Structure of the hydrated α-Al2O3 (0001) surface

Science

Volumn 288, Issue 5468, 2000, Pages 1029-1033

  Eng, Peter J ^a   Trainor, Thomas P ^a   Brown, Gordon E ^a   Waychunas, Glenn A ^a   Newville, Matthew ^a   Sutton, Stephen R ^a   Rivers, Mark L ^a  

^a UNIVERSITY OF CHICAGO   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ALUMINUM OXIDE; OXYGEN; WATER;

ARTICLE; CHEMICAL STRUCTURE; HYDRATION; PHYSICAL CHEMISTRY; PRIORITY JOURNAL; SPECTROSCOPY; SURFACE PROPERTY; WATER VAPOR; X RAY ANALYSIS;

EID: 0034640360     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.288.5468.1029     Document Type: Article

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22. CTR diffraction is done by measuring diffuse scattering between Bragg points as a function of perpendicular momentum transfer into the substrate. This diffuse CTR scattering aligns normal to the surface and arises from breaking bulk translation symmetry due to the presence of an interface or surface. Surface relaxations and reconstructions and the presence of overlayer atoms modulate the signal along these rods of scattering. For an introduction to the technique see I. K. Robinson and D. J. Tweet, Rep. Prog. Phys. 55, 599 (1992); G. Renaud, Surf. Science Rep. 32, 1 (1998); P. Fenter et al., Geochim. Cosmochim. Acta 64, 1221 (2000)
23. 3) followed by multiple rinses with MilliQ water and heating to 350°C in air. After cooling to room temperature, it was immediately subjected to an extensive wash with MilliQ water and blown dry with an argon jet. Similarly prepared wafers have been studied by our group using photoemission and atomic force microscope, as reported in (5)
24. In previous work (5-9), the hydration reaction was found to be dissociative, leading to surface hydroxyl groups
25. The structure factor is determined by taking the square root of the background-subtracted integrated intensity corrected for active area, polarization, step size, and Lorentz factor [I. K. Robinson, in Handbook on Synchroton Radiation, vol. 3, G. Brown and D. E. Moncton, Eds. (Elsevier, Amsterdam, 1991), pp. 221-266]
26. 3 (space group R3̄c) consists of a distorted hexagonal close-packed layer sequence of oxygens, with aluminum occupying two-thirds of the octahedral holes. The oxygen stacking sequence runs along the c axis, and a unit cell consists of six oxygen layers, giving six formula units per unit cell. The Al atoms are staggered along the c direction about a plane centered between the oxygen layers, and the oxygen atoms are slightly displaced in-plane from their ideal positions. The staggered positions of the Al atoms lead to two sets of Al-O bond lengths. The Al that is displaced in the positive direction along the c axis has three short Al-O bonds (1.86 Å) to the oxygen layer above and three long Al-O bonds (1.97 Å) to the oxygen layer below. The reverse is the case for the Al that is displaced in the negative direction along the c axis. The cell parameters used in this work (a = 4.757 Å, c = 12.988 Å) are from [A. Kirfel and K. Eichhorn, Acta Crystallogr. A 46, 271 (1990)], with bulk isotropic Debye-Waller factors from [N. Ishizawa, T. Miyata, I. Minato, F. Marumo, S. Iwai, Acta Cystallogr. B 36, 228 (1980)]
27. 3 (space group R3̄c) consists of a distorted hexagonal close-packed layer sequence of oxygens, with aluminum occupying two-thirds of the octahedral holes. The oxygen stacking sequence runs along the c axis, and a unit cell consists of six oxygen layers, giving six formula units per unit cell. The Al atoms are staggered along the c direction about a plane centered between the oxygen layers, and the oxygen atoms are slightly displaced in-plane from their ideal positions. The staggered positions of the Al atoms lead to two sets of Al-O bond lengths. The Al that is displaced in the positive direction along the c axis has three short Al-O bonds (1.86 Å) to the oxygen layer above and three long Al-O bonds (1.97 Å) to the oxygen layer below. The reverse is the case for the Al that is displaced in the negative direction along the c axis. The cell parameters used in this work (a = 4.757 Å, c = 12.988 Å) are from [A. Kirfel and K. Eichhorn, Acta Crystallogr. A 46, 271 (1990)], with bulk isotropic Debye-Waller factors from [N. Ishizawa, T. Miyata, I. Minato, F. Marumo, S. Iwai, Acta Cystallogr. B 36, 228 (1980)]
28. For each of the three chemical terminations, there are six crystallographically distinct terminations depending on where the unit cell is cut. Because they are chemically equivalent, the six terminations must be equally probable, and therefore our model consisted of equal weighting of these terminations, with the same fit parameters (displacements, Debye-Waller factors, and occupancies) used for each
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30. The (00L) rod is sensitive only to ordering in the direction perpendicular to the surface and is unaffected by the degree of lateral ordering. The dominant free parameter in the overlayer model is the z position of the layer. The Debye-Waller factor acts as a measure of layer disorder even though the actual structure is likely to be more complex. A more accurate description of the layer involves a continuous electron density distribution normal to the surface; however, extensive specular reflectivity data are required to uniquely determine such a distribution
31. 3 (0001) surface has p3 symmetry, with Al atoms positioned at the centers of the threefold axis and the oxygens arranged about the threefold axis. To maintain symmetry in our surface model, we displaced the Al atoms only along the z direction and we constrained the oxygen atoms to maintain trigonal symmetry
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44. J. P. Fitts, T. P. Trainor, D. Grolimund, G. E. Brown Jr., G. A. Parks, unpublished data
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46. Supported by Department of Energy (DOE) grant DEFG03-93ER14347-A007 (Stanford University), DOE grant DE-FGO2-94ER14466, National Science Foundation grant EAR-9906456, and the W. M. Keck foundation (GeoSoilEnviroCARS, University of Chicago). Use of the Advanced Photon Source was supported by the DOE, Basic Energy Sciences, Office of Energy Research, under contract W-31-109-ENG-38. We thank J. Fitts, D. Grolimund, F. Sopron, and N. Lazarz for their assistance with the measurements and J. V. Smith for constructive comments on the manuscript