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Unraveling the Origin of Structural Disorder in High Temperature Transition Al2O3: Structure of #-Al2O3 Libor Kovarik, Mark Bowden, Dachuan Shi, Nancy M Washton, Amity Andersen, Jianzhi Hu, Jaekyoung Lee, Janos Szanyi, Ja-Hun Kwak, and Charles H. F. Peden Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 22 Sep 2015 Downloaded from http://pubs.acs.org on September 23, 2015

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Chemistry of Materials

Unraveling the Origin of Structural Disorder in High Temperature Transition Al2O3: Structure of θ-Al2O3 Libor Kovarik1*, Mark Bowden1, Dachuan Shi2, Nancy M. Washton1, Amity Andersen1, Jianzhi Hu2, Jaekyoung Lee3, Janos Szanyi2, Ja-Hun Kwak3, Charles H. F Peden2

1

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory,

P.O. Box 999, Richland, Washington 99352 2

Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999,

Richland, Washington 99352 3

Department of Chemical Engineering, UNIST, Ulsan, Korea

*Corresponding author: [email protected] 1

ACS Paragon Plus Environment


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ABSTRACT The crystallography of transition Al2O3 has been extensively studied in the past due to 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 due to 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 also provide new perspectives on structural complexity that can emerge from intergrowth of closely related structural polymorphs.

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INTRODUCTION Transition Al2O3 derived from dehydration of boehmite is a group of oxides with a high degree of inherent structural disorder. The way in which this structural disorder manifests as a function of various heat treatment conditions is a highly relevant topic in many technological fields, with perhaps the highest relevance in the field of catalysis, where transition Al2O3 is heavily used as catalysts and catalytic support materials 1-3, and where detailed understanding of structure and structural disorder could provide a much needed foundation for rationalization of unique surface chemistry of these materials 4-8. In the case of transition Al2O3 heat-treated at relatively high temperatures (>900°C), the two phases that form are δ-Al2O3 and θ-Al2O3 9-11. To understand high temperature transition Al2O3, it is important to understand the defective nature of the individual phases and, perhaps more importantly, the way the individual crystallographic phases coexist in common microstructures 9, 12, 13. The crystallography of what is now known as δ-Al2O3 is rather intriguing as it is becoming recognized that the interpretation of structure in terms of a unique crystallographic structure is not appropriate.

Instead it is necessary to describe the

structure as a complex intergrowth of closely related polymorphs belonging to the δAl2O3 family 13. While this interpretation is relatively new, it should be pointed out that numerous studies have previously reported several crystallographic symmetries and lattice parameters for δ-Al2O3 9-11 , and did not provide descriptions in terms of a unique structural polymorph. A clarification regarding the actual nature of δ-Al2O3 has been obtained and discussed in Kovarik et al.

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based on quantitative High Angle Annular



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Dark Field (HAADF) observations. It was shown that there are two main closely related polymorphs that coexist in a common intergrowth that macroscopically appears as a single phase. The two closely related polymorphs were denoted as δ1-Al2O3 and δ2-Al2O3 13

, and were found to intergrow along the [100] direction. In addition to these two

polymorphs, there are other crystal symmetries and lattice parameters that have been previously proposed for δ-Al2O3

9-11

, but verification of these structures and

determination of their crystallographic nature is yet to be performed. At higher temperatures, δ-Al2O3 gradually transforms to θ-Al2O3, which is commonly accepted to possess a monoclinic symmetry and a structure that is isomorphous to βGa2O3. The models available in the literature have minor differences, which are mostly due to partial occupancies and lattice parameters. The model of Zhou and Snyder

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suggests that the structure has partial occupancies at selected atomic sites while the models proposed by Yamaguchi et al. and Repelin et al.

14, 15

consider full occupancy at

Al sites. Numerous studies have shown that one of the factors leading to poor crystallinity in θ-Al2O3 can be ascribed to twinning, which is often found at relatively high densities 9,13,16. Other polymorphs or “microstructures” aside from δ-Al2O3 and θ-Al2O3 have also been reported to evolve during high temperature aging of boehmite/γ-Al2O3 precursors 17, 18

. Perhaps the most intriguing in this regard is a disordered structure of Al2O3 that has

been reported to coexist with δ-Al2O3 and θ-Al2O3. As shown by Kovarik et al.

13

, the

disordered structure is unique as it has only 1D crystallographic periodicity, which makes it clearly distinguishable from the existing models of δ-Al2O3 and θ-Al2O3.

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The current study is motivated by the paucity of information detailing structural characteristics of high-temperature treated transition Al2O3. Addressing the structural complexity in these materials, we find that θ−Al2O3 is much more complex than that previously described in the literature. In particular, we show that even after extensive high temperature treatment, θ-Al2O3 is a disordered phase that deviates from its commonly accepted structure of β-Ga2O3. The disorder is accomplished by structural intergrowth of a crystal type of β-Ga2O3 and crystal that is closely related to δ-Al2O3. The reported analysis on θ-Al2O3 provides new crystallographic insight into the origin of poor crystallinity in transition aluminas, and also affords a new perspective on the structural complexity that can arise from the intergrowth of closely related structural polymorphs.

METHODS Two sources of boehmite were used to prepare transition Al2O3 in this work: one laboratory synthesized and the other obtained commercially. The laboratory-synthesized samples, prepared from alkoxides, were in the form of morphologically well-defined platelets defined by rhombus facets. The size of particles was approximately 10-20 nm in thickness, and approximately 50 nm across the short rhombus diagonal. Detailed information about the synthesis protocol and morphology of the particles, as obtained from TEM tomographic measurement, can be found in our previous work 19. In addition to the lab-synthesized samples, we also studied morphologically well-defined transition alumina obtained by calcination of a commercial boehmite material supplied by Sasol. The microstructural investigation was performed with an aberration corrected FEI Titan 80-300 electron microscope in Scanning Transmission Electron Microscopy

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(STEM) mode. The probe convergence angle was set to approximately 18 mrad and the inner detection angle on HAADF detector was approximately 3 times higher than the probe convergence angle. The TEM sample preparation involved dispersion of the alumina materials in ethanol and sonication for approximately 3-5 minutes in order to improve the dispersion of the particles, and then subsequent application of a small drop of the ethanol slurry onto TEM grid. Selected set of samples were heat treated inside of TEM with Aduro ProtochipsR heating holder, and imaged after the in-situ heat treatment conditions. Simulation of HAADF images was performed with a computer code developed by E. Kirkland

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. The calculations were performed with microscope parameters that closely

correspond to the experimental conditions (E=300kV, cs=5µm, convergence angle 18mrad, inner collection angle 70 mrad and outer collection angle 240 mrad). The simulated images were convoluted with a Gaussian of FWHM=0.08 nm to account for spatial incoherence of the imaging system identical to previous simulations13 27

Al direct polarization Nuclear Magnetic Resonance (NMR) experiments were

conducted on a Varian/Agilent VNMRs system utilizing a 1.6 mm HXY probe operating in DR mode tuned to 221.421829 MHz. A calibrated π/20 pulse of 0.25 µs, a recycle delay of 2s, a spinning speed of 38 kHz, and a constant 20oC temperature were used to collect 1024 transients. Time domain free induction decays were apodized with 25 Hz Lorentzian broadening after zero filling. All spectra were referenced to 1M AlCl3 at 0 ppm. X-Ray Diffraction (XRD) patterns were collected utilizing a PANalytical MPD Bragg-Brentano diffractometer equipped with Cu Kα radiation, a variable divergence slit, and a post-diffraction monochromator. Quantitative Rietveld refinements were carried

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out with the Bruker TOPAS software and published structures for δ-Al2O3 13 and θ-Al2O3 12

. Density Functional Theory (DFT) calculations were performed with the Vienna Ab-

initio Simulation Package (VASP) 21, 22. A detailed description of the calculations can be found in our previous work 13.

RESULTS STRUCTURAL OSBERVATIONS OF θ-Al2O3 θ-Al2O3 is one of the most stable transition alumina phases, and is commonly reported to form at temperatures exceeding 900°C. The activation temperature for its formation varies depending on size and morphology of the alumina particles and the nature of the precursor. In the systems studied here, we found that θ-Al2O3 formed at temperatures in excess of ~1000°C. The rate of formation was rather low at 1000°C and, even after prolonged isothermal exposure, the specimens were dominated by δ-Al2O3. At higher temperatures, the formation of θ-Al2O3 was greatly accelerated. An example of θ-Al2O3 formation during isothermal heat treatment at 1050°C is shown in Fig.1(a). During the isothermal hold at 1050°C, the diffraction peaks of θ-Al2O3 are seen to gradually replace the peaks of δ-Al2O3, but interestingly, even after prolonged heat treatment for 48 hours, the diffraction pattern shows a significant amount of δ-Al2O3. As shown in Fig.1(a), the amounts of θ-Al2O3 and δ-Al2O3 can be readily estimated from XRD patterns in the 2θ range of 44-48°. In this range, which covers the “cube planes” of the oxygen sublattice, the diffraction peaks on the outside (highest splitting) correspond to θ-Al2O3 and the peaks on the inside correspond primarily to δ-Al2O3. At 1100°C, θ-Al2O3 became the dominant phase within several hours of calcination but in the current study, a specimens 7



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corresponding exclusively to θ-Al2O3 were not synthesized under any heat treatment conditions. At the higher temperatures of 1100°C, the transformation of δ-Al2O3 to θAl2O3 is competing with the formation α-Al2O3, which then can quickly become a dominant phase. An example of XRD pattern from the lab-synthesized sample at 1100°C and with some of the highest proportions of θ-Al2O3 is shown in Fig.1(b). Under these relatively severe conditions a small volume fraction of α-Al2O3 ( < 1%) can now be identified.

Figure.1. (a) Section of the XRD pattern for alumina sample (Sasol) heat-treated at 1050 °C. (b) XRD pattern of lab-synthesized alumina sample heat-treated at 1100°C for 3 hours with θ-Al2O3 as the main component of the microstructure.

The microstructure of samples heat-treated at 1100°C was extensively examined using HAADF imaging. At this temperature, many of particles were found to have transformed to θ-Al2O3. When quantitatively analyzing HAADF images from several different crystallographic orientations, it was recognized that the available structural

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models based on β-Ga2O3 did not correctly reproduce the expected features of the microstructure. The observation from the [010]θ direction is shown in Fig.2(a,b). In this orientation, the observed symmetry and periodicity of the microstructure is generally consistent with the model of θ-Al2O3 with β-Ga2O3 structure type. However, the relative intensities of Al atomic columns in the images (the image contrast is dominated by Al columns) are much less consistent with the expected model of θ-Al2O3 with β-Ga2O3 structural type. Specifically, the intensity of octahedral Al sites are found to be consistently higher than those of tetrahedral Al sites, which is not expected based on the model of θ-Al2O3. According to the model θ-Al2O3, both Aloct and Altet sites should be equally occupied and thus should have very similar intensities. HAADF simulations of the expected contrast from θ-Al2O3 with β-Ga2O3 structure type is shown in Fig.2(c). The observations revealing the largest inconsistency with θ-Al2O3 of β-Ga2O3 structural type were obtained from [001]θ and are shown in Fig.2(d,e). When imaging along this direction, it was found that the structure contained a high density of welldefined defects, which led to an aperiodic appearance of the structure. In the HAADF images, the defects can be recognized as high intensity features that do not follow the lattice arrangement of β-Ga2O3. It was determined that the image features correspond to Al atomic columns that are well aligned in the viewing direction, which is consistent with [110]SPINEL. In between the aperiodic features, small pockets in the structure can be identified that are consistent with the model of θ-Al2O3 with β-Ga2O3. These small regions are highlighted with rectangles in Fig.2(e), and the similarity with β-Ga2O3 structure type is

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evident from HAADF image simulations shown in Fig.2(f). Importantly, the HAADF simulations also provide a reference frame for understanding how the aperiodic image features are incorporated in the overall structure. It enables us to conclude that these high intensity image features correspond to Al in octahedral sites.

Figure 2. Detailed atomic level view of θ-Al2O3 microstructure (3 hours @ 1100°C, labsynthesized sample) as investigated with HAADF imaging. (a,b) HAADF images from the [010] zone. (c) [010] HAADF image simulation of Al2O3 with β-Ga2O3 structure. (d,e) HAADF images from the [001] zone. The microstructure is found to be aperiodic. (f) Corresponding [001] HAADF image simulation of the β-Ga2O3 structure.

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The disordered structure of θ-Al2O3 was consistently observed for samples exposed to temperatures exceeding 1000°C, under both in-situ and ex-situ heating conditions. For heat treatment temperatures of ~1000-1050°C, it was regularly found that the disordered θ-Al2O3 is present in a common microstructure with δ-Al2O3, and that the coexistence of both structures is accomplished by formation of coherent and semi-coherent phase boundaries. A typical example of disordered structure of θ-Al2O3 coexisting with δ-Al2O3 is shown in Fig.3(a). The observations in Fig. 3 also provide a useful reference regarding the relative image intensities in θ-Al2O3 and δ-Al2O3. Based on the direct comparison, we recognize that the aperiodic features (Al octahedral columns) in θ-Al2O3 have a comparable intensity with the most evident columns in δ-Al2O3, which implies that these are highly/fully occupied rows of Al. However, some aperiodic features in θ-Al2O3 with lower intensity columns can also be identified, suggesting that the occupation of Al can vary significantly. This is especially the case for a heat treatment condition where θ-Al2O3 evolved to a greater extent, such as under high temperature conditions shown in Fig.2. Varying intensity of the aperiodic image features implies that the rows of octahedral Al are not continuous through out the structure. The spatial arrangement of the Al aperiodic features in HAADF images is not random, but instead follows a set of specific constraints. The most evident constrain is that the high intensity features are arranged on lattice planes that have spacing of approximately 5.75 Å, as shown in Fig.3(b,d). The normal of these planes is consistent with [100]SPINEL. Additionally, within each of these planes, the high intensity features are separated by distances that are restricted to N×aγ√2/4, where N can be any integer greater than or 11



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equal to 2 and aγ is the cubic unit cell dimension ascribed to γ-Al2O3. Note that if N=2, the contrast in this projection would be fully consistent with δ-Al2O3 as described in our earlier work13. Indeed, there are local regions in the structure that can be identified to have N=2, indicating that these structural pockets may be locally consistent with δ-Al2O3. We also observe that the distance between the high intensity spots from adjacent planes is restricted to that found in δ-Al2O3 13. While the high-intensity image features dominate the contrast, other less prominent characteristic features can also be recognized in the images. This includes a) subtle intensity variations around the high intensity features, and b) the underlying periodic arrangements of other lower intensity columns, which are in general consistent with Oxygen/Aluminum position in β-Ga2O3 structural type (as shown in Fig.1c). The underlying periodicity for the less intense image features is evident in the FFT shown in Fig.3(d) where they give rise to sharp diffraction spots. The spots corresponding to (004) and (220), indexed with respect to a cubic spinel oxygen sublattice, are marked on this figure. On the other hand, the aperiodic features in the images give rise to highly diffuse intensities in FFT. It should be noted that the FFT from HAADF images emphases the diffuse scattering due to the Z-contrast nature of the images. Importantly, it was also found that the degree of disorder within θ-Al2O3 can vary from region to region even within the same crystal. For samples heat-treated at approximately 1000°C, for example, we observed areas which contained high-intensity image features with a much enhanced degree of ordering. As shown in Fig.3(e), the basic structural motifs of the microstructure remain very similar within each of these regions, and give rise to the complex patterns of intergrowth. 12


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Figure.3(a) An example of a structure of θ−Al2O3 and δ−Al2O3 forming in a 1000°C heat treated sample (lab-synthesized sample). (b) Detailed atomic view displaying the aperiodic θ−Al2O3, and the characteristic distances identified for aperiodic features in the structure. The distances are given in spinel reference frame. (c) Intensity profile along the [100] direction. (d) Corresponding FFT spectrum from the HAADF image of the aperiodic structure (e) region with a higher degree of periodicity.

To further advance the structural description of θ-Al2O3, we also employed complementary characterization techniques. In this respect, NMR spectroscopy was found to be particularly valuable, as it provides a quantitative measure regarding

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percentage of octahedral and tetrahedral Al sites, which is a critical constraint for describing the structure. High resolution 27Al MAS-NMR spectra from three alumina samples heat treated for extended periods of time at 1050oC and 1100oC are shown in Fig.4(a). The highresolution NMR spectra show well-resolved tetrahedral and octahedral Al sites; tetrahedrally coordinated Al are found in the range of 55–80 ppm and octahedrally coordinated Al are found in the -5–20 ppm range. The amount of Al in tetrahedral and octahedral sites were then determined based on the integral peak intensities. Depending on the extent of heat-treatment for this set of three samples, the percentage of tetrahedral sites were found to vary from ~40% to ~43%, with the highest amount determined for samples heat-treated at 1050oC. The experimentally determined values of 40%-43% reflect the total amount of tetrahedral sites within the samples, which mostly consists of θ-Al2O3 but also contain some portion of δ-Al2O3. In order to quantify the tetrahedral/octahedral sites just within θ-Al2O3, further calculations were required. To determine the amount of tetrahedral sites in θ-Al2O3, we consider the overall value of tetrahedral sites in the sample to be a weighted average of tetrahedral sites from δ-Al2O3 and θ-Al2O3. The quantification of tetrahedral sites in θ-Al2O3 thus requires an independent determination of volume fraction of individual phases, and the knowledge about the amount of tetrahedral sites in δ−Al2O3. In our approach, the volume fraction of δ−Al2O3 and θ-Al2O3 was determined by XRD Rietveld analysis. Detailed description of the Rietveld refinement is included in the supplementary information in Table S1. and Fig.S1. The amount of tetrahedral sites for δ−Al2O3 was taken as 37.5% from a previously published data 13.

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Graphical representation from the quantification of tetrahedral sites within the θAl2O3 is shown in Fig.4(b). In this graph, we plot the overall value of tetrahedral sites as determined from (NMR), against the volume fraction of θ-Al2O3 as determined from XRD Rietveld analysis. When plotted together with theoretically derived values (assuming the presence of θ-Al2O3 with different amount of tetrahedral sites in the 4050% range) it then becomes evident what is the actual amount of tetrahedral sites in θAl2O3 in the complex microstructure. As shown in Fig.4(b) with iso-concentration lines, θ-Al2O3 is expected to have ~42% (3hrs@1100°C), ~43% (16hrs@1100°C) and ~46%(48hrs@1050°C) of tetrahedral sites. In all cases, the amount of tetrahedral sites is below 50%, which is the amount of presently accepted model of θ-Al2O3. Importantly, we find that θ-Al2O3 that formed at the lower temperature contain a higher proportion of tetrahedral Al.

Figure.4 (a) 27Al NMR spectra of heat-treated samples. (b) Proportion of tetrahedral sites plotted as function of volume fraction of θ-Al2O3 for samples and heat-treatment conditions. The solid lines indicate a percentage of tetrahedral sites in two-phase δ−Al2O3 θ-Al2O3 microstructure for a given θ-Al2O3 model with various occupancies of 15



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tetrahedral/octahedral sites. The lines provide a guide for determination of the tetrahedral/octahedral in θ-Al2O3.

STRUCTURAL INTERPRETATION The experimental observations presented in this work revealed that boehmite-derived θ-Al2O3 is a disordered phase that deviates from the commonly accepted structure of βGa2O3. While the structure contains local regions that are consistent with β-Ga2O3, it also contains a high density of local regions that have a bonding environment that appears to be more closely related to δ-Al2O3. As both regions are observed within a single grain and create a seamless intergrowth at the atomic scale, we therefore propose to interpret θphase as a disordered composite phase containing elements of β-Ga2O3 and a newly proposed δ3-Al2O3. A schematic illustration for the interpretation of the disordered structure of θ-Al2O3 as a structural intergrowth of β-Ga2O3 and variant of δ3-Al2O3 is shown in Fig.5. The newly proposed variant of δ3-Al2O3 has lattice parameters and symmetry that is consistent with β-Ga2O3. In this respect it is different from the previously known δ1-Al2O3 and δ2Al2O3 crystallographic variants, but because it adopts bonding environment that is closely consistent with the two variants of δ-Al2O3, we designate it as part of δ-Al2O3 family. The intergrowth of β-Ga2O3 and δ3-Al2O3 is facilitated by a high degree of structural compatibility, which stem not only from symmetry compatibility of the unit cells but also from continuity of oxygen lattice and Al octahedral/tetrahedral atoms through both structures. In the β-Ga2O3 type structure, octahedral and tetrahedral Al sites form a

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continuous chains parallel to b. The chains of Al octahedral sites is illustrated in Fig.5a with green bars; the tetrahedral chains are omitted for visual clarity. The octahedral/tetrahedral chains of Al in “β-Ga2O3 structure” continue into δ3−Al2O3 structure, assuring structural compatibility. However, inside δ3-Al2O3 the chain are not continuous because additional Al rows that are aligned in perpendicular direction are introduced. The presence of such perpendicular Al columns, which are labeled as orange bars in Fig.5(a), leads to the main structural modifications, driving the ratio of tetrahedral/octahedral sites in the overall structure towards lower values. Due to structural compatibility, both β-Ga2O3 and δ3−Al2O3 regions can seamlessly intergrow on (100)θ (010)θ and (001) , thereby creating

a composite disordered

structure. Fig.5(b) illustrates a schematic representation of the composite disordered structure as viewed along [001]. The orange rectangles in this graph correspond to a rows of Al in octahedral sites, as previously shown in Fig.5(a), and they are predicted to appear as bright features in HAADF images.

The provided description suggests that the

regions around the orange rectangles have a bonding environment that is consistent with δ3-Al2O3, while the remaining parts of structure are consistent with the model of β-Ga2O3. The schematic in Fig. 5(b) reproduces the features of aperiodic intergrowth typically observed within the experimental observations; notably the intergrowth leads to arrangement of octahedral columns on planes approximately 5.75 Å apart, and separated by N×aγ√2/4 (N≥2) along [010]θ depending on the extent of the intergrowth.

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Fig.5. Crystallographic interpretation of θ−Al2O3 structure as unique intergrowth of Al2O3 with “β−Ga2O3” structure and a structure that share close similarity of δ−Al2O3.

The above description provides a conceptual explanation for the atomic arrangement in θ-Al2O3. But in order to provide a full description of the structure, we built several models of δ3-Al2O3 that would reflect the structural description, and also reflect constrains from HAADF and NMR observations. Here we show, that a structural model of δ3-Al2O3 that can fully fit these constrains is closely related to δ2-Al2O3; the newly proposed model of δ3-Al2O3 can be considered as an un-twinned version of δ2Al2O3. The structure is based on 37.5/62.5% of tetrahedral/octahedral sites, and in general maintains similar bonding environment of δ1-Al2O3/δ2-Al2O3 while adopting different lattice symmetry. In building the new structure of δ3-Al2O3, we find that the actual unit cell lattice parameter must double along the x and quadruple along the z direction, as compared to θ−Al2O3. The resulting unit cell then has 160 atoms, and has a space group A2/n (No.15).

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The newly proposed structure is shown in Fig.6, and it is compared with the previously derived structures of δ1-Al2O3 and δ2-Al2O3 13. The DFT derived lattice parameters of δ3Al2O3 are a=12.03Å, b=11.30 Å, c=11.34 Å and β=102.6°. Full crystallographic information is provided in the supplementary information in Table S2.

Figure.6 Graphical representation of the δ1-Al2O3, δ2-Al2O3, and δ3-Al2O3 variants. The δ3-Al2O3 can be regarded as “un-twinned” variant of the previously known structure of δ2-Al2O3.

To demonstrate that the proposed composite structure of θ-Al2O3 accounts for the HAADF observed structural features in Fig.3, we have performed HAADF image simulations. For the [001] projection, which is the direction that discloses the structural disorder, the simulations successfully reproduce the high intensity image features in positions between Al tetrahedral sites, and also confirm the subtle intensity variations around the tetrahedral sites. For the [010] projection, the model reproduces the intensity differences observed between octahedral and tetrahedral columns shown in Fig.2. These

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HAADF simulations, documenting the images contrast features, are presented in the supplementary information in Fig.S2. The proposed composite crystal model of θ-Al2O3 can also fully account for NMR derived values for tetrahedral and octahedral sites. As it pointed out in previous discussions, the relative amount of tetrahedral sites in θ-Al2O3 will depend on the extent of intergrowth between the two structural components. The local bonding of δ3-Al2O3 will contain 37.5/62.5% of tetrahedral/octahedral sites, while the local bonding of βGa2O3, will contain 50/50% of tetrahedral/octahedral sites 9, 13. The amount of these two structural components can be estimated from STEM images by measuring the average spacing of the aperiodic columns. The relationship between the average spacing of the high intensity columns and the amount of tetrahedral sites is provided in Table S3. For example, in the case of the sample heated at 1100 °C for 3 hours (Fig. 2), it was found that the high intensity columns had an average spacing of approximately N~3, which would correspond to ~42% of tetrahedral sites, which agrees with the NMR analysis. To further support the intergrowth model, we used DFT calculations to estimate the enthalpy of formation for δ3−Al2O3, and compared this with previously reported values for δ1−Al2O3, δ2−Al2O3 and “Al2O3” with the β−Ga2O3 structure. As shown in Table.1, all of these crystals have a very similar enthalpy of formation, which thus provides a further support for favorability of these polymorphs to coexist in common intergrowth. Table 1 also includes calculated lattice parameters. While expressed in different coordinate systems, the lattice parameters of δ3−Al2O3 are seen to be highly compatible with the lattice of δ−Al2O3. For the intergrowth with the polymorph of β-Ga2O3, it is seen

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that some lattice strain may be required to maintain both structures in structural intergrowth.

Table.1 DFT derived enthalpies of formation and lattice parameters. The lattice parameters are expected to be consistently overestimated by 1%. Lattice parameters

Space

Al(Td)

∆H-∆Halpha

Group

[%]

[eV/Al2O3]

a [Å]

b [Å]

c [Å]

Beta

δ1−Al2O3

P212121

37.5

0.051

7.99

16.10

11.80

-

δ2−Al2O3

P21

37.5

0.052

8.00

8.05

11.80

δ3−Al2O3

A2/n

37.5

0.054

12.09

11.30

11.41

102.6

C2/m

50

0.049

11.554

2.940

5.669

103.9

Al2O3 with Ga2O3

DISCUSSION The structural analysis performed in this current study has implications on several important topics in transition Al2O3, one of which is the mechanism of polymorphic transformations. In general, the polymorphic transformations are known to proceed very gradually, and have been often conceptually explained by gradual reordering of Al on selected atomic sites. Such gradual evolution is for example well captured in XRD patterns, where it has been frequently shown how individual diffraction peaks progressively evolve during the course of Alumina transformations 12, 23, 24. There have been several attempts to rationalize the polymorphic transformations using maximal subgroup/supergroup symmetry relationship

21


9, 25

. In view of our previous


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work on δ−Al2O3

13

and the current study on θ-Al2O3, however, the previously proposed

pathways for δ−Al2O3 to θ-Al2O3 transformations does not seem to be applicable. Given the structural information derived in this study, it is instead suggested that the δ−Al2O3 to θ-Al2O3 transformations initiate in the δ3-Al2O3 segments of complex δ−Al2O3 (δ1,δ2,δ3) structure. The nucleation of θ-Al2O3 is envisioned to simply proceed by a short range reordering of selected Al sites. From the analysis of both structures, it can be shown that such reordering would involve atomic exchange between only limited number of first neighbor tetrahedral and octahedral sites. The subsequent growth of θ−Al2O3, which will require transformation of the remaining complex structure of δ−Al2O3, is then expected to proceed by a mechanism that involves a longer-range rearrangements of Al atoms. The disordered composite phase (DCP) of θ-Al2O3 has been observed after high temperature treatment and after relatively long times (e.g. 1050 °C for 48 hours), which leads us to believe that the extent of disorder at this stage is thermodynamically rather than kinetically controlled. In other words, θ-Al2O3 that adopts the β-Ga2O3 structure, without δ3−Al2O3 structural elements, is not expected to form after high temperature calcination and prolonged times. Stabilization of disorder in θ-Al2O3 at high temperatures, in particular by entropy contributions, is consistent with our limited observations of low and high temperature treated samples; we find that the relative amount of octahedral Al in θ-Al2O3 increases with increasing temperatures, which would be expected if this disorder was entropically stabilized. Stabilization of the structure by entropy is also supported by the fact that bonding environment of β-Ga2O3 and δ3−Al2O3 can be readily changed. Given the current models, the change would require only small local reordering (simple 22


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shuffle rearrangement) of selected Al atoms between neighboring octahedral/tetrahedral sites, and this should be relatively simply achieved at higher temperatures. An important finding of this study is the derivation of new crystal structure of δ3−Al2O3. The terminology for this structure was chosen to reflect its structural similarity with other δ−aluminas. (The crystal share the same amount of 37.5/62.5% octahedral/ tetrahedral site with previously identified δ1/δ2−Al2O3). It is interesting to note that a crystallographic variant with identical lattice parameters, monoclinic symmetry and space group A2/n (No.15) was previously identified in thermally treated plasma sprayed Al2O3 and was previously denoted as θ′′-Al2O3 9, 25. While the structural information has not been provided in the previous work of Levin at al.

25

, it seems very likely that the

currently derived δ3−Al2O3 represents the same structural polymorph. In addition to the presently derived model of transition aluminas, it is expected that other structural variants based on the general motif of δ-Al2O3, such as for example those obtained from twinning, and applying other symmetry operations, can form in the microstructure of high temperature treated aluminas. In this respect, it would be sensible to examine if any of such structures can be related to the other previously proposed polymorphs of θ′-Al2O3, λ-Al2O3 and δ′-Al2O3 9, 10, 17.

CONCLUSIONS The current study has addressed crystallographic structure of θ-Al2O3 formed during high temperature treatment of boehmite. It has been shown that the microstructure is much more complex than has been previously envisioned in the literature. The structure contains a high density of well-defined structural defects that give rise to

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crystallographic aperiodicity. By correlating imaging and spectroscopy results with DFT calculations, we proposed a model for θ-Al2O3 as a disorder intergrowth of two crystallographic variants at the unit cell level. One of the variant is based on the known structural model of β-Ga2O3. The other variant, designated as δ3-Al2O3, has not been previously known and was fully crystallographically derived as a part of this study. Overall, the reported study affords new perspective on the structural disorder that emerges during high temperature treatment of transition Aluminas.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website/

XRD Rietveld refinement, Crystallographic information of δ3-Al2O3, HAADF image simulations of intergrowth structure of θ-Al2O3, the relationship between the average distances of aperiodic features in HAADF images and the amount of tetrahedral sites.

AUTHOR INFORMATION Corresponding Author * [email protected]

NOTES

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The authors declare no competing financial interests.

ACKNOWLEDGEMENTS The research described in this paper is part of the Chemical Imaging Initiative at Pacific Northwest National Laboratory (PNNL). It was conducted under the Laboratory Directed Research and Development Program at PNNL, a multiprogram national laboratory operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract No. DE-AC05-76RLO1830. Some of us (DS, JH, JS, JHK and CHFP) were supported by the U.S. DOE, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences. The work was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL.

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SYNOPSIS TOC:

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