Hexaborides: a review of structure, synthesis and processing

Hexaboride compounds are a subset of borides with many interesting thermal, electrical, magnetic and physical properties. As such, interest has grown in this class of ceramics not only to understand their peculiar characteristics, but also to apply them to modern scientific and engineering problems. Hexaborides are currently used or show promise for a variety of applications such as electron emitters, thermoelectric materials, coatings, single-photon detectors and superconductors. A shared crystal structure amongst all hexaborides permits them to readily form solid solutions, allowing for fine-tuning of electrical, optical and thermal properties with mixed-metal compounds. Recent advances in characterization techniques have allowed researchers to gain new insights and renewed interest in some of the more exotic hexaboride behaviors such as surface conductivity in SmB6 or magnetic ordering in CeB6. Like many other non-oxide ceramics, borides present certain synthesis and processing challenges. This paper presents an overview of the structure, synthesis and processing of hexaboride compounds in an effort to guide researchers with the first step in pursuing these interesting and unique materials.