Showing papers in "The Journal of Physical Chemistry in 1970"

Composition and surface structure of the (0001) face of .alpha.-alumina by low-energy electron diffraction

Abstract: ~ The (0001) crystal face of ci-alumina undergoes. a change of surface structure upon heat treatment in vacuum above 1250°C. This order-order phase transformation [(Al(OOOl) (1 x 1) _.,_rotated ( ~31 x ~ 31)] is accompanied by a change in the chemical composition of the surface, i. e. by the loss of oxygen. The structural change is reversible and either surface structure can be obtained alternately by heat treatment of the samples in oxygen (>lo-4 torr) or in the presence of excess aluminum on the surface. The large unit cell which characterizes the high-temperature oxygen-deficient surface structure is indicative of a marked mismatch between the surface layer and the underlying (0001) crystal orientation. Evidence is presented to s,how that the aluminum cation in the high temperature surface structure is in a reduced valence state and that the structure in composed of a cubic overlayer on the hexagonal ~~alumina substrate. It appears that compounds with unusual oxidation states which would not be stable in the solid state may be stabilized in the sur'face environment.