Structural Intergrowth in Î´-Al2O3

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C: Physical Processes in Nanomaterials and Nanostructures

Structural Intergrowth in #-Al2O3 Libor Kovarik, Mark E. Bowden, Dachuan Shi, János Szanyi, and Charles H. F. Peden J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10135 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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The Journal of Physical Chemistry

Structural Intergrowth in -Al2O3

Libor Kovarik,1,2* Mark Bowden,1,2 Dachuan Shi,2 Janos Szanyi,2 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

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ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

*Corresponding author: [email protected] ABSTRACT Metastable polymorphs of Al2O3 accommodate a large amount of structural disorder that has proven difficult to quantify. In the case of -Al2O3, the nature of crystallographic disorder has recently started to emerge as a planar structural intergrowth. In this work, which uses aberration corrected HAADF STEM imaging and ab initio calculations, we reveal a new type of structural intergrowth in -Al2O3, and show that it is an important part of the complex/defected -Al2O3 microstructure. The newly identified intergrowth is different from the previously known type due to intergrowth direction and variant selection. In its most basic representation, the new intergrowth consists of three variants of 2-Al2O3, 3-Al2O3 and 4-Al2O3. As a part of this work, we derive the crystallography and structural relationship between the variants, and we discuss the overall crystallography of Al2O3 in the context of previous structural interpretations. Finally, it is shown that the individual -Al2O3 variants can be related to other metastable polymorphs that are known to evolve from -Al2O3.

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INTRODUCTION Transition aluminas are metastable forms of Al2O3 that can adopt a variety of crystallographic and structural forms (often referred to as polymorphs) depending on their origin and synthesis protocol.1-3 For transition aluminas derived from a Boehmite precursor, there are three structural forms known as -Al2O3, -Al2O3 and -Al2O3. The structural form of -Al2O3 is kinetically the most favored, and evolves at temperatures as low as 500°C. Since it is the least stable form, a sufficient thermal activation, usually in the 800-900°C temperature range, leads to a -Al2O3 -> -Al2O3 transformation. The transformation can be rather gradual, and it is very common that both -Al2O3 and -Al2O3 are present in the microstructure over a significant time/temperature range.4 The formation of -Al2O3 requires the highest activation energy of the metastable polymorphs, and its formation is predominantly observed

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at temperatures exceeding 1000°C. All three structural forms of transition aluminas are characteristic for their defective microstructure.5-9 While the general types of structural disorder are now recognized, the specific crystallographic information and the mechanism of transformation remains still actively studied. In the case of -Al2O3, the main type of structural disorder has been identified as planar structural intergrowth, arising from intergrowth of two closely related crystallographic variants of 1-Al2O3 and 2Al2O3.7 The planar intergrowth is commonly found in other classes of materials. For example, it is found in open framework zeolites,10 oxides such as -MnO2,11 hydroxides such as kyolinite,12 or intermetallic phases of NiTiPt(Pd).13 The variants of 1-Al2O3 and 2-Al2O3 were fully crystallographically described in the work of Kovarik et. al.7 It has been shown that the variant of 1-Al2O3 has orthorhombic symmetry and P212121 (No.19) space group, which is consistent with previous considerations for -Al2O314,15. The variant of 2-Al2O3 has been shown to be monoclinic, belonging to space group P21 (No.4.).7 In addition to the well resolved variants of 1-Al2O3 and 2-Al2O3, there are also several other models proposed for -Al2O3, including variants with tetragonal, orthorhombic and monoclinic symmetry.1,8,14,16 While some of these models have been questioned,15 it is currently not clear whether or how many of these additional variants do indeed exist and should be included within -Al2O3 family. The variant

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of -Al2O3 with tetragonal symmetry lattice parameters a=aγ and c=3aγ (space group P41), as proposed in the original work of Lippens et al.,1 has drawn the highest interest. Other models include orthorhombic symmetry with lattice parameters a=2aγ, b=2aγ, c=1.5aγ or a=aγ, b=aγ, c=1.5aγ , which can be found in the work of Jayram and Levi,14 and Wang et al., respectively.16 In the latter paper,16 two additional monoclinic crystallographic polytypes, with P2 space group, each of which with different lattice parameters were proposed. Interestingly, recent work by Kovarik et al.8 also proposed a monoclinic 3-Al2O3. The existence of this variant was proposed based on identification of structural defects within -Al2O3. Unlike the other previous studies, the recent proposal provided a full crystallographic description. Given the outstanding questions regarding the structure of -Al2O3, we revisit the crystallographic analysis of -Al2O3 in this paper. The present work shows that the structure of -Al2O3 is indeed more complex, and goes beyond the intergrowth of 1-Al2O3 and 2-Al2O3. The additional complexity is found to arise from a new type of intergrowth that can be rationalized in terms of crystallographic variants of 2Al2O3, 3-Al2O3 and 4-Al2O3. The newly identified structural complexity, and the other previous structural interpretations are discussed and reconciled.

METHODS

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The transition aluminas investigated in this work were synthesized via dehydration of Boehmite. This included Boehmite samples that were laboratory prepared from alkoxide precursors, and Boehmite samples obtained from Sasol. Detail description of the synthesis procedure for the laboratory prepared Boehmite samples can be found in our previous work.17 Microstructures of -Al2O3 were obtained from heat treatments performed in a tube furnace in the temperature range of 900-1050°C. Selected samples were also heat treated with a MEMS based Aduro Protochips heating holder inside the S/TEM. The microstructural observations were performed with a probe corrected FEI Titan 80-300 transmission electron microscope (TEM). The observations were obtained with Scanning Transmission Electron Microscopy (STEM) using a HAADF detector. The probe convergence angle was 18 mrad, and the inner detection angle on the HAADF detector was 3 times higher than the probe convergence angle. Simulations of HAADF images were performed with a computer code developed by E.J. Kirkland,18 and the calculations utilized microscope parameters that closely correspond to the experimental conditions (E=300kV, cs=5m, convergence angle 18mrad). The simulated images were convoluted with a Gaussian of FWHM=0.08 nm to account for spatial incoherence of the imaging system.

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Density Functional Theory (DFT) calculations were performed with the Vienna Ab-initio Simulation Package (VASP).19,20 A detailed description of the calculations can be found in our previous work.7 RESULTS The current work builds on our earlier study,7 which showed -Al2O3 to be an intergrowth consisting of two closely related crystallographic variants of 1-Al2O3 and 2-Al2O3. In particular, it is shown here that -Al2O3 is more complex and that additional crystallographic variants and intergrowth directions need to be considered. To provide a comprehensive description of -Al2O3, we briefly summarize the previously known crystallography of 1-Al2O3 and 2-Al2O3 intergrowth, and then follow this with a series of new findings revealing the additional structural complexity in -Al2O3. An example of the previously known 1,2-Al2O3 intergrowth is shown Figure 1(a). The intergrowth consists of two variants, 1-Al2O3 and 2-Al2O3, which share a high level of structural similarity. In particular, the structural similarities are evident from the evaluation of projected atomic positions in HAADF images. Only subtle differences between the variants can be identified due to distribution of vacant Al octahedral/tetrahedral sites. The vacant sites give rise to characteristic contrast (dark lines) in the HAADF images. The corresponding HAADF simulations shown in Figure 1(b,c), and structural models of projected unit cells, with the vacant sites

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highlighted as rectangular regions, are shown in Figure 1(d,e). Both variants of 1Al2O3 and 2-Al2O3 are assigned different colors in Figure 1 for easy interpretation. Due to the structural similarities, both variants can switch from one to another at the unit cell level. The intergrowth direction is [010] when expressed in 1-Al2O3, and [100] when expressed in 2-Al2O3 coordinate systems.7 It is noted that the observations of structural intergrowth can be resolved only from the zone axis that is perpendicular to the intergrowth direction. Other zones, will result in overlapping projections of 1-Al2O3 and 2-Al2O3, which may appear to be periodic in HAADF images.7

Figure1. (a) Structural intergrowth of 1-Al2O3 and 2-Al2O3 revealed along the cube [100]FCC_O direction. The intergrowth structure is denoted as 1,2-Al2O3. (b,c) HAADF simulation of 1-Al2O3 and 2-Al2O3. (d,e) Crystal projection of 1-Al2O3 and 2-Al2O3 (only Al displayed). The newly identified type of structural intergrowth in -Al2O3 is shown in Figure 2(a). The structure is different from 1,2-Al2O3, not only due to the different 8


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combination of crystallographic variants, but also due to a different intergrowth direction. In the example shown in Figure 2(a), which depicts the structure along the cube [100]FFC(O) orientation, there are two intergrowing variants of 2-Al2O3 and 3Al2O3 that can be identified. The variant of 2-Al2O3 can be readily identified based on the known projected atomic positions. The variants of 2-Al2O3 is present in [100]2 and [001]2 orientations; both of these orientations can seemingly change from one to another along the intergrowth direction. The corresponding HAADF simulations together with the crystallographic model of 2-Al2O3 are included in Figure 2(b,c,d,e).

The other component of the newly identified intergrowth microstructure is 3Al2O3, which is based on an identical structural motif as 2-Al2O3, but has a modified stacking sequence. Detailed account of 3-Al2O3 structure, including symmetry and its relationship to 2-Al2O3 can be found in our earlier work.8 The lattice parameters, as derived from DFT, are: a=12.09Å, b=11.30Å, c=11.41Å, and β=102.6°; the structure belongs to a monoclinic (A2/n) space group. Note that 3-Al2O3 was previously considered as a hypothetical structure, while in this study it is verifiably identified as an independent structure.

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The variant of 3-Al2O3 can be recognized in HAADF images from the characteristics sequence of structural layers. There are two degree of freedom of how 3-Al2O3 can be accommodated within the planar intergrowth. In Fig.2, the variants are accommodated in [110] and [1-10]. HAADF image simulations and crystallographic models of 3-Al2O3 supporting the interpretation of the newly identified planar disorder is shown in Figure 2 (f,g,h,i). The orientation relationship of intergrowth structures is [100]2/[110]3 and (001)2/(001)3.

Figure 2. (a) Newly identified structural intergrowth in 2,3-Al2O3 as revealed along a generic [100] direction. (b,c) HAADF simulation of 2-Al2O3. (d,e) Crystal projection of 2-Al2O3 (only Al displayed). (f,g) HAADF simulation of 3-Al2O3. (h,i) Crystal projection of 3Al2O3 (only Al displayed).

An important, but perhaps more subtle point about the structural complexity within the newly identified intergrowth is that 2-Al2O3 segments contain stacking faults. An example of stacking faults within the structure of 2-Al2O3 is shown in

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Figure 3(a). The faults are differentiated with different colors for easy visualization. Based on an analysis of projected atom positions in HAADF images, and coordination environment in 2-Al2O3, it can be concluded that the translational vector of the fault correspond to diagonal translation R= ½[101]. We note that an alternative interpretation for the stacking faults can be made by employing an independent variant, denoted as 4-Al2O3. The 4-Al2O3 variant has triclinic symmetry, with lattice parameters: a=aγ,, b=aγ, c=1.5aγ, =71.9°, β =72.1°, ~90°, and space group P-1. Only inversion symmetry is identified in this structure. A detailed crystallographic description of 4-Al2O3 is provided in the supplementary information. Comparison of simulated HAADF contrast for 2-Al2O3 and 4-Al2O3, together with the depiction of crystal structures, is provided in Figure 3(b,c,d,e). As shown in Figure 3(c), the HAADF simulations of 4-Al2O3 fully reproduce the experimental observations. The interpretation of faults in terms of 4-Al2O3 is also supported on the basis of DFT calculations. We find that the proposed variant of 4-Al2O3 to be enthalpically stable. The DFT derived enthalpy of formation, which is defined with respect to Halpha as (H4-Halpha), is 0.053eV/Al2O3. This is essentially energetically degenerate with 2-Al2O3 (0.052eV/Al2O3), suggesting that such structure/faults introduce only a negligible energetic penalty, and is thus favorable to form.

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The extent of 4-Al2O3 within the overall intergrowth structure remains to be fully established. The presence of 4-Al2O3 can only be identified from the [100]FFC(O), which has been challenging to align in the studied samples, thus leading to only relatively limited observations. Nevertheless, due to its favorable enthalpy of formation, it can be expected that 4-Al2O3 is, at a minimum, as prevalent as 3-Al2O3, which has only a slightly higher H3-Halpha of 0.054eV/Al2O3.

Figure 3 (a) Stacking faults within the structure of 2-Al2O3 represent another form of stacking disorder in 2,3-Al2O3. The stacking faults within 2-Al2O3 can be interpreted as an independent variant of 4-Al2O3 as described in the text. (b,c) HAADF simulation of 2-Al2O3 and 4-Al2O3. (d,e) Crystal projection of 2-Al2O3 and 4Al2O3 (only Al displayed).

The extent of 3-Al2O3 intergrowth within the 2,3,4-Al2O3 can be evaluated from [110]FFC(O), which is more readily accessible than [100] FFC(O). As shown in Figure 4(a,b,c), the variants of 3-Al2O3 can be distinguished from 2,4-Al2O3 based on the

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orientation of two closely spaced Al octahedral columns (dumbbells). In 2-Al2O3 and 4-Al2O3 the microstructure consists of oppositely inclined dumbbell motifs, while the microstructure of 3-Al2O3 consists of identically inclined dumbbell motifs. (Note that variants of 2 and 4 share identical projection and cannot be distinguished in [110]FFC(O)). The dark regions in the images are pores, which are present in high densities throughout the microstructure 17. For the presently studied microstructures that were observed following a heat treatment of Boehmite at 900-1050°C, we find that the faulted 2-Al2O3/4-Al2O3 is the primary component of 2,3,4-Al2O3 intergrowth. It is common that 2,4-Al2O3 represents 70-90%, as shown in Figure 4(a,b). Such a higher occurrence of 2,4 is consistent with higher enthalpic stability when compared to 3-Al2O3. In addition to the enthalpic considerations, other factors, such as accommodation of strain from multiple variants, are also expected to play important role in the variant selection. For example, the relatively small shear distortions for 2 would be expected to favor accommodation of this variant in common intergrowth microstructure.

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Figure 4(a,b). HAADF observations of 2,3,4-Al2O3 in the [110]FFC(O) direction (as defined in terms of spinel model for -Al2O3 or FCC oxygen sublattice). (c) HAADF image simulations depicting that the variants of 2-Al2O3 and 4-Al2O3 can be readily distinguished from 3-Al2O3 based on the sequence of dumbbell motifs. The dark regions in the images are pores, which are present in high densities throughout the microstructure. The intergrowth structure of 2,3,4-Al2O3 has long escaped identification, including our previous work.7 The failures to previously recognize this intergrowths structure can be related to the fact that unambiguous identification requires resolving the structure on [100]FFC(O) zones, which can be difficult to access in these highly defective materials. HAADF/HRTEM images from the more readily accessible [110]FFC(O) zones can be readily mistaken with either the highly-twinned -Al2O3 or 1,2-Al2O3. The comparison of 2,3,4-Al2O3 and -Al2O3 from the [110] zone is shown in Figure 5(a,b). For both structures, the HAADF images display dumbbell motifs (corresponding to two highly occupied Al octahedral sites) as the dominant

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image features. Distinction of the two structures can be made based on the relatively weak and often noisy intensity contrast from less occupied sites in the horizontal plane between the pair of dumbbell motifs. As shown in Figure 5(b), a pair of tetrahedral Al sites can be resolved between the dumbbell motifs in -Al2O3 8. This is not the case for 2,3,4-Al2O3. For 2,3,4-Al2O3, lower occupancy of these tetrahedral sites, and additional presence of octahedral sites together with the oxygen sublattice, leads to difficulty of resolving individual atoms between the pair of octahedral dumbbell motifs. While the HAADF image contrast of the two structures is highly similar, it should be noted that 2,3,4-Al2O3 could be also distinguished from -Al2O3 based on subtle differences in lattice parameters. The comparison of 2,3,4-Al2O3 and 1,2-Al2O3 along the orientation is shown in Figure 5(c,d). The two structures share similarities due to the identical arrangements of highly occupied Al octahedral sites that provide the dominant contrast in HAADF images. The differences can be recognized from the subtle intensity variation from less occupied Al sites, which lead to different image symmetry of the projected structures. In particular, the 2,3,4-Al2O3 has apparent mirror symmetry at the plane of high intensity octahedral sites, while this is not the case for 1,2-Al2O3, as shown in Figure 5 (e). It is noted that, since 1,2-Al2O3 is observed 45 degrees with respect to the intergrowth direction, the deviation from

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the apparent mirror symmetry depends on the proportion of 1-Al2O3 and 2-Al2O3. The deviation increases with increasing content of 1-Al2O3. It is important to note that both structures of 2,3,4-Al2O3 and 1,2-Al2O3 appear periodic in one of the [110]FFC(O) orientation. For 2,3,4-Al2O3, the periodic appearance is a consequence of identical projected potential for 2-Al2O3, 3-Al2O3 and 4-Al2O3. The periodic appearance of 1,2-Al2O3 is a consequence of intergrowth being inclined 45 degrees with respect to the viewing direction, and the resulting images thus represent overlap of both variants.

Figure 5. Comparison of HAADF image contrast from -Al2O3 and -Al2O3 and Al2O3 as obtained on FCC(O) as defined in terms of FCC oxygen sublattice. (a) HAADF image of -Al2O3. (b) HAADF image of -Al2O3. (c) HAADF image of -Al2O3 that is 90 rotated with respect to orientation in panel a. (d) HAADF image of -Al2O3. (e) HAADF image simulation of -Al2O3 crystallographic variants for orientation in (c,d). The high degree of structural similarity between 1,2-Al2O3 and 2,3,4-Al2O3 permits coexistence of both in the overall -Al2O3 microstructure. The coexistence is

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achieved by formation of domains within the common microstructure. The presence of domains is especially prominent in the early stages of -Al2O3 formation, when the domains size is smaller than the size of prior Boehmite precursor nanoparticles. An example of domain microstructure for -Al2O3 is shown in Figure 6. Since all intergrowth domains are accommodated within the common microstructure along the common tetragonal distortion of the oxygen sublattice, there are two rotational degree of freedom for 2,3,4-Al2O3 and one for 1,2-Al2O3. In addition to rotationally related domains, it is important to note that domains with identical orientation, but with a translational offset (antiphase boundaries), are a common feature of microstructure. The domain boundaries of 1,2-Al2O3 and 2,3,4-Al2O3 can be both sharp (welldefined) or diffuse, as shown in Figure 6. The diffuse boundaries are especially prominent for inclined boundaries, as shown in Figure 6(a,b). On the other hand, many vertical and horizontal domain boundaries are often found to be well defined. In several instances, we identify that these well-defined interfaces, as between 1,2Al2O3 and 2,3-Al2O3, assume bonding environment that is consistent with the bonding in -Al2O3.8 An example of well-defined interfaces is shown in Figure 6(c).

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Figure 6 (a,b,c). Example of intergrowth domains within the microstructure of Al2O3. The microstructure consists of domains of 1,2-Al2O3 and 2,3,4-Al2O3 that can seemingly change from one type to another.

DISCUSSION VARIANTS OF -Al2O3 The present work significantly revises our understating of crystallography and structural complexity in -Al2O3. Most importantly, this work revises the number of variants present in the -Al2O3 family. There are now four closely related and experimentally confirmed structural variants identified under -Al2O3 terminology. However, given the experimental difficulty of fully resolving the microstructure of transition Al2O3 phases, an important question is whether the current count now fully captures the overall structural complexity of -Al2O3. To probe whether there are any other possible stacking sequences (variants of Al2O3) which have not been experimentally observed in this work, we constructed several possible models based on the common motif of -Al2O3, and then tested their 18


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energetic preferences with ab-initio calculations. In exploring the possible stacking sequences (variants of -Al2O3), we identified one additional relatively stable configuration. This configuration is created by faulting of 3-Al2O3, with the translation vector corresponding to R= ½[100]. We name the hypothetical variant as 5*-Al2O3. The 5*-Al2O3 variant has a monoclinic symmetry, with the lattice parameters of a=√2aγ,, b=√2aγ, c=1.5aγ, β =104.6°, and belonging to the space group A2 (No.12). The variant of 5*-Al2O3 is shown in Figure 7, together with 2-Al2O3, and 3-Al2O3 and 4-Al2O3. The possible formation of this hypothetical variant can be expected on the basis of energetic favorability (H-Halpha), which is 0.057eV/Al2O3, a value that is slightly higher, but in general comparable with other -Al2O3 variants.

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Figure 7. Comparison of experimentally derived 2-Al2O3, 3-Al2O3, 4-Al2O3 and theoretically hypothesized 5*-Al2O3 (only Al displayed). All four variants are based on an identical structural motif, and can fully intergrow along the vertical direction. In the quest for other possible structural forms of -Al2O3 , we also examined whether one can construct a structural models that would satisfy the symmetry and periodicity of -Al2O3 proposed in the literature.1,14,16 Formulating the models within the constraints of -Al2O3 structural motifs, however, our attempts to construct/identify energetically favorable configurations were not successful. Most notably, the unsuccessful search considered tetragonal P41 model with a triple lattice parameter along the c direction, a model that has been controversial but regularly included in discussions for -Al2O3. 1,15 We find that the -Al2O3 structural motif and the stacking constraints, cannot lead to crystal structure with four-fold symmetry, thus excluding the P41 model. Nevertheless, it is interesting to note that the intergrowth structures of 2,3,4-Al2O3 accommodates four 90 degree related structural motifs of -Al2O3, which would be generally present in the P41 model, although this is without maintaining four fold symmetry. We also note that there has been previously proposed a tetragonal model with triple lattice parameter of 3a along the c direction, and P41212 space group for Fe2O3.21 This latter model shares some similarities with the current structural motif of -Al2O3, but in general is distinguishable from the -Al2O3 family of variants.

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Importantly, we find that the enthalpy of formation (H-Halpha) for -Al2O3 with P41212 is 0.085 eV/Al2O3, which is far less favorable than any other -Al2O3 variant, and thus unlikely configuration. Additional variants that we tried to construct with the basic structural motif of -Al2O3 included orthorhombic structures with larger lattice parameters a=2aγ, b=2aγ, c=1.5aγ and a=aγ, b=aγ, c=1.5aγ as introduced by Jayram and Levi,14 and Wang, respectively16, and monoclinic crystallographic polytypes with P2 space group, with lattice parameters of a=aγ, b=2aγ, c=1.5aγ and a=aγ, b=aγ, c=1.5aγ, as reported by Wang et al.16 The lattice periodicity of a=2 aγ or b=2 aγ is not consistent with structural motif of -Al2O3. Interestingly, the monoclinic lattice with a=aγ, b=aγ, c=1.5aγ and P2 space group can be considered as misinterpretation of 2-Al2O3, which has P21space group.

RELATIONSHIP OF -Al2O3 VARIANTS TO OTHER METASTABLE Al2O3 The high number of confirmed and realistic -Al2O3 variants, as shown in Table 1, raises a question about whether any of these variants can be related to the presently unresolved transition alumina structures, which have been reported to form from different precursors, and designated under different names than Al2O3.22-24

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Table.1 List of transition -Al2O3 variants that form the basis of planar structural disorder in -Al2O3. Inter-growth

1,2-Al2O3 2,3,4Al2O3

Var. nam e

[eV/Al2O]

1 2 3 4

0.050 0.052 0.054 0.053

5*

0.057

HHalpha

Space group

a [A]

b [A]

P212121 7.99 16.10 P21 8.00 11.80 A2/n 12.10 11.30 P 8.00 8.05 A2

12.19 11.30

c [A]







11.80 90 8.05 90 11.41 90 13.10 71.9

90 ~90 102.6 72.1

90 90 90 ~90

11.40

104.6

90

90

The fact that 3-Al2O3 shares identical lattice parameters and space group with the structural polymorph of -Al2O3 has been previously pointed out by Kovarik et al.8 This association thus naturally leads to the conclusion that these two are identical structures but reported under different names. This association also naturally implies that the structure of 3-Al2O3 has a broader applicability, and forms as an independent bulk phase in plasma sprayed aluminas.15,24 The association of 4-Al2O3 with any other unresolved transitional polymorphs is not immediately evident. There are no reports of transition Alumina variants with triclinic symmetry and lattice parameters of a=aγ,, b=aγ, c=1.5aγ, =71.9°, β =72.1°, ~90°, and space group P-1. Nevertheless, we point out that the 4-Al2O3 potentially share some similarities with the unresolved structure of -Al2O3. The polymorph of -Al2O3 was considered to be monoclinic with lattice parameters a=3aγ√2/2, b=2aγ, c=1.5aγ, β=115° and space group P21/c.15,22 While this is very different from the

22


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triclinic symmetry of 4-Al2O3, it can be shown that both structures show diffraction similarities on one of [100] FFC(O). For the hypothetical variant of 5*-Al2O3, we considered association with Al2O3 as a possible choice. As discussed by Levin and Brandon,15,23 -Al2O3 has monoclinic structure with lattice parameters: a= aγ√3/2, b= aγ√2, c= aγ√3/2, β =94°, and C2/m space group. -Al2O3 is the last known transition Alumina polymorphs that evolve from -Al2O3, however, the characteristic 1/6(002) and 1/3(222) of Al2O3 can not be reproduced with 5*-Al2O3, and thus this association cannot be made.

CONCLUSIONS The current work has addressed the structural complexity of -Al2O3. We find that -Al2O3 can adopt two different intergrowth structures that can be differentiated by their variant selection and intergrowth direction. The newly identified type, which has been designated as 2,3,4-Al2O3, represents a linear combination of three independent variants of 2-Al2O3, 3-Al2O3 and 4-Al2O3. All new variants were crystallographically described. The results of this work are discussed in the context of previous -Al2O3 interpretations in the literature. We also discuss how the individual -Al2O3 variants can be related to other metastable polymorphs that are known to evolve from -Al2O3.

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ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Full crystallographic information for δ4-Al2O3 and δ5*-Al2O3 are provided as cif files.

AUTHOR INFORMATION Corresponding Author * [email protected]

NOTES The authors declare no competing financial interests.

ACKNOWLEDGEMENTS The research described in this paper was 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 24


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national laboratory operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract No. DE-AC05-76RLO1830. DS, JS 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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TOC 126x100mm (140 x 140 DPI)


Structural Intergrowth in Î´-Al2O3

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