Role of Impurities in the Kinetic Persistence of Amorphous Calcium Carbonate: A Nanoscopic Dynamics View

Amorphous calcium\ncarbonate is found in the early stages of biomineral\nformation. Its moldability and its kinetic persistence against crystallization\nmake it of potential interest to the development of biomimetic materials.\nThe presence of water and of inorganic impurities such as Mg²⁺ has been reported to increase its kinetic stability. Yet, the precise\nmechanisms of stabilization as well as the link between structural\ndynamics and crystallization kinetics are still missing. Here, we\nreport the vibrational and diffusive dynamics of water and other atomic\nconstituents in a series of Mg²⁺-doped solid amorphous\ncalcium carbonates revealed using coherent X-ray and incoherent neutron\nscattering. This unique combination of techniques presents a novel\nview on the nanoscopic dynamic behavior of these amorphous materials:\nthe presence of Mg²⁺ results in a higher frequency of structural\nrearrangements within the material. However, this increased dynamic\nbehavior does not translate into a higher crystallization kinetics\nbecause of a concomitant stiffening of the water hydrogen bond network.