Crystal growth

Crystal Growth is a major stage of crystallization that occurs after nucleation. It involves the addition of new atoms, ions and other materials to the crystal base to form the lattice of the crystal.

Nucleation

Nucleation is the first stage in the crystallization process. It is a localized physical process that involves the budding of lattice points. Nucleation is a slow process because in order for it to start, the atoms or ions have to be able to become attracted to one another. Nucleation in the case of crystals occurs when there is a supersaturated system and the sites start to cluster on the nanometer scale. These sites when stable become the nuclei or the start of the crystal. The sites have a higher concentration of the solution that is being used to make the crystal, and only once they are stable will they start to be nuclei. However, if the nuclei sites are unstable, they will redissolve back into the solution to try again later. The nuclei need to be at a critical size in order for nucleation to continue over to the crystal growth stage. There are two ways that a solution can go through nucleation: homogeneous nucleation and heterogeneous nucleation.

Nucleation isn't only a process for crystallization; it also occurs elsewhere in nature. Clouds use nucleation to form water droplets. Carbon dioxide bubbles nucleate when you release the pressure on the container holding them. The most visible effect of nucleation happens when you introduce Mentos to Diet Coke.

Homogeneous Nucleation

Homogeneous nucleation occurs when the solution is made up entirely of a uniform content. An example of this is purified water. This typically takes the longest as purified solutions have a lower energy state that they need to reach in order to nucleate. This energy state is lower than their normal ground state energy and it has to be drained off in order for the nucleation to happen. Thus it requires a large amount of energy to get started.

Examples of Homogeneous Nucleation

• Purified Water freezes at -40 degrees Celsius
• Perfect Diamonds have no impurities

Heterogeneous Nucleation

Heterogeneous nucleation happens when there are impurities in the solution that is being used for crystallization. These impurities can be naturally in there or artificially put in, but either way, they form the center for a nucleation point. The impurities create a site where surface energy is lower and easier to overcome for the case of nucleation sites.

Heterogeneous nucleation is the most common form of nucleation in nature. Water forms ice crystals when there is a speck of dust to nucleate around, and others nucleate as well.

The most common form of nucleation used for the artificial growth of crystals is this point where some sort of impurity is introduced into the solution to act as something for the crystals to grow off of. This can be as simple as a piece of string, or as complex as scratches in the container holding the crystallization solution.

Examples of Heterogeneous Nucleation

• Normal water freezes at 0 degrees Celsius. This is due to dust particles being there to nucleate around.
• Important in condensation of rain droplets
• Causes Snowflakes
• Causes Hail