  LaB6 has been used as a commercial electron emitter for decades. Despite the
large number of studies on the work function of LaB6, there is no comprehensive
understanding of work function trends in the hexaboride materials family. In
this study, we use Density Functional Theory (DFT) calculations to calculated
trends of rare earth hexaboride work function and rationalize these trends
based on the electronegativity of the metal element. We predict that alloying
LaB6 with Ba can further lower the work function by ~0.2 eV. Interestingly, we
find that alloyed (La, Ba)B6 can have lower work functions than either LaB6 or
BaB6, benefitting from an enhanced surface dipole due to metal element size
mismatch. In addition to hexaborides we also investigate work function trends
of similar materials families, namely tetraborides and transition metal
nitrides, which, like hexaborides, are electrically conductive and refractory
and thus may also be promising materials for electron emission applications. We
find that tetraborides consistently have higher work functions than their
hexaboride analogues as the tetraborides having less ionic bonding and smaller
positive surface dipoles. Finally, we find that HfN has a low work function of
about 2.2 eV, making HfN a potentially promising new electron emitter material.
