Fast and slow components in the crystallization of a model multicomponent system, NaKCa(NO3): the role of composition fluctuations.

We use calorimetrically detected crystal nucleation and growth studies to broaden the discussion of fluctuation-induced nucleation processes to include composition fluctuations in ionic complex-forming systems. We use the model system Ca(NO(3))(2)-KNO(3) with NaNO(3) introduced as a third component, so that crystallization kinetics can be controlled by change of alkali cation at constant mole fraction of Ca(NO(3))(2). At fixed NaNO(3) content, we find separate and thermodynamically anomalous kinetics for the crystallization of NaNO(3) and Ca(NO(3))(2),which we attribute to the importance of slow concentration fluctuations in the latter case. The "nose" of the time-temperature-transformation TTT curve for crystallization of the Ca(NO(3))(2) occurs at much higher temperatures and longer times than that for NaNO(3) and the shape of the curve is different. Above the metastable liquidus surface of NaNO(3), supercooled ternary melts can persist for long times. Suppression of the fast NaNO(3) crystallization, by replacement of Na(+) by K(+), is a prerequisite for easy vitrification in this system.