ИСТИНА

The growing need for alternative energy sources demands new and better materials for energy generation, storage, and conversion. New thermoelectric materials with high electric conductivity, low thermal conductivity, and high thermoelectric power are of high interest due to their potential application in freon-free refrigerators, waste-heat converters, and direct solar thermal energy converters. Clathrate crystals attract attention due to their unique structure that includes a nanoscale framework of ordered atoms, with another kind of atoms occupying guest positions, forming no covalent bonds to the framework. Recently a new type of Zintl phases, such as Sn-In-As-I clathrates with advanced thermoelectric properties, has been proposed [1]. However, potential application of such clathrate-based materials is limited by instability of their surfaces in air due to oxidation. Therefore, the information on the surface oxidation kinetics is of high demand. The oxidation process can also be employed to create a passivation layer with well-defined structure and composition. It may be supposedly formed by the compound’s native or thermal oxide under optimized oxidation conditions. Apart from the application issues, the fundamental properties of the Zintl phases are also of high importance. It has been already established that the compounds formed in the Sn-In-As-I system exhibit great diversity of crystal structures; the phonon spectra of the crystals are unusual. The information on the clathrate structure, electronic properties and their reactivity also ensures further development of theoretical chemistry. We report here the atomic mechanism of Sn-In-As-I clathrate oxidation by molecular oxygen probed by NAP XPS at different pressure and temperature. By variation of oxygen pressure (10-4- 0.1 mBar) we found the conditions for the fast formation of a thin oxide layer at the surface. The oxidation process even by molecular oxygen seems to be rather complex due to multicomponent nature of the system. Real time measurements of the surface composition and observation of the different spectral features related to different chemical states of Sn, In and As allow us to suppose the atomic mechanism of the oxidation. The oxidation process occurs in two steps, the rate of the first one is strongly depends on oxygen pressure, with essential surface enrichment in tin promoted by temperature increase. The oxide layer obtained at room temperature does not passivate the surface. The passivation is achieved at temperature higher than 150OC.