in B-substituted ceramics. Similar behavior is found in both sections, including the same sequence of structural phase transitions and transformations from normal ferroelectric to relaxor state and similar evolution of properties with concentration. The results clearly demonstrate that the role of crystal chemical factors in the development of relaxor behavior and electromechanical performance of lead-oxide materials is not less important than that of quenched random fields. The mechanisms of the observed behavior are discussed in the frame of existing theoretical models.