The Journal of Solid State Chemistry is pleased to present this special issue on “Polar Inorganic Materials: Design Strategies and Functional Properties”. Polar materials, i.e., materials that exhibit a dipole moment, and are found in one of ten polar crystal classes (1, 2, 3, 4, 6, m, mm2, 3m, 4mm, 6mm), are of academic and commercial interest attributable to their technologically relevant functional properties. These materials are used in burglar alarms, pollution monitors, and thermal detectors, as well as non-volatile memories, capacitors, actuators, and non-linear optical technologies. Polar materials represent a vast and growing field, and the concept papers, reviews, and research articles published in this special issue span a range of topics and include design strategies, synthesis, characterization, functional properties, and theoretical calculations.
Many of the contributions in this special issue involve the design and synthesis of new polar compounds, as well as advanced characterization techniques. Advances in polar materials will only come about through the synthesis of new compounds, oxides, oxyfluorides, halides, chalcogenides, and organic–inorganic hybrid materials, in combination with detailed physical property measurements, as well as theoretical calculations. A major challenge with polar materials is their design and synthesis—how does one create, synthesize, a new polar material? The contributions that describe new polar materials reflect a growing sophistication and skill with respect to answering this fundamental question. Specific design strategies are described by numerous authors and provide profound insights into these materials.
Technologically important functionalities that are observed exclusively in polar materials include pyroelectricity, ferroelectricity, and multi-ferroic behavior. Piezoelectricity and second-harmonic generation may also be observed in polar materials, however, with these phenomena polarity is not strictly required, i.e., a material may exhibit piezoelectric and second-harmonic generating phenomena without being polar. With the properties exclusive to polar materials, the pyroelectric effect may be defined as the change in the spontaneous polarization of the material as a function of temperature, whereas a ferroelectric can be formally defined as a pyroelectric material that has reversible, or ‘switchable’ polarization . That is, in a ferroelectric material the macroscopic polarization, or dipole moment, can be reversed in the presence of an external electric field. Thus all ferroelectrics exhibit pyroelectric behavior, but the converse is not true. For multi-ferroic materials, a material is considered multi-ferroic if at least two primary ferroic properties occur in the same material . This definition has been recently expanded . The interest with multi-ferroic materials stems from their applications in information storage. Suggestions have been made that on a single multi-ferroic bit, information could be independently encoded by both polarization and magnetization .
As research in polar materials continues to grow, the proper characterization of the aforementioned functionalities is crucial. This characterization, e.g., pyroelectric current, ferroelectric hysteresis loops, magneto-electric behavior, etc., enables us to better understand the host of structure–property relationships associated with polar materials. Through this understanding, these design methodologies may be refined toward the ‘rational’ synthesis of new polar materials. Characterization of polar materials, whether in bulk, as single crystals, or as thin films are described in many of the contributions. As the design, synthesis, and characterization of new polar materials continues to expand, an equally impressive renaissance has occurred involving theoretical calculations. Here first-principle calculations have replaced semi-empirical ball-and-stick models. These calculations provide great insight with respect to suggesting new polar materials, as well as their relative stability. A number of contributions to this special issue discuss theoretical advances with respect to designing and understanding polar materials, as well as their associated functional properties.
The special issue is organized as follows—the first few contributions are reviews or concept papers on polarization, multi-functional behavior in polar materials, as well as designing new polar compounds. Polarity, in specific structure types, is then discussed by several contributors. We move on to specific polar materials—borates, mixed-metal oxides, oxyfluorides, halides, and chalcogenides. As can be clearly seen from the articles in this special issue, we are in a position to build and expand upon past discoveries, and develop a deeper understanding of polar inorganic materials with respect to their design strategies, synthesis, structure–property relationships, and theoretical understanding. Without a doubt the future for polar inorganic materials will be exciting and full of surprises!
Finally, I would like to extend my personal thanks to all the authors who contributed to this special issue, as well as to the editorial staff of the Journal of Solid State Chemistry for their efforts in seeing this issue to completion.