Structural study of calcium phosphonates: a combined synchrotron powder diffraction, solid-state NMR and first-principle calculations approach

Abstract

The structures of four Ca-phosphonate phases are reported here: Ca(C₆H₅–PO₃H)₂ (1), Ca(C₆H₅–PO₃)·2H₂O (2), Ca(C₄H₉–PO₃H)₂ (3) and Ca(C₄H₉–PO₃)·H₂O (4). Structural models were obtained ab initio by using a combined synchrotron powder diffraction, solid-state nuclear magnetic resonance, and gauge including projector augmented wave (GIPAW) calculation approach. The ¹H, ¹³C, ³¹P and ⁴³Ca NMR parameters calculated from these structural models were found to be in good agreement with the experimental values, thereby indicating the high accuracy of the DFT-optimized structures. Correlations between the NMR parameters and structural features around the phosphonate were then analyzed, showing in particular the high sensitivity of the ³¹P asymmetry parameter η_CS and the ⁴³Ca isotropic chemical shift to changes in local structure around the phosphonate groups and the Ca²⁺, respectively. Finally, the NMR data of a new mixed Na–Ca phosphonate phase, Ca_1.5Na(C₄H₉–PO₃)₂, are reported.