title: Water as the Key to Proto-Aragonite Amorphous CaCO3.
abstract: 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.
authors.affiliation: Department of Chemistry, Physical Chemistry, University of Konstanz, Universitätsstrasse 10, Box 714, 78457, Konstanz, Germany.
authors.forename: Masoud
authors.initials: M
authors.lastname: Farhadi-Khouzani
authors.affiliation: Department of Chemistry & School of Biomedical Engineering, Dalhousie University, Halifax, Canada.
authors.forename: Daniel M
authors.initials: DM
authors.lastname: Chevrier
authors.affiliation: Department of Chemistry & School of Biomedical Engineering, Dalhousie University, Halifax, Canada.
authors.forename: Peng
authors.initials: P
authors.lastname: Zhang
authors.affiliation: Department of Materials and Environmental Chemistry, Stockholm University, 10691, Stockholm, Sweden.
authors.forename: Niklas
authors.initials: N
authors.lastname: Hedin
authors.affiliation: Department of Chemistry, Physical Chemistry, University of Konstanz, Universitätsstrasse 10, Box 714, 78457, Konstanz, Germany. denis.gebauer@uni-konstanz.de.
authors.forename: Denis
authors.initials: D
authors.lastname: Gebauer
keywords: NMR spectroscopy; amorphous calcium carbonate; biomineralization; polyamorphism; water