Whereas prototypical Al(2)O(3) is not a glass former, amorphous Al(2)O(3) can be formed as thin films through vapor deposition and can serve as a structural model for the Al(2)O(3) glass. The first two-dimensional solid-state NMR experiments for amorphous Al(2)O(3) thin film reveal that four- and five-coordinated species are predominant (95%), while six-coordinated species are minor. Such a species distribution is remarkably similar to what has been predicted theoretically for Al(2)O(3) melts. Upon annealing to 800 degrees C the five-coordinated species becomes negligible, indicating the onset of crystallization of Al(2)O(3).

Revealing the extent of disorder in amorphous oxides is one of the remaining puzzles in physical chemistry, glass sciences, and geochemistry. Here, we report the 27 Al NMR results for amorphous Al 2 O 3 thin films obtained from two different deposition methods (i.e., physical vapor-deposition and atomic layer-deposition), revealing two distinct amorphous states defined by a fraction of five-coordinated Al ( [5] Al). The fractions of [4] Al and [5] Al are dominant (∼92-95%) in both films. While the overall similarity between these two states suggests a narrow stability of available amorphous states, the fraction of [5] Al in atomic layer-deposited thin films is apparently larger and thus more disordered than that in physical vapor-deposited films. Such results require that varying extents of disorder exist in the amorphous oxides prepared under different processing conditions. As the [5] Al site (<1%) in crystalline Al 2 O 3 is known to control its catalytic ability over [4] Al and [6] Al, the significant fractions (∼40%) of [5] Al in our amorphous thin films suggest that amorphous Al 2 O 3 may be potentially useful as a new class of catalysts.

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