X-Ray Standing Wave Imaging

The spatial distribution of elemental profiles can be determined directly by Fourier inversion from X-ray standing wave data because XSW is directly phase sensitive (unlike traditional crystallography). The complex structure factor of the fluorescing atom can be written as: F = F_Hexp(i2pP_H), where F_H and P_H are the coherent fraction and position respectively. The density profile can be determined directly without reference to any model structures for a sufficiently complete set of measurements.

To illustrate the power of this approach, we determined the impurity ion site distribution in the muscovite lattice along [00L]. Making use of the known muscovite structure, XSW imaging directly revealed the Mn, Fe and Ti were located exclusively in the octahedral site (O), while Ba would found exclusively in the interalyer site (I). We also directly confirmed the internal consistency of this approach by comparing the known and measured site distributions of the major elements, including (Si, Al, and K). This confirmed that Si is in the tetrahedral site (T), while Al is in both tetrahedral and octahedral sites, and K is exclusively in the interlayer site.  

This approach has been used recently to image the site distributions of various adsorbates at the mineral-electrolyte interface, including Sr²⁺, Zn²⁺ and Y³⁺ at the rutile surface, as well as selenite and arsenate at the hematite surfaces.  In particular, this approach revealed an unexpected difference in the lateral adsorption sites of Zn²⁺ at the rutile-water interface, which was found to adsorb at two distinct sites, as compared with that observed for Sr²⁺ and Y³⁺ which adsorbed solely in a single site.
  

Derived lateral ion site distributions (a-d) on the rutile(110) surface (e) for Ti (from the substrate lattice) and adsorbed Zn, Sr, and Y.  Color bar (a, inset) indicates density scale.  Schematic structures are shown for Zn2+, which adsorbs at two sites (f), and Sr2+ and Y3+ that adsorb at the tetradentate site (g).  Ion heights above the surface Ti-O plane, z, are noted below each image.

References

Cheng L., Fenter P., Bedzyk M. J., and Sturchio N. C., "Fourier-expansion Solution of Atom Distributions in a Crystal using X-ray Standing Waves", Physical Review Letters 90(25), 255503 (2003).

Z. Zhang, P. Fenter, L. Cheng, N. C. Sturchio, M. J. Bedzyk, M. L. Machesky, D. J. Wesolowski, "Model-Independent Imaging of Adsorbed Cations at the Crystal-Water Interface", Surface Science Letters, 554(2-3) L95-L100 (2004).

J. G. Catalano, Z. Zhang, C. Park, P. Fenter, M. J. Bedzyk, “Bridging Arsenate Surface Complexes on the Hematite (012) Surface”, Geochimica et Cosmochimica Acta, 71 1883-1897 (2007).

J. G. Catalano, Z. Zhang, P. Fenter, M. J. Bedzyk, “Inner-Sphere Adsorption Geometry of Se(IV) at the Hematite (100)-Water Interface”, Journal of Colloid and Interface Science, 297 665-671 (2006).