RAMAN-SPECTRA OF LA2O3 DISPERSED ON GAMMA-AL2O3

Raman spectra have been obtained for La2O3 dispersed on Al2O3, but onl y after La2O3 coverages reach one theoretical monolayer or higher. Sha rp peaks at 104, 191, and 408 cm(-1) were observed, which are in excel lent agreement with previous studies of bulk La2O3; however, two addit ional broader, but intense bands not previously reported were also obs erved at 322 and 935 cm(-1). These latter two bands disappear at tempe ratures of 450 K or higher, but they returned with full intensity upon cooling to 300 K. A sample of bulk La2O3 (a-form) with a low surface/ volume ratio also exhibited weak peaks at these latter two positions. Raman spectra taken with different exciting laser lines showed that th e bands at 322 and 935 cm(-1) can be attributed to a laser-excited lum inescence. It is inferred that these luminescence bands are evidence f or the presence of oxygen anion vacancies. After exposure to NO at 300 K, two peaks developed at 747 and 1047 cm(-1) which can be assigned t o a surface nitrate species. (C) 1998 Academic Press.