Crystal Morphology and Phase Transformation of LiAlO2: Combined Experimental and First-Principles Studies

Layered α-LiAlO 2 matrix often suffers from detrimental structural and morphological changes during its lifetime, especially under H 2 -containing fuel inlet environments of molten carbonate fuel cell systems. However, the underlying mechanism of this phase and the morphological transformations has rarely been explored. In this study, transmission electron microscopy (TEM) techniques were used to determine the changes in structure and morphology of LiAlO 2 samples lifted by a focused ion beam. In accordance with X-ray diffraction analysis, TEM images show that under H-rich conditions the rhombohedral α-LiAlO 2 transforms to tetragonal γ-LiAlO 2 phase with the appearance of the (101)-terminated octahedral-shaped γ-LiAlO 2 crystallites. We further support, and plausibly rationalize, the observed transformations using density functional theory (DFT) calculation. The DFT computed surface energies of γ-LiAlO 2 reveal that the {101} surface becomes the lowest energy surface upon H-adsorption, thus leading to formation of observed octahedral geometry. Contrary to stabilization of γ-LiAlO 2 surfaces upon H passivation, DFT revealed H adsorption on α-LiAlO 2 surfaces to be energetically unfavorable. This contrasting behavior of α-LiAlO 2 and γ-LiAlO 2 under H-rich environments could be a potential driving force for the observed α-LiAlO 2 to γ-LiAlO 2 phase transformation.