54. Crystalline bodies

A common feature of any solid body is its ability to preserve not only its volume as a liquid, but also its shape. Primarily solid bodies are in the crystalline state.

In the crystal, atoms or molecules occupy certain, ordered positions. This results in the proper external shape of the crystal. For example, a particle of ordinary table salt has flat faces that make up right angles with each other. This can be seen by looking at the salt with a magnifier. And how geometrically correct is the shape of a snowflake! (fig. 1). It also reflects the geometric correctness of the internal structure of a crystalline solid body - ice.

However, the proper external shape is not the only or even the most important consequence of the ordered structure of the crystal. The main thing is the dependence of physical properties on the direction chosen in the crystal. First of all, it is noticeable that the crystal has different mechanical strength in different directions. A piece of mica is easily split in one direction into thin plates (fig. 2). It is much more difficult to split it in the direction perpendicular to the plates. It is just as easy to split in one direction the graphite crystal. When you write with a pencil, this splitting occurs continuously and thin layers of graphite remain on paper. This is because the graphite crystal lattice has a layered structure. The layers are formed by a series of parallel flat grids consisting of carbon atoms (fig. 3). The atoms are located at the tops of the correct hexagons. The distance between the layers is relatively large: approximately twice as long as the side length of the hexagon. The bonds between the layers are therefore less strong than the bonds inside them.

Many crystals conduct heat and electric current in different directions. The optical properties of the crystals are also dependent on the direction. Thus, a quartz crystal refracts light in different ways depending on the direction of the rays falling on it.

The dependence of physical properties on the direction inside the crystalline body is called anisotropy.
All crystalline bodies are anisotropic.

Most solid bodies are crystals. In particular, metals have a crystal structure. However, if we take a relatively large piece of metal, then, at first glance, its crystal structure does not appear either in the outer appearance of the piece, or in its physical properties. Metals in their usual state do not reveal anisotropy.

The thing here is that usually the metal consists of a huge number of small crystals fused with each other. Under a microscope or even with a magnifying glass, they are easy to see, especially when the metal is freshly broken. The properties of each crystal are different in different directions, but the crystals are oriented towards each other chaotically. As a result, none of the directions is highlighted and the properties of metals are the same in all directions, which means that metals are isotropic.

A solid body, consisting of a large number of small crystals, is called polycrystalline. In the case of a monocrystal, the whole piece of substance is one crystal.

With extreme precautions, you can grow a large metal crystal, a single crystal. Under normal conditions, the polycrystalline body is formed as a result of the fact that many crystals simultaneously begin to grow until they come into contact with each other, forming a single body.

Polycrystals are not just metals. An ordinary piece of sugar also has a polycrystalline structure.