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Of the various phases of transition alumina, iota-alumina ({iota}-Al{sub 2}O{sub 3}) is the least well known. It is considered to be the end member of the mullite series in the limit of zero Si content. The structural details of {iota}-Al{sub 2}O{sub 3} are not available and its physical properties are totally unknown. Based on an appropriately modified structure of a high alumina content mullite phase close to the 9-1 mullite, we have successfully constructed a structural model for {iota}-Al{sub 2}O{sub 3}. The simulated x-ray diffraction (XRD) pattern of this model agrees well with a measured XRD pattern obtained from samples that claim to belong to {iota}-Al{sub 2}O{sub 3}. {iota}-Al{sub 2}O{sub 3} is a highly disordered ultralow-density phase of alumina with a theoretical density of 2854 kg/m{sup 3}. The calculated total energy per Al{sub 2}O{sub 3} is much higher than {alpha}-Al{sub 2}O{sub 3} and the other well-known disordered phase, {gamma}-Al{sub 2}O{sub 3}. Using this theoretically constructed model, we have calculated the elastic, thermodynamic, electronic, and spectroscopic properties of {iota}-Al{sub 2}O{sub 3} and compared them with those of {alpha}-Al{sub 2}O{sub 3} and {gamma}-Al{sub 2}O{sub 3}. It is shown that the mechanical strength of {iota}-Al{sub 2}O{sub 3} is much weaker with bulk andmore » shear moduli that are only 57 and 42% of {alpha}-Al{sub 2}O{sub 3}. Phonon dispersion results and the subsequently calculated thermodynamic properties point to the fact that {iota}-Al{sub 2}O{sub 3} is an alumina phase preceding {gamma}-Al{sub 2}O{sub 3} in the processing of alumina before reaching {alpha}-Al{sub 2}O{sub 3}. Electronic structure calculations show it to be an insulator with a direct band gap of only 3.0 eV at the {Gamma} point and a higher ionic bonding character than {alpha}-Al{sub 2}O{sub 3} and {gamma}-Al{sub 2}O{sub 3}. It has a calculated static dielectric constant of 2.73 and a corresponding refractive index of 1.65 that are in agreement with reported data. Also calculated are the Al-K, Al-L{sub 3}, and O-K edges of the x-ray absorption near edge structure in {iota}-Al{sub 2}O{sub 3}, showing sensitive dependence on its local bonding environment. However, only the weighted average of these spectra can be directly compared with the measured ones which are currently unavailable.« less

physical review. b, condensed matter and materials physics

Structure and properties of the low-density phase ι-Al<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mn>2</mml:mn></mml:msub></mml:math>O<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow /><mml:mn>3</mml:mn></mml:msub></mml:math>from first principles

Structure and properties of the low-density phase ι-Al2O3from first principles