Metal Hexaboride Work Functions: Surface Configurations and the Electrical Double Layer from First-Principles


Metal hexaborides are ionic crystals having exceptional thermochemical stability and unique electrochemical behavior, including low work functions, making these materials ideal for electron emission applications. Here, the importance of specific interfacial properties for the development of efficient cathode devices is examined. In particular, the electrostatic behavior of BaB6, CaB6, LaB6, and SrB6 surfaces having various terminal configurations and surface coverage is studied using a repeated symmetric slab model with self‐consistent field calculations. The calculated work functions for each system are found minimal for a monolayer cation coverage and monotonically approach the experimental value for pure boron as cations are subsequently removed. Using three patterns to sample the surface space on stoichiometric slabs with identical surface composition, it is found that configurational changes can substantially alter the calculated work function. Further examination of the bonding charge densities suggest the low work functions typically observed in these materials are mainly due to the electrical double layer formed at the surface, rather than an inherent bulk property. This provides evidence suggesting that a variety of experimental observations may be due to surface features resulting from different preparatory methods. Significant electron density shifts are observed only few atomic layers deep leaving most of the bulk material unaltered.

Advanced Electronic Materials