Structural characterization of amorphous alumina and its polymorphs from first-principles XPS and NMR calculations

Abstract

We have calculated x-ray photoemission (XPS) spectra and nuclear magnetic resonance (NMR) chemical shifts for amorphous alumina from first principles. We generate models for amorphous structures at three different densities by means of a stochastic quenching procedure. We analyze these structures by calculating radial distribution functions, angle distributions functions, bond lengths, and coordination numbers. Our amorphous models compare well with previous molecular dynamics simulations and experiments. We include in our study, the stable phase of alumina $\ensuremath{\alpha}$, some of the transition phases, that is, $\ensuremath{\theta}$, $\ensuremath{\gamma}$, and $\ensuremath{\kappa}$, and the hypothetical bixbyite structure for comparison. Our results reproduce both XPS spectra and NMR chemical shifts and suggest that the XPS failure to resolve the different local environments in the different phases of alumina is due to the strong ionicity of the Al-O bond. Our calculated NMR chemical shifts show that the local environments are well resolved. We estimate the broadening of the NMR peaks due to local atomic environment differences in the amorphous phase to be as large as 35 ppm.