AMORPHOUS SOLIDS: STRUCTURE, PROPERTIES, EXAMPLES - CHEMISTRY - 2025

The amorphous solids are those without an ordered structure long range. They are the opposite of what is known as a crystalline solid. Its particles associate in a disorderly way, similar to that of liquids, but with enough force to coalesce into a solid structure.

This amorphous character is more common than you might think; it is in fact one of the possible states that condensed matter can adopt. By this it is understood that any compound capable of solidifying and, therefore, crystallizing, can also agglomerate in a disorderly way if the experimental conditions allow it.

Cotton candy is an example of an amorphous solid. Source: Pixabay.

The above said usually applies to pure substances, whether elements or compounds. But it is also valid in the case of mixtures. Many solid mixtures are amorphous, such as cotton candy, chocolate, mayonnaise, or mashed potatoes.

The fact that a solid is amorphous does not make it less valuable than a crystalline one. Structural disorder sometimes endows it with unique properties that it would not exhibit in a crystalline condition. For example, in the photovoltaic industry amorphous silicon is preferred over crystalline for certain small-scale applications.

Structure of amorphous solids

Difference between a crystalline structure and an amorphous structure. Source: Gabriel Bolívar.

The structure of an amorphous solid is messy; it lacks periodicity or structural pattern. The image above illustrates this point. A corresponds to a crystalline solid, while B represents an amorphous solid. Note that in B the purple rhombuses are arranged arbitrarily, even though both in A and B exist the same type of interactions.

If you also look at B, you will see that there are spaces that appear to be empty; that is, the structure has defects or irregularities. Therefore, part of the microscopic or internal disorder of an amorphous solid is due to its particles being "arranged" in such a way that the resulting structure has many imperfections.

At first mention was made of the scope in the ordering degree of amorphous solids. In B there are just a couple of rhombuses that appear to be neatly aligned. There may be ordered regions; but only at close range.

An amorphous solid is then said to be made up of immeasurable tiny crystals of different structures. The sum of all these structures ends up becoming labyrinthine and meaningless: the overall structure becomes amorphous, made up of endless crystalline blocks scattered everywhere.

Properties

The properties of an amorphous solid vary depending on the nature of its constituent particles. However, there are certain general characteristics that can be mentioned. Amorphous solids can be vitreous, when they present similar aspects to crystals; or gelatinous, resinous, or dusty.

Since their structures are disordered, they do not generate reliable X-ray diffraction spectra. Likewise, their melting points are not precise, but rather cover a range of values.

For example, the melting point for an amorphous solid can range from 20 to 60 ° C. Meanwhile, crystalline solids melt at a specific temperature, or in a narrow range if they contain many impurities.

Another characteristic of amorphous solids is that when they break or fracture, they do not originate geometric fragments with flat faces, but irregular fragments with curved faces. When they are not vitreous, they appear as dusty and opaque bodies.

Preparation

More than an amorphous solid, this concept should be treated as an 'amorphous state'. All compounds (ionic, molecular, polymeric, metallic, etc.) are capable, up to a certain point, and if experimental conditions allow, of forming amorphous and non-crystalline solids.

For example, in organic syntheses solid compounds are initially obtained as powdery masses. Its content of impurities is so high that they affect its molecular order in the long run. That is why when the product recrystallizes over and over again, the solid becomes more and more crystalline; it is losing its amorphous character.

This does not mean however that amorphous solids are necessarily impure materials; several of them are amorphous by their own chemical nature.

A pure substance can solidify amorphously if its liquid is suddenly cooled, in such a way that its particles do not crystallize, but adopt a glassy configuration. The cooling is so fast that the particles do not have enough time to accommodate the crystalline blocks that barely manage to "be born".

Water, for example, is capable of existing in a glassy, ​​amorphous state, and not just as ice.

Examples of amorphous solids

Minerals and plastics

Obsidian is one of the few amorphous minerals that are known. Source: Pixabay.

Virtually any crystalline material can conform to an amorphous form (and vice versa). This happens with some minerals, which for geochemical reasons could not formally establish their conventional crystals. Others, on the other hand, do not form crystals but glass; such is the case with obsidian.

On the other hand, polymers tend to solidify amorphously, since their molecules are too large to define an ordered structure. This is where resins, rubbers, polystyrene foam (anime), plastics, Teflon, Bakelite, among others, come in.

Biological tissue

Biological solids are mostly amorphous, such as: organ tissue, skin, hair, cornea, etc. Likewise, fat and proteins form amorphous masses; However, with proper preparation, they can crystallize (DNA crystals, proteins, fats).

Glasses

Glass, an amorphous solid

Although it has been left almost last, the most representative amorphous solid is by far the glass itself. Its composition is essentially the same as that of quartz: SiO ₂. Both quartz crystal and glass are three-dimensional covalent networks; only that the glass lattice is messy, with Si-O bonds of different lengths.

Metallic glass sample

Glass is the quintessential amorphous solid, and materials that take on a similar appearance are said to have a glassy state.

Carbon and metals

We have amorphous carbon, activated carbon being one of the most important for its absorbent capacities. Also, there is amorphous silicon and germanium, with electronic applications where they act as semiconductors.

And finally, there are amorphous alloys that do not establish a crystalline structure due to the disparity of their metal atoms.

References

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