Crystalline vs Polycrystalline
Although we define solids as crystalline or amorphous, in nature there are few examples of these pure forms. Most of the time, they are mixed together or form variations. Polycrystalline is such a variation from the crystalline solids. Here, we will look into the differences between crystalline and polycrystalline in detail.
Crystalline can be a crystal, composed of crystal, or resemble a crystal. Crystalline solids or crystals have ordered structures and symmetry. The atoms, molecules, or ions in crystals are arranged in a particular manner, thus have a long-range order. In crystalline solids, there is a regular, repeating pattern; thus, we can identify a repeating unit. By definition, “a crystal is a homogenous chemical compound with a regular and periodic arrangement of atoms. Examples are halite, salt (NaCl), and quartz (SiO2). But, crystals are not restricted to minerals: they comprise most solid matters such as sugar, cellulose, metals, bones, and even DNA.”1 Crystals are naturally occurring as large crystalline rocks on earth, such as quartz, granite. Crystals are formed by living organisms too. For example, calcite is produced by mollusks. There are water-based crystals in the form of snow, ice or glaciers. Crystals can be categorized according to their physical and chemical properties. They are covalent crystals (e.g. diamond), metallic crystals (e.g. pyrite), ionic crystals (e.g. sodium chloride), and molecular crystals (e.g. sugar). Crystals can have different shapes and colors. Crystals have an aesthetic value, and it is believed to have healing properties; thus, people use them to make jewelry.
Other than being a crystal, some solids may resemble a crystal by adopting some of its properties. For example, these can be sparkly, transparent or clear, or have the structure similar to a crystal.
In nature, most of the time, crystals appear to have disrupted their long-range order. Polycrystalline are solids that are composed of many numbers of small crystals. These are arranged in different orientations and are bound by highly defective boundaries. The crystals in a polycrystalline solid are microscopic, and they are known as crystallites. These are also known as grains. There are solids, which are composed of a single crystal like gems, silicon single crystals. However, these occur very rarely in nature. Most of the time solids are polycrystalline. In a structure like this, number of single crystals is held together by a layer of amorphous solids. Amorphous solid is a solid, which lacks a crystalline structure. That is, it does not have long range ordered arrangement of atoms, molecules or ions within the structure. Therefore, in a polycrystalline structure, the long-range order has been disrupted. For example, all the metals and ceramics are polycrystalline. In these, the order and the orientation are very random. It can be determined from the way the polycrystalline solid has grown or by the processing conditions.
What is the difference between Crystalline and Polycrystalline?
• Polycrystalline solids are composed of many numbers of crystalline solids.
• Crystalline solids or crystals have ordered structures and symmetry, but, in a polycrystalline structure, the long-range order has been disrupted.
• Crystalline structure is uniform and has no boundaries, but polycrystalline structure differs from this. It does not have a continuous structure, and it has boundaries between grains.
• Crystalline structure is hard to produce, and it is rare in nature in contrast to polycrystalline structure.
1 Wenk, H. R., Bulakh A., “Minerals: their constitution and origin”, University press, Cambridge, 2004