Condensed Matter Seminars

Relaxors have been extensively studied for a variety of applications as ideal dielectric materials because they often exhibit extremely large dielectric and piezoelectric constants while the dielectric loss and temperature dependence are small. Although it is now widely believed that heterogeneity embedded and appearing in relaxors are relevant to various phenomena specific to relaxors, there is still no established microscopic theory for relaxors. The prototypical relaxors Pb(Mg1/3Nb2/3)O3 (PMN) consists of Pb2+ on the A site of the ABO3 Perovskite structure and Mg2+ and Nb5+ on the B site. To keep the charge neutrality, Mg2+ and Nb5+ have to form a solid solution with a ratio of 1:2 so as to have an average valence of 4+. However, it is likely that a large difference between the ionic radius of Mg2+ and that of Nb5+ prefers the 1:1 solid solution resulting in an alternating arrangement of Mg2+ and Nb5+. The 1:2 state and the 1:1 state are mutually exclusive, thus the system falls in a state of frustration. The concept of frustration has been studied almost exclusively in magnetism, e.g., a geometrical frustration in an antiferromagnetic triangular lattice, though phenomena related to frustration are widely seen in nature. Since there is no unique ground state in a frustrated system, the system becomes unstable among various different states, which may cause large fluctuations leading to a extremely large susceptibility against an external field and to a novel exotic phase. We now consider that heterogeneous structures appearing in relaxors can originate from such a frustration. In this presentation, a review is given on a series of neutron and x-ray scattering experiments on spatial structures and dynamics of polar nano regions in relaxors. We then would like to discuss how such experimental results can be understood in the frame work of frustration. We also discuss what we will be able to study the microscopic mechanism of relaxors by controlling the frustration through lattice distortion, charge imbalance and dimensionality.