Abstract:
We have determined the structure of the {101} face of ammonium dihydrogen phosphate (ADP) in aqueous solution by the use of surface x-ray diffraction. We find that a layer of ammonium ions terminates the {101} face of ADP, followed by a layer of phosphate ions. For a proper description of the experimental data, relaxation in these two layers needs to be included. The ammonium layer is relaxed inward by 0.34 +/- 0.13 Angstrom while the dihydrogen phosphate layer is relaxed 0.27 +/- 0.11 Angstrom outward. Comparing our results with earlier experiments on potassium dihydrogen phosphate KDP {101}, we find both similarities and differences between these two isomorphous crystals. The most striking difference is the reversal of the direction of relaxation of the outmost layer. Hydrogen bonding in ADP is a likely cause for this difference.