Complex Cooperative Phenomena in
Disordered Ferroelectrics and Dielectrics
Dr. George Samara
Sandia National Laboratory
1515 Eubank S.E., MS 1421
Albuquerque, NM 87123
(505) 845-0011
There has been much recent interest in the dynamic and static properties of systems in which randomly competing interactions cause the formation of a glass-like state at low temperatures. Much of this work has dealt with disordered magnetic systems, e.g., systems containing mixtures of competing ferromagnetic/antiferromagnetic interactions, which lead to the formation of spin glass state. A large body of work has also dealt with structural glass systems. In discussing the properties of structural glass systems, we need to distinguish between amorphous solidsóthe so-called topological glasses which have no crystallographic long-range order, e.g., amorphous silica and many polymersóand structurally site-disordered crystalline solids (or orientational glasses). Our interest is in the latter class of solids of which so-called relaxor ferroelectrics are a subclass.
On cooling, these systems exhibit a slowing down of the relaxation of their orientational degrees of freedom, ultimately resulting in a frozen-in, frustrated multipole state with no long-range orientational order. A universal signature of such disordered solids is a relatively sharp, frequency-dependent peak in the temperature-dependent susceptibility. The peak defines a dynamics freezing, or glass transition, temperature. Despite the fairly general character of the phenomena associated with the orientational freezing process, many rather fundamental questions have arisen about the physics involved. Among these questions are the following: What is the nature of the transition? What are the consequences of the failure of the system to reach thermal equilibrium? Detailed theoretical understanding of these and other questions has proven difficult because of the random and, often, frustrated nature of the interactions.
In ferroelectrics, relaxor (R) behavior results from either frustration- or compositionally- induced disorder. The latter type of disorder and related random fields are believed to be responsible for the relaxor properties of mixed perovskite oxides. A newly recognized feature in the response of some of these materials is the observation of a spontaneous, first-order R-to-normal ferroelectric (nFE) transition in the absence of a poling electric field. The physics of the relaxor behavior and R-nFE transition in these systems is not well understood. The usual way to study the properties has been to vary the composition and degree of disorder to induce relaxor behavior. However, this approach introduces complications such as added randomness, compositional fluctuations, lattice defects and changed interatomic forces. Consequently, there is always considerable vagueness in the interpretation and understanding of experimental results. Alternatively hydrostatic pressure has been found to be a ìcleanerî variable in studying such systems. By applying pressure to a sample of fixed composition one varies only the interatomic interactions and balance between long- and short-range forces, making it easier to get to the essential physics. A limited amount of work has been done using this complementary approach which holds much promise.