Surface science studies of metal hexaborides

Abstract

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 LaB 6 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 LaB 6 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 O 2 , H 2 O and CO.

Surface properties ofSmB6from x-ray photoelectron spectroscopy

“…The surface termination of hexaborides has been systematically studied, motivated by the exceptionally low work function of LaB 6 and its use as a thermionic emitter [34]. LaB 6 has been found to be metal terminated [35], for SmB 6 partial or full Sm termination has been suggested [35,36].…”

Termination-dependent surface in-gap states in a potential mixed-valent topological insulator:SmB6

“…[16][17][18] Due to this reason, the topological nature arising from the intrinsic surface states is not easily confirmed experimentally. Another difficulty with SmB 6 is the strong correlation effect of f electrons.…”

Distribution of boundary planes in a (La0.67Nd0.33)B6polycrystalline bulk prepared by spark plasma sintering

“…It is generally accepted that for a promising thermionic cathode candidate, a smaller work function is indispensable to acquire a higher thermionic emission current density. 5,32 Meanwhile, abundant literature has reported that the work functions (Φ) of hexaborides are closely related to the indices of crystalline interfaces; the order of work functions for dif- 5,11,12,[33][34][35][36][37][38] In virtue of the GBPD results illustrated in Fig. 2a and 3a, it can be clearly observed that the most frequently occurring boundary planes are, successively, {001}, {011} and {111} planes, and boundaries are more frequently terminated by {001} planes rather than any other orientations.…”

“…8 Moreover, the size and types (energy, angle) of grain boundaries are significant to both the electron emission mechanism 9 and the carrier transport pathway; 10 with respect to the work function (defined as the minimum energy it takes for the electrons to be extracted from a metal), some theoretical studies suggest that substitution of lanthanum with barium atoms on the LaB 6 (100) surface could astonishingly reduce the work function to 1.43 eV. 11,12 With respect to the Richardson constant, it has been proven that such a constant could differ between either different emitting materials or various crystallographic faces of the same material. 13 In brief, to obtain an electron emissive material with higher performance, characterization of the occurrence frequency of boundary (including grain boundary and phase boundary) planes in the mixed LaB 6 structure is an important issue that should not be ignored.…”