Structural analysis of clean LaB_6(100), (111), and (110) surfaces via quantitative low-energy electron diffraction

摘要

LaB6 has very important features for electron emission, but its proposed surface structures of the fundamental planes contradict the work function values. In this study, we determined the surface structures of the LaB6 single crystal via quantitative low-energy electron diffraction analysis. On the (100) plane, the surface was terminated by a La atom, which relaxed toward the bulk crystal side. On the (111) and (110) planes, the surfaces were terminated by a La atom and a cluster of B, but the cluster had smaller number of B atoms than the B-6 octahedron. Because the B-6 octahedron is quite stable, it is likely that the surfaces are terminated by a B-6 octahedron. In contrast, our structural analysis revealed that the surface of the (111) plane was terminated by a B-3 triangle. The surface of the (110) plane has a c(2 x 2) periodicity, and we obtained three possible models according to our analysis. We cannot tell which of these models exists on the surface, and they can coexist on the surface. These three models have similar structures; however, model 1 has B-4 termination, model 2 has B(4 )and B(3 )mixed termination, and model 3 has B-3 termination. The determined structures for the (111) and (110) planes are very different from previously suggested models. Based on our surface analysis, the work function values of the above surface planes of the LaB6 single crystal can be reasonably explained.