Structural interpretation of the diaspore–corundum and boehmite–γ-Al<sub>2</sub>O<sub>3</sub>transitions

Ervin, G.

doi:10.1107/s0365110x52000216

Cited by 104 publications

(52 citation statements)

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“…Lippens & DeBoer (1964) assumed, in view of the lamellar texture, that the cubic close-packed oxygen layers in the ~7 structure are one-dimensionally disordered in the direction parallel to the c axis of the precursor bayerite (which is the [111] direction of the spinel). Ervin (1952) observed that the X-ray powder diffraction patterns of all the transition aluminas have in common the strong line at d = 1.39 A (20 = 67-3°), see Figs. 2 and 3 (all 20's refer to Cu Ka~ radiation).…”

Section: Aumnalmentioning

confidence: 98%

Structures and transformation mechanisms of the η, γ and θ transition aluminas

Zhou¹,

Snyder²

1991

Acta Crystallogr B Struct Sci

759

546

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617B(4,4) = 2rr2At 2 (Yamamoto et al., 1984) AbstractThe defect crystal structures of r/-, y-and 0-alumina obtained from dehydroxylation of well crystallized bayerite and boehmite have been derived from the analysis of their X-ray powder diffraction patterns and from the Rietveld refinement of their neutron powder diffraction patterns. Profile analysis of the various reflection zones in these defect spinel structures shows different coherent domain sizes which can be associated with the tetrahedral and octahedral aluminium and the oxygen sublattices. These observations have been used to define the nature of the crystal structures, and to give insight into the transformation mechanisms. The very large surface energies of these phases are evident in the observation of * Present address: Materials Data, Inc., PO Box 791, Livermore, CA 94550, USA.t To whom all correspondence should be directed.0108-7681/91/050617-14503.00 a nearly three-coordinated surface AI atom in the 7/ phase, and are the reason for the stability of the defect spinel structures of the transition aluminas. The reduction of surface area and the ordering of the tetrahedral AI sublattice which occurs on heating causes the spinel framework to collapse so that the structure, which exhibits tetragonal character at the early stage of the transition, settles into monoclinic 0-alumina displacively at the later stage, and eventually transforms to hexagonal corundum reconstructively. Thus 0-alumina should be considered the ultimate rather than the intermediate structural form into which the transition aluminas could evolve on the way to corundum. The overall crystal structure of the transition aluminas should therefore be viewed intrinsically as spinel deformed rather than as tetragonally deformed. Crystal data at room temperature: r/-alumina, cubic, Fd3m, a = 7.914 (2) A, RB = 6.24, Rp = 6-50, R, = 8.43%; y-alumina, cubic, Fd3m, a = © 1991 International Union of Crystallography 618 r/, ,/ AND 0 TRANSITION ALUMINAS 7.911 (2),~, Rs = 10.53, Rp = 7-61, R,. = 10.25%; 0-alumina, monoclinic, C2/m, a = 11.854(5), b = 2-904 (1), c = 5-622 ,~, /3 = 103.83 (7) °, Re = 15-02, Rp = 9"37, R,. = 11"93%.

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Section: Aumnalmentioning

confidence: 98%

Structures and transformation mechanisms of the η, γ and θ transition aluminas

Zhou¹,

Snyder²

1991

Acta Crystallogr B Struct Sci

759

546

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“…Most authors admit the existence of six well-defined, crystalline transition aluminas, 1,2 usually designated with the prefixes g, x, d, u, h, and k. Transition aluminas form by dehydration of the Al trihydrates and monohydrates, which decompose on heating in different ways, 1 giving different sequences of transition aluminas before finally yielding corundum. The structures of transition aluminas are based in a more or less perfect cubic closepacking of the oxygen anions, 3 in whose interstices the Al cations are stuffed. The g and h forms are the most disordered and are pseudocubic, with the cations occupying disorderly the cation positions in the spinel structure.…”

Section: Introductionmentioning

confidence: 99%

The infrared dielectric properties of η–Al₂O₃

1996

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The infrared complex permittivity function of pseudo-cubic, disordered, spinel-type variety of alumina, h -Al 2 O 3 , obtained by spray pyrolysis, has been determined from its IR reflectance spectra, measured at near to normal incidence on pressed powder pellets. The optical constants obtained therefrom have been verified by using them in the simulation of the corresponding absorption spectra for KBr-diluted pellets of this material, and these are in excellent agreement with the experimental spectra. All calculations are based on a procedure for the estimation of the effective dielectric function of a mixture incorporating percolation features, which has been recently developed by the authors.

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“…The tetrahedral-like parameters of resonance (c), when it first appears, suggest that a significant proportion of the iron is located in the tetrahedral sites of a transition alumina, either the y-alumina, or, more likely, 0-alumina (diffuse X-ray peaks suggestive of the latter appear simultaneously with the appearance of resonance (c) and disappear as heating progresses). According to Ervin [10], the transition aluminas may all be regarded as structurally related to y-A1203, differing only in the degree of ordering of the aluminium cations; furthermore, 0-alumina, the most ordered form of y-alumina, is closely related to corundum, having its aluminium atoms in the same configuration, but inscribed in a face-centred cubic oxygen network rather than the hexagonal oxygen network of corundum [11]. This suggests that the transformation to corundum could involve a change in the oxygen packing while essentially retaining the cation configuration of the precursor phase, which achieves a more typical corundum arrangement only slowly on heating.…”

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confidence: 99%