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[MIS17-P04] Conversion from a stable crystal to a metastable crystal by grinding
Keywords:phase conversion, cluster incoporation, crystal grinding
Materials have some crystal structures. In general cases, the crystal structure of the ground state is unique. Other structures with higher free energy are thermodynamically metastable. When stable and metastable crystals coexist in a system, the stable crystal grows and the metastable one shrinks. Although temporal appearance of metastable crystal is well known as the Ostwald’s step rule, eventually large stable crystals form in the system in equilibrium.
In the case of a chiral crystal, the chiral pair of crystal structures are realized with the same probability since the pair of structures are energetically equal. In 2005, Viedma discovered the conversion from racemic mixture of chiral sodium chlorate crystals to crystals with single chirality in a saturated solution by stirring with glass beads. In this system, the racemic mixture of crystals is the equilibrium state, and the single chirality state is the one of metastable states. This phenomenon is distinguished from the ordinary Ostwald ripening and called Viedma ripening. One of the hypotheses, which explain Viedma ripening, is the acceleration of crystal growth by the cluster incorporation. If the mechanism of Viedma ripening works effectively, it might be possible to convert from crystals of a stable structure to crystals of a metastable structure.
We demonstrate the conversion from crystals of a stable structure to crystals of a metastable structure using the theoretical idea of Viedma ripening. We introduce the generalized Becker-Döring model, which includes the cluster incorporation to the crystal of the same structure. When the amount of metastable crystals is large enough for that of stable crystals, metastable crystals grow and stable ones disappear at the steady state. If the cluster incorporation mechanism does not work, the amount of the metastable crystal is not amplified from any initial state. The system only reaches the equilibrium as well as the case without crystal grinding. Grinding effectively breaks crystals, and supplies small clusters, and then, the metastable crystals completely consumes the stable crystals by the acceleration of crystal growth due to the small cluster incorporation.
In the case of a chiral crystal, the chiral pair of crystal structures are realized with the same probability since the pair of structures are energetically equal. In 2005, Viedma discovered the conversion from racemic mixture of chiral sodium chlorate crystals to crystals with single chirality in a saturated solution by stirring with glass beads. In this system, the racemic mixture of crystals is the equilibrium state, and the single chirality state is the one of metastable states. This phenomenon is distinguished from the ordinary Ostwald ripening and called Viedma ripening. One of the hypotheses, which explain Viedma ripening, is the acceleration of crystal growth by the cluster incorporation. If the mechanism of Viedma ripening works effectively, it might be possible to convert from crystals of a stable structure to crystals of a metastable structure.
We demonstrate the conversion from crystals of a stable structure to crystals of a metastable structure using the theoretical idea of Viedma ripening. We introduce the generalized Becker-Döring model, which includes the cluster incorporation to the crystal of the same structure. When the amount of metastable crystals is large enough for that of stable crystals, metastable crystals grow and stable ones disappear at the steady state. If the cluster incorporation mechanism does not work, the amount of the metastable crystal is not amplified from any initial state. The system only reaches the equilibrium as well as the case without crystal grinding. Grinding effectively breaks crystals, and supplies small clusters, and then, the metastable crystals completely consumes the stable crystals by the acceleration of crystal growth due to the small cluster incorporation.