Structural Transitions in Alumina and Aluminate Glasses and Melts: Implication for Crystallization of Magma Ocean

Abstract

Understanding the structure and disorder of prototypical oxide glasses provides improved insights into geochemical processes involving magmas in the Earths surfaces and interiors. Al2O3, one of the archetypical covalent oxides, is not a glass-former, unlike most other covalent oxides such as SiO2, GeO2, and B2O3, hence an amorphous phase cannot be obtained by conventional melt-quenching. Amorphous Al2O3 can be formed as thin films through varying deposition processes, such as physical vapor deposition (PVD) and atomic layer deposition. It can also be synthesized through hydrolysis of Al-organic ligands by sol-gel process. Whereas full understanding of atomic arrangements of amorphous Al2O3 has remained an unsolved problem due to the lack of suitable experimental probes of amorphous oxide surfaces, progress in high-resolution 2-dimenional solid-state NMR techniques (e.g. 3QMAS) enables us to explore previously unknown details of Al coordination environments in oxide glasses. Lee et al. (2009) reported the first Al-27 3QMAS NMR spectra for physically and chemically deposited Al2O3 glass and the species distribution is remarkably similar to what has been predicted theoretically for Al2O3 melts (Lee et al., 2009

Lee et al., 2010). In contrast to a small sample volume typically ~nm-µm-thick films of amorphous Al2O3 through thin film deposition, sol-gel synthesis of Al2O3 glass using aluminum lactate as an attractive precursor yields much larger sample volume, shedding light on a new opportunity to collect an NMR spectrum with significantly improved signal/noise ratio. This allows us to explore the detailed temperature-induced structural transitions in amorphous alumina. Hydrolysis of Al-lactate at room temperature was accompanied by the annealing of the transparent xerogel at approximately 720 K. 27Al 3QMAS NMR spectrum for the sol-gel synthesized Al2O3 glass exhibits well resolved Al coordination environments, characterized with mostly [4,5]Al and a minor fraction of [6]Al. This species distribution shows the remarkable similarity to that of thin films, suggesting narrow stability of amorphous states in the amorphous Al2O3. The unique temperature-induced structural changes in amorphous Al2O3 have been observed. The fraction of [5]Al decreases with increasing annealing time and temperature. We show, for the first time, that two distinct processes (structural transitions within glass and crystallization) can indeed be experimentally observed by probing the fraction of [5]Al. The results also demonstrate that structural transition relevant to the annihilation of [5]Al within amorphous network is kinetically favored compared with structural rearrangement during crystallization of amorphous oxides. We also found that the estimated activation energy for each process may depend on the presence of potential residual carbon phases (e.g., residual organic ligand) and presence of hydration shell as has been suggested from the previous studies on crystallization kinetics. Despite the uncertainty in the estimated activation energy barrier, ln k decreases linearly with increasing 1/T suggesting that each process can be described with Arrhenius behaviors. The current results highlight the first detailed estimation of crystallization kinetics of the archetypal Al2O3 on the basis of its atomic scale structural evolution. Additionally, the experimental results, for the first time, revealed the presence of two distinct mechanisms for structural rearrangement upon crystallization in the prototypical single-component oxide glasses. The current results and method can shed light on a new opportunity to study crystallization kinetics of diverse natural and multi-component silicate glasses and melts. The potential result may yield atomic-level understanding of Earths chemical evolution.