2016, Farhadi-Khouzani, Masoud, Chevrier, Daniel M., Zhang, Peng, Hedin, Niklas, Gebauer, Denis

Temperature and pH value can affect the short-range order of proto-structured and additive-free amorphous calcium carbonates (ACCs). Whereas a distinct change occurs in proto-vaterite (pv) ACC above 45 °C at pH 9.80, proto-calcite (pc) ACC (pH 8.75) is unaffected within the investigated range of temperatures (7-65 °C). IR and NMR spectroscopic studies together with EXAFS analysis showed that the temperature-induced change is related to the formation of proto-aragonite (pa) ACC. The data strongly suggest that the binding of water molecules induces dipole moments across the carbonate ions in pa-ACC as in aragonite, where the dipole moments are due to the symmetry of the crystal structure. Altogether, a (pseudo-)phase diagram of the CaCO3 polyamorphism in which water plays a key role can be formulated based on variables of state, such as the temperature, and solution parameters, such as the pH value.

2016, Farhadi-Khouzani, Masoud, Chevrier, Daniel M., Zhang, Peng, Hedin, Niklas, Gebauer, Denis

Temperatur und pH-Wert können die Nahordnung in proto-strukturiertem, amorphem Calciumcarbonat (ACC) in Abwesenheit von Additiven beeinflussen. Während im Proto-Vaterit(pv)-ACC bei pH 9.80 bei >45 °C eine ausgeprägte Veränderung stattfindet, ist dies beim Proto-Calcit(pc)-ACC innerhalb des untersuchten Temperaturbereichs von 7 bis 65 °C nicht der Fall. Mit IR- und NMR-Spektroskopie sowie EXAFS konnte gezeigt werden, dass die temperaturbedingten Veränderungen auf die Bildung von Proto-Aragonit(pa)-ACC zurückzuführen sind. Die experimentellen Befunde sprechen zudem dafür, dass bei pa-ACC durch die Bindung von Wassermolekülen ein Dipolmoment innerhalb der Carbonationen induziert wird, vergleichbar mit dem Effekt der Kristallsymmetrie auf die Carbonationen in der Aragonitstruktur. So kann in Abhängigkeit von Zustandsgrößen wie der Temperatur und Lösungsparametern wie dem pH-Wert die Polyamorphie von CaCO3 anhand eines (Pseudo-)Phasendiagramms, in dem Wasser eine Schlüsselrolle spielt, dargestellt werden.

2015, Farhadi-Khouzani, Masoud, Chevrier, Daniel M., Güttlein, Patricia, Hauser, Karin, Zhang, Peng, Hedin, Niklas, Gebauer, Denis

Amorphous calcium carbonate (ACC) is known to play a prominent role in biomineralization. Different studies on the structure of biogenic ACCs have illustrated that they can have distinct short-range orders. However, the origin of so-called proto-structures in synthetic and additive-free ACCs is not well understood. In the current work, ACC has been synthesised in iso-propanolic media by direct precipitation from ionic precursors, and analysed utilising a range of different techniques. The data suggest that this additive-free type of ACC does not resemble clear proto-structural motifs relating to any crystalline polymorph. This can be explained by the undefined pH value in iso-propanolic media, and the virtually instantaneous precipitation. Altogether, this work suggests that aqueous systems and pathways involving pre-nucleation clusters are required for the generation of clear proto-structural features in ACC. Experiments on the ACC-to-crystalline transformation in solution with and without ethanol highlight that polymorph selection is under kinetic control, while the presence of ethanol can control dissolution re-crystallisation pathways.