Perspective on emerging views on microscopic origin of relaxor behavior

Relaxors are complex oxide ferroelectrics that are of interest because of their attractive dielectric and electromechanical properties. A broad frequency-dependent dielectric permittivity peak is usually considered as a characteristic feature of relaxors. In spite of decades long research, a microscopic model of relaxors has not been conclusively established. In recent years, this field has experienced renewed interest driven by mainly two factors: (a) discovery of new Pb-free relaxors in response to toxicity concerns about Pb-based materials in electronics and (b) advancements in experimental and theoretical techniques that provided new microscopic insights. The objectives of the current review are the following. First, we will provide a description of some important Pb-free relaxors with particular reference to their dielectric properties. Second, we will review the classical microscopic models of relaxors and discuss how these models have evolved over the last decade. The opportunities in this regard provided by the emergence of new Pb-free relaxors will be highlighted.