Of the various phases of transition alumina, iota-alumina (ι-Al2O3) 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 ι-Al2O3 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 ι-Al2O3. 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 ι-Al2O3. ι-Al2O3 is a highly disordered ultralow-density phase of alumina with a theoretical density of 2854 kg/m3. The calculated total energy per Al2O3 is much higher than α-Al2O3 and the other well-known disordered phase, γ-Al2O3. Using this theoretically constructed model, we have calculated the elastic, thermodynamic, electronic, and spectroscopic properties of ι-Al2O3 and compared them with those of α-Al2O3 and γ-Al2O3. It is shown that the mechanical strength of ι-Al2O3 is much weaker with bulk and shear moduli that are only 57 and 42% of α-Al2O3. Phonon dispersion results and the subsequently calculated thermodynamic properties point to the fact that ι-Al2O3 is an alumina phase preceding γ-Al2O3 in the processing of alumina before reaching α-Al2O3. Electronic structure calculations show it to be an insulator with a direct band gap of only 3.0 eV at the Γ point and a higher ionic bonding character than α-Al2O3 and γ-Al2O3. 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-L3, and O-K edges of the x-ray absorption near edge structure in ι-Al2O3, 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.
Sitaram Aryal, Paul Rulis, Lizhi Ouyang, and W. Y. Ching, "Structure and properties of the low-density phase ι-Al2O3 from first principles," Phys. Rev. B 84, 174123 – 2011 https://doi.org/10.1103/PhysRevB.84.174123