Unraveling the Origin of Structural Disorder in High Temperature Transition Al₂O₃: Structure of θ-Al₂O₃

Abstract

The crystallography of transition Al2O3 has been extensively studied in the past, because of the advantageous properties of the oxide in catalytic and a range of other technological applications. However, existing crystallographic models are insufficient to describe the structure of many important Al2O3 polymorphs, because of their highly disordered nature. In this work, we investigate structure and disorder in high-temperature-treated transition Al2O3 and provide a structural description for θ-Al2O3 by using a suite of complementary imaging, spectroscopy, and quantum calculation techniques. Contrary to current understanding, our high-resolution imaging shows that θ-Al2O3 is a disordered composite phase of at least two different end-members. By correlating imaging and spectroscopy results with density functional theory (DFT) calculations, we propose a model that describes θ-Al2O3 as a disordered intergrowth of two crystallographic variants at the unit-cell level. One variant is based on β-Ga2O3, and the other on a monoclinic phase that is closely related to δ-Al2O3. The overall findings and interpretations afford new insight into the origin of poor crystallinity in transition Al2O3, and we also provide new perspectives on structural complexity that can emerge from intergrowth of closely related structural polymorphs.

Copper‐alumina nanocomposites derived from CuAlO2: Phase transformation and microstructural coarsening

“…In the θ structure, 50% of the aluminum cations are in the octahedral interstices, while the other 50% are located in tetrahedral interstices. This changes the δ‐structured aluminas, which have 62.5% of the aluminum cations in the octahedral sites . This ordering brings the structure closer to the α‐Al 2 O 3 structure, in which all the aluminum cations are located in the octahedral sites, with a 2/3 occupancy.…”

“…The diffraction patterns were indexed and checked for consistency with the crystal structures of copper and all the known polymorphs of alumina (eg, γ, δ, θ, κ, and α). The matrix phase in the nanocomposite region was identified unambiguously as θ‐alumina (monoclinic, a = 11.79 Å, b = 2.92 Å nm, c = 5.62 Å nm, β = 103.97°). This result was also confirmed by X‐ray diffraction.…”

“…This changes the d-structured aluminas, which have 62.5% of the aluminum cations in the octahedral sites. 23 This ordering brings the structure closer to the a-Al 2 O 3 structure, in which all the aluminum cations are located in the octahedral sites, with a 2/3 occupancy. One possible explanation for the reversal in the phase transformation sequence is as follows.…”

High field 27Al MAS NMR and TPD studies of active sites in ethanol dehydration using thermally treated transitional aluminas as catalysts

“…Likewise, the Al O at about 16 ppm observed from the 800 to 1000 °C samplescan be attributed to the nucleation sites with local structure similar to that of the α- (Fig. 1), the assignments of the most upfieldAl O peak centered at about 5 ppm and the most upfieldAl T peak centered at about 64 ppm are not possible due to the lacking of characteristic XRD patterns [35,37]for other phases such as δor θ-Al 2 O 3 phase at calcination temperature from 500 to 850 °C and thus left unassigned. Further dedicated research is needed to understand the nature of these two unassigned peaks.…”

Unveiling the role of the hexagonal polymorph on SrAl2O4-based phosphors

“…It corresponds to the g-Al 2 O 3 phase with an isotropic chemical shi at about 12 ppm 30 and unassigned alumina (Al un ) phase that appears at the lowest isotropic chemical shi of about 7 ppm. [31][32][33] In addition, the pentahedral (Al P ) coordination is observed; around ca. 37.8 ppm.…”