DIRECT OBSERVATION OF LAB6(001) SURFACE AT HIGH-TEMPERATURES BY X-RAY AND ULTRAVIOLET PHOTOELECTRON-SPECTROSCOPY, LOW-ENERGY ELECTRON-DIFFRACTION, AUGER-ELECTRON SPECTROSCOPY, AND WORK-FUNCTION MEASUREMENTS

The properties of the LaB6(001) surface which is in equilibrium with oxygen (ultrahigh vacuum - 10-6 Torr) at high temperatures (room temperature ∼1500 °C) have been investigated by XPS, UPS, LEED, AES, work-function measurements, and so on. In the case of the clean surface (experiments under ultrahigh vacuum), the chemical composition of the surface does not change up to ∼1500 °C, although lanthanum ions which form the outermost layer of the surface begin vigorous thermal motion at ∼1100 °C. A surface electronic state observed at room temperature undergoes no essential change even at ∼1400 °C. Under an oxygen atmosphere, the surface is covered by an oxide and has a high work function below a critical temperature Tc which depends on the oxygen pressure PO2. Above Tc, the surface oxide, which is rich in lanthanum content, evaporates (the activation energy of the evaporation is 7.8 eV), and the chemical composition and work function of the surface become identical to those of the clean surface under ultrahigh vacuum. When PO2 is 1×10-6 Torr, for example, Tc is 1325 °C.