Parasitic crystallization of colloidal electrolytes: growing a metastable crystal from the nucleus of a stable phase

Abstract

Despite its lower stability and higher nucleation barrier, a metastable charge-disordered colloidal phase manages to parasitically crystallize from nuclei of the stable charge-ordered phase due to its enhanced kinetic crystal growth.

Fcc vs. hcp competition in colloidal hard-sphere nucleation: on their relative stability, interfacial free energy and nucleation rate

“…However, the estimated γ from our Seeding simulations cannot be ascribed to any specific crystal orientation rather than to a curved interface which contains different contributions of distinct crystal orientations. This behaviour of γ with pressure was already known for the fcc crystal phase [29,55,86,138], and it has also been recently reported for oppositely charged colloidal hard-spheres [107]. We find that at high pressure (i.e., p * > 14), the interfacial free energy of both polymorphs is rather similar (within the uncertainty of our calculations), however, at low pressure (i.e., p * < 13), γ is slightly lower for fcc clusters.…”

“…A well-known example is the Ostwald step rule [143] proposed in 1897 (and later reexamined by Stranski and Totomanow [144]), where it was postulated that the crystal phase that nucleates from the fluid needs not be the one that is thermodynamically most stable, but the one with the lowest free energy barrier. An opposite behaviour recently reported by us, is parasitic crystallization [107], in which the nucleating phase is the thermodynamically most stable one, but the post-critical crystal growth occurs through an out-of-equilibrium process by which a different (metastable) parasitic phase from the nucleating one and/or the most stable one, grows from the critical nucleus. These different possible intricate crystallization scenarios highlight how polymorphic competition can also take place beyond the nucleus formation.…”

“…We determine the optimalq 4 andq 6 thresholds as a function of pressure with the mislabelling criterion as shown in Ref. [107] to distinguish among fluid-like and solid-like particles (q 6 ), and to discriminate between fcc-like and hcp-like solid particles (q 4 ). The optimalq 6 andq 4 thresholds as a function of pressure are described by the following linear equations:q 6 t,f cc =0.133+0.01819p * for distinguishing between fcc-like and fluid-like particles,q 6 t,hcp =0.14883+0.…”

“…From MI simulations at r w < r o w , where stable crystal slabs were already formed, we set a marginally higher pressure than the melting one, and we simulate until the full system crystallizes. To discriminate between fcc-like and hcp-like particles, we employ theq 4 order parameter [126] within the mislabelling scheme [55,107,137]. We find that among all the studied crystal faces, the fcc (111) (or equivalently hcp (0001)) plane is the only crystal face that enables stacking disordered growth of both polymorphs (see Fig.…”

“…In a similar way, at about 2000 bar, ice I and III equally compete in stability and propensity to nucleate [104,105], although between these two polymorphs ice stacking mixtures cannot be attained. But polymorphic competition is not at all an exclusive feature of water, also in colloidal suspensions of oppositely charged colloids [106], different crystal polymorphs with positionally-charge order and disorder tightly play to nucleate from the same metastable fluid at moderate temperature and high pressure [89,107]. Additionally, in Lennard-Jones supercooled fluids, it has been shown how polymorphic selection can be finely tuned in order to promote either body-centered cubic (bcc) or face-centered-cubic (fcc) crystallization [97].…”