Structure of the Fluoroapatite (100)-Water Interface
High resolution specular X-ray reflectivity has provided a direct view of the structure of the fluorapatite (FAp)-water interface, through measurements of the structure of FAp(100) growth surfaces. Key observations from this work are:
- Fap growth surface termination is revealed by the sharp intensity dip just above the (100) Bragg peak (B), characteristic of “termination interference” associated with the Ca-PO4 termination, as shown in A.
- The outermost surface layer was found to be deficient in either the Ca and/or F ions with a nominal composition of θCa = 1.67 ± 0.33 with fixed θF = 1.0 (with respect to the ideal θCa = θF = 2). Surface structural relaxations included
- ΔzCa = 0.01 ± 0.14 Å
- ΔzP = - 0.34 Å ± 0.12 Å
- ΔzF = 0.38 Å ± 0.37 Å
- Two distinct layers above the mineral surface were observed at heights of 2.64 ± 0.09 Å and 4.17 ± 0.14 Å, with 3.5 ± 1.3 and 4.1 ± 2.2 water molecules per unit mesh, which is substantially smaller than expected for a dense water layer (~6.5 waters per unit mesh).
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Figure: (A) Perspective view of fluorapatite (100) with Ca ions (green), P (red), oxygen (blue) and F (yellow). (B) High resolution X-ray reflectivity data compared to various interfacial models. (C) Derived interfacial electron density profiles. Blue curve corresponds to the best-fit model. |
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Reference: C. Park, P. Fenter, Z. Zhang, L. Cheng, and N. C. Sturchio, "Structure of Naturally Grown Fluorapatite (100)–Water Interface by High-Resolution X-ray Reflectivity", American Mineralogist 89, 1647-1654 (2004).
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