CRYSTAL STRUCTURE: STRUCTURE, TYPES AND EXAMPLES - CHEMISTRY - 2025

The crystalline structure is one of the solid states that atoms, ions or molecules can adopt in nature, which is characterized by having a high spatial ordering. In other words, this is evidence of the "corpuscular architecture" that defines many bodies with glassy and shiny appearances.

What promotes or what force is responsible for this symmetry? The particles are not alone, but interact with each other. These interactions consume energy and affect the stability of solids, so that the particles seek to accommodate themselves to minimize this energy loss.

So their intrinsic natures lead them to place themselves in the most stable spatial arrangement. For example, this can be the one where the repulsions between ions with the same charges are minimal, or where some atoms -such as metallic ones- also occupy the largest possible volume in their packings.

The word "crystal" has a chemical meaning that can be misrepresented for other bodies. Chemically, it refers to an ordered structure (microscopically) that, for example, can consist of DNA molecules (a DNA crystal).

However, it is popularly misused to refer to any glassy object or surface, such as mirrors or bottles. Unlike true crystals, glass consists of an amorphous (disordered) structure of silicates and many other additives.

Structure

In the image above, some emerald gems are illustrated. Just like these, many other minerals, salts, metals, alloys, and diamonds exhibit a crystalline structure; but, what relationship does its ordering have with symmetry?

If a crystal, whose particles could be observed with the naked eye, are applied symmetry operations (invert it, rotate it at different angles, reflect it on a plane, etc.), then it will be found that it remains intact in all dimensions of space.

The opposite occurs for an amorphous solid, from which different orders are obtained by subjecting it to a symmetry operation. Furthermore, it lacks structural repetition patterns, which shows the randomness in the distribution of its particles.

What is the smallest unit that makes up the structural pattern? In the upper image, the crystalline solid is symmetrical in space, while the amorphous one is not.

If squares were drawn to enclose orange spheres and symmetry operations were applied to them, it would be found that they generate other parts of the crystal.

The above is repeated with smaller and smaller squares, until finding the one that is asymmetrical; the one preceding it in size is, by definition, the unit cell.

Unit cell

The unit cell is the minimum structural expression that allows the complete reproduction of the crystalline solid. From this it is possible to assemble the glass, moving it in all directions in space.

It can be considered as a small drawer (trunk, bucket, container, etc.) where the particles, represented by spheres, are placed following a filling pattern. The dimensions and geometries of this box depend on the lengths of its axes (a, b and c), as well as the angles between them (α, β and γ).

The simplest of all unit cells is that of the simple cubic structure (upper image (1)). In this, the center of the spheres occupies the corners of the cube, four at its base and four at the ceiling.

In this arrangement, the spheres only occupy 52% of the total volume of the cube, and since nature abhors a vacuum, not many compounds or elements adopt this structure.

However, if the spheres are arranged in the same cube in such a way that one occupies the center (cubic centered in the body, bcc), then there will be a more compact and efficient packing (2). Now the spheres occupy 68% of the total volume.

On the other hand, in (3) no sphere occupies the center of the cube, but the center of its faces does, and they all occupy up to 74% of the total volume (face-centered cubic, cc).

Thus, it can be appreciated that other arrangements can be obtained for the same cube, by varying the way in which the spheres are packed (ions, molecules, atoms, etc.).

Types

Crystal structures can be classified according to their crystal systems or the chemical nature of their particles.

For example, the cubic system is the most common of all, and many crystalline solids are governed by it; however, this same system applies to both ionic and metallic crystals.

According to its crystalline system

In the previous image the seven main crystal systems are represented. It can be noted that there are actually fourteen of these, which are the product of other forms of packaging for the same systems and make up the Bravais networks.

From (1) to (3) are crystals with cubic crystal systems. In (2) it is observed (by the blue stripes) that the sphere in the center and that of the corners interact with eight neighbors, so the spheres have a coordination number of 8. And in (3) the coordination number is 12 (to see it you need to duplicate the cube in any direction).

Elements (4) and (5) correspond to simple and face-centered tetragonal systems. Unlike the cubic, its c-axis is longer than the a and b axes.

From (6) to (9) are the orthorhombic systems: from simple and centered on the bases (7), to those centered on the body and on faces. In these α, β and γ are 90º, but all the sides are of different lengths.

Figures (10) and (11) are the monoclinic crystals and (12) is the triclinic one, the last one presenting inequalities in all its angles and axes.

Element (13) is the rhombohedral system, analogous to the cubic one but with an angle γ different from 90º. Finally there are the hexagonal crystals

The displacements of the elements (14) originate the hexagonal prism traced by the green dotted lines.

According to its chemical nature

- If the crystals are made up of ions, then they are ionic crystals present in the salts (NaCl, CaSO ₄, CuCl ₂, KBr, etc.)

- Molecules like glucose form (whenever they can) molecular crystals; in this case, the famous sugar crystals.

- Atoms whose bonds are essentially covalent form covalent crystals. Such are the cases of diamond or silicon carbide.

- Likewise, metals such as gold form compact cubic structures, which constitute metallic crystals.

Examples

K

NaCl (cubic system)

ZnS (wurtzite, hexagonal system)

CuO (monoclinic system)

References

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2. Pressbooks. 10.6 Lattice Structures in Crystalline Solids. Retrieved on May 26, 2018, from: opentextbc.ca
3. Crystal Structures Academic Resource Center.. Retrieved on May 24, 2018, from: web.iit.edu
4. Ming. (2015, June 30). Types Crystal Structures. Retrieved on May 26, 2018, from: crystalvisions-film.com
5. Helmenstine, Anne Marie, Ph.D. (January 31, 2018). Types of Crystals. Retrieved on May 26, 2018, from: thoughtco.com
6. KHI. (2007). Crystalline Structures. Retrieved on May 26, 2018, from: folk.ntnu.no
7. Paweł Maliszczak. (April 25, 2016). Rough emerald crystals from Panjshir Valley Afghanistan.. Retrieved on May 24, 2018, from: commons.wikimedia.org
8. Napy1kenobi. (April 26, 2008). Bravais lattices.. Retrieved on May 26, 2018, from: commons.wikimedia.org
9. User: Sbyrnes321. (November 21, 2011). Crystalline or amorphous.. Retrieved on May 26, 2018, from: commons.wikimedia.org