Editing Lead(II) iodide (section)

== Crystallization ==

Lead iodide prepared from cold solutions usually consists of many small hexagonal platelets, giving the yellow precipitate a silky appearance. Larger crystals can be obtained by exploiting the fact that [[solubility]] of lead iodide in water (like those of [[lead(II) chloride|lead chloride]] and [[lead(II) bromide|lead bromide]]) increases dramatically with temperature. The compound is colorless when dissolved in hot water, but crystallizes on cooling as thin but visibly larger bright yellow flakes, that settle slowly through the liquid — a visual effect often described as "golden rain".<ref name="gold" /> Larger crystals can be obtained by [[autoclave|autoclaving]] the {{chem|Pb|I|2}} with water under pressure at 200&nbsp;°C.<ref name="zhu"/>

Even larger crystals can be obtained by slowing down the common reaction. A simple setup is to submerge two [[Beaker (glassware)|beaker]]s containing the concentrated reactants in a larger container of water, taking care to avoid currents. As the two substances [[diffusion|diffuse]] through the water and meet, they slowly react and deposit the iodide in the space between the beakers.<ref name="tub" />

Another similar method is to react the two substances in a [[gel]] medium, that slows down the diffusion and supports the growing crystal away from the container's walls. Patel and Rao have used this method to grow crystals up to 30&nbsp;mm in diameter and 2&nbsp;mm thick.<ref name="patrao" />

The reaction can be slowed also by separating the two reagents with a permeable membrane. This approach, with a [[cellulose]] membrane, was used in September 1988 to study the growth of {{chem|Pb|I|2}} crystals in zero gravity, in an experiment flown on the [[Space Shuttle Discovery]].<ref name=Scaife/>

{{chem|Pb|I|2}} can also be crystallized from powder by [[Sublimation (phase transition)|sublimation]] at 390&nbsp;°C, in near [[vacuum]]<ref name="forn" /> or in a current of [[argon]] with some [[hydrogen]].<ref name=Liu/>

Large high-purity crystals can be obtained by [[zone melting]] or by the [[Bridgman–Stockbarger technique]].<ref name="matuz" /><ref name="forn" /> These processes can remove various impurities from commercial {{chem|Pb|I|2}}.<ref name=Tonn/>