Surface science studies of metal hexaborides

Over 30 years of surface science research on metal hexaborides are reviewed. Of this class of compounds, lanthanum hexaboride has been the subject of the majority of the studies because of its outstanding properties as a thermionic emitter. The use of LaB₆ cathodes as an electron source stems from the unusually low work function of ∼2.5 eV for the (100) surface combined with a low evaporation rate at high temperatures. Of particular interest has been the determination of the surface geometric and electronic structure responsible for the low work function and how the work function is affected by various adsorbates. The low-index faces of single crystals of LaB₆ and other hexaborides have been studied with a variety of ultrahigh vacuum surface science methods to gain a better understanding of the structure and properties of the clean surfaces as well as their interactions with gases such as O₂, H₂O and CO.