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Calcium carbonates and in particular amorphous calcium carbonate (ACC) are of great interest for bone substitution, due to their excellent biocompatibility and ability to release some of the bone mineral constitutive ions. Despite thorough investigations on the ACC precipitation mechanism in the presence of a wide variety of additives to stabilise it, their influence on the composition and structuration of the resulting ACC is rarely finely characterised. In this article, pyrophosphate (Py) was studied as an additive, due to its calcite inhibitor role and the good bioactive properties it demonstrated in vitro and in vivo. A series of pyrophosphate-stabilised ACC (PyACC) powders were synthesised by co-precipitation in water with various amounts of Py, and the structure, composition and multiscale organisation of the resulting compounds were thoroughly characterised. XRD, FTIR spectroscopy and FEG-SEM showed a reduction of the amount and size of calcite crystallites when increasing the Py content, crystallites co-existing with an increasing quantity of the amorphous phase, up to obtaining fully amorphous materials above a certain Py amount. Beyond the stabilising role of Py, and considering SAXS, PDF and solid-state NMR analyses, we proposed an experimental model for PyACC based on a vaterite-like (or proto-vaterite) cluster of 1 nm surrounded by a 2 nm-outer shell containing pyrophosphate ions and pseudo-structural water stabilising the cluster. This comprehensive study and the resulting model are predominant prerequisites to understand the underlying mechanisms of PyACC evolution and could contribute to the rational design of tunable amorphous and bioinspired inorganic bone substitute materials. [ABSTRACT FROM AUTHOR]

α-Canaphite (CaNa2P2O7·4H2O) is a layered calcium disodium pyrophosphate tetrahydrate phase of significant geological and potential biological interest. This study overcomes the lack of a reliable protocol to synthesize pure α-canaphite by using a novel simple and reproducible approach of double decomposition in solution at room temperature. The pure α-canaphite is then characterized from the atomic to the macroscopic level using a multitool and multiscale advanced characterization strategy, providing for the first time full resolution of the α-canaphite monoclinic structure, including the hydrogen bonding network. Synchrotron X-ray diffraction and neutron diffraction are combined with multinuclear solid state NMR experimental data and computational modeling via DFT/GIPAW calculations. Among the main characteristics of the α-canaphite structure are some strong hydrogen bonds and one of the four water molecules showing a different coordination scheme. This peculiar water molecule could be the last to leave the collapsed structure on heating, leading eventually to anhydrous α-CaNa2P2O7 and could also be involved in the internal hydrolysis of pyrophosphate ions as it is the closest water molecule to the pyrophosphate ions. Relating such detailed structural data on α-canaphite to its physico-chemical properties is of major interest considering the possible roles of canaphite for biomedical applications. The vibrational spectra of α-canaphite (deuterated or not) are analyzed and Raman spectroscopy appears to be a promising tool for the identification/diagnosis of such microcrystals in vitro, in vivo or ex vivo. [ABSTRACT FROM AUTHOR]