This is a repository copy of Crystallization of citrate-stabilized amorphous calcium phosphate to nanocrystalline apatite : a surface-mediated transformation.

CaSO 4 minerals (i.e. gypsum, anhydrite and bassanite) are widespread in natural and industrial environments. During the last several years, a number of studies have revealed that nucleation in the CaSO 4 -H 2 O system is non-classical, where the formation of crystalline phases involves several steps. Based on these recent insights we have formulated a tentative general model for calcium sulfate precipitation from solution. This model involves primary species that are formed through the assembly of multiple Ca 2+ and SO 4 2ions into nanoclusters. These nanoclusters assemble into poorly ordered (i.e. amorphous) hydrated aggregates, which in turn undergo ordering into coherent crystalline units.The thermodynamic (meta)stability of any of the three CaSO 4 phases is regulated by temperature, pressure and ionic strength with gypsum being the stable form at low temperatures and low to medium ionic strengths, and anhydrite the stable phase at high temperatures and lower temperature at high salinities. Bassanite is metastable across the entire phase diagram but readily forms as the primary phase at high ionic strengths across a wide range of temperatures, and can persist up to several months. Although the physicochemical conditions leading to bassanite formation in aqueous systems are relatively well established, nanoscale insights into the nucleation mechanisms and pathways are still lacking. To fill this gap, and to further improve our general model for calcium sulfate precipitation, we conducted in situ scattering measurements at small-and wide-angles (SAXS/WAXS) and complemented these with in situ Raman spectroscopic characterization.Based on these experiments we show that the process of formation of bassanite from aqueous solutions is very similar to the formation of gypsum: it involves the aggregation of small primary species into larger disordered aggregates, only from which the crystalline phase develops. These data thus confirm our general model of CaSO 4 nucleation and provide clues to explain the abundant occurrence of bassanite on the surface of Mars (and not on the surface of Earth).

1Metal ions are frequently incorporated into crystalline materials to improve their 2 electrochemical properties and to confer new physicochemical properties. Naturally-3 occurring phosphate apatite, which is formed geologically and in biomineralization processes, 4 has extensive potential applications and is therefore an attractive functional material. In this 5 study, we generate a novel building block for flexible optoelectronics using bio-inspired 6 methods to deposit a layer of photoactive titanium-modified hydroxyapatite (TiHA) 7 nanoparticles (NPs) on conductive polypyrrole(PPy)-coated wool yarns. The titanium 8 concentration in the reaction solution was varied between 8-50 mol% with respect to the 9 phosphorous, which led to titanate ions replacing phosphate in the hydroxyapatite lattice at 10 levels up to 17 mol%. PPy was separately deposited on wool yarns by oxidative 11 polymerization, using two dopants: (i) antraquinone-2-sulfonic acid to increase the 12 conductivity of the PPy layer and (ii) pyroglutamic acid, to reduce the resistivity of the wool 72TiHAs were synthesized using increasing amounts of the titanium precursor to reach the 73 molar percentage of Ti atoms with respect to P of 8%, 17%, 25% and 50%. The obtained were used to collect diffraction patterns on samples heated at 700°C for 6 hours. 92Complementary analysis was performed on raw powders using the Diamond Light Source 96The structural and microstructural analysis of the samples was performed using the FullProf TiHA NPs were studied after thermal transformation as described by XRD and Raman 119 spectroscopy. Each sample was placed on a potassium-bromide substrate producing minimal 120 background signal, and the potassium-bromide substrate containing the sample was placed on

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