CHARACTERIZATION OF CLEAN AND OXIDIZED (100)LAB6

We have studied clean and oxidized (100)LaB6 grown from aluminum melts by making Auger, LEED, evaporation and work function measurements on well-defined surfaces. The clean surface shows no La enrichment when initially heated as high as 1700°C. Its LEED pattern is 1 × 1, indicating no surface reconstruction. Langmuir evaporation studies up to temperatures of 1700°C show only La and B evaporating non-congruently, and LaO. The activation energy for B evaporation from LaB6 (and from CeB6 and EuB6 also) is abot 5.5 eV, very close to that from elemental B. The rare-earth activation energies, however, vary, being highest for the rare-earth whose pure metal vapor pressure is lowest. Oxidation was carried out at room temperature using O2 pressures up to 10-7 Torr and at 1000°C using O2 pressures up to 10-4 Torr. At room temperature oxygen adsorption proceeeds to a saturated value indicated from LEED behavior to be about one monolayer. It produces a monotonic work function increase, which also saturates (at 1.40 V), varying linearly with the oxygen uptake. Oxidation at 1000°C is much more extensive than at room temperature, involving at least several monolayers, and results in a work function increase of 2.42 V. Results are discussed in terms of a terminal plane composed of La atoms, and adsorbed oxygen which, when given sufficient mobility, prefers bonding to La atoms at sites atop the B octahedra. © 1978.