Experimental study on the transition from periodit precipitation to fractal crystal aggregates

我们绘制了形态相图，它是琼脂浓度和碘离子与铅阴离子比例的函数。为了了解碘化铅的结晶过程，我们进行了原位实验，测量了结晶的传播速度。结果发现，在条带结构和分枝结构中的传播速度是相同的，说明由于琼脂浓度的不同而导致的扩散率差异不会引起形态转变。通过对分枝结构形成过程的观察，发现大晶体表面的异质形核与分枝结构的形成密切相关。从扫描电子显微镜(SEM)观察到，带状结构和分枝结构中的晶体数密度相差三个数量级。此外，从扫描电子显微镜观察发现，琼脂结构的不同取决于琼脂浓度的不同并不会导致数密度的差异。通过一系列实验发现，碘化铅成核过程不是简单的，而是相当复杂的，不仅是碘化铅单体的浓度，而且过量的阴离子和离子也起着重要作用。这些实验结果表明，琼脂浓度对成核过程的细微影响导致了带状结构和分枝结构之间的形态转变。

We made a morphological phase diagram as a function of agar concentration and a ratio of iodide ion and lead anion. In order to understand the crystallization process of lead iodide, we carried out in-situ experiments by which the propagation velocity of crystallization is measured. As a result, it was found that the propagation velocities both in the banding structure and the branching structure are the same, suggesting that the diffusivity difference due to concentration difference of agar does not cause the morphological transition. From the observation of formation process of branching structure, it is found that the heterogeneous nucleation on the surface of large crystals is deeply related to the formation of branching structure. From the observation by the scanning electron microscope (SEM), crystal number density in banding structure is different from that in branching structure by three orders of magnitude. In addition, from the SEM observation, agar structure difference depending on the agar concentration is found not to cause the number density difference. From a series of experiments, it is found that the lead iodide nucleation process is not simple, rather complex that not only the monomer concentration of lead iodide but also the excess anion and ion play important roles. As a result of these experiments, it is concluded that the subtle effect of agar concentration on the nucleation process causes the morphological transition between the banding structure and branching structure.