Crystal structure

Crystals can be divided into four categories according to their structural particles and forces: ionic crystals, atomic crystals, molecular crystals and metal crystals.
Solids can be divided into three categories: crystalline, amorphous and quasicrystalline.
Solid matter with regular geometric shape, fixed melting point and anisotropy is a basic form of matter existence. Whether a solid substance is a crystal can generally be identified by X-ray diffraction.
The particles (atoms, ions, molecules, atomic clusters) in the internal structure of the crystal are regularly and repeatedly arranged in the three-dimensional space, forming a lattice of a certain form, which is a geometric polyhedron of a certain shape in appearance. The plane that forms a certain geometric polyhedron is called crystal plane. Due to different growth conditions, the crystal may be slightly skewed in shape, but the included angle between crystal planes of the same crystal is certain, which is called the principle of crystal plane angle invariance.
Synthetic bismuth single crystal
According to its internal structure, the crystal can be divided into seven crystal systems and 14 lattice types. All crystals have certain symmetry, and there are 32 kinds of symmetrical element systems. The corresponding symmetrical action groups are called crystal system point groups. According to the nature of the force between internal particles, crystals can be divided into four typical crystals, such as ionic crystals, atomic crystals, molecular crystals, and metal crystals, such as salt, diamonds, dry ice, and various metals. The same crystal also has the difference between single crystal and polycrystalline (or powder crystal). In practice, there are also mixed crystals. When it comes to crystals, we have to start with crystals. As we all know, all substances are composed of atoms or molecules. As we all know, there are three forms of aggregation of matter: gas, liquid and solid. However, do you know how many kinds of solids can be classified according to their internal structural characteristics? The research shows that solid can be divided into three categories: crystal, amorphous and quasicrystal.
geometry
Crystals usually have regular geometric shapes, just like those specially made by someone. The arrangement of the atoms inside is very regular and strict, much more orderly than the square array of soldiers. If any atom in the crystal is translated along a certain direction for a certain distance, the same atom will be found. The arrangement of atoms in glass, pearl, asphalt, plastic and other amorphous materials is disorderly. Quasicrystals are a new class of substances found, and their internal arrangement is different from both crystals and amorphous materials.
crystal
What kind of substance can be counted as crystal? First, except for liquid crystals, crystals are generally solid. Secondly, the atoms, molecules or ions that make up a substance have regular and periodic arrangements. Such a substance is a crystal.
However, it is difficult to distinguish crystal, amorphous and quasicrystal with the naked eye only from the appearance. So, how can we quickly identify them? One of the most commonly used techniques is X-ray technology. Using X-ray to analyze the structure of solids, you will soon find that crystal, amorphous and quasicrystal are three different types of solids.
In order to describe the crystal structure, we regard the atoms constituting the crystal as a point, and then connect these points representing atoms with imaginary line segments to draw a lattice space structure as shown in the figure. This geometric space lattice used to describe the arrangement of atoms in crystals is called lattice. Because the arrangement of atoms in the crystal is regular, a minimum unit that can fully express the lattice structure can be taken out of the lattice, which is called the unit cell. Many cells with the same orientation form grains. Objects composed of grains with different orientations are called polycrystals. All cells in a single crystal have the same orientation. Common single crystals such as single crystal silicon and single crystal quartz. The most common is polycrystal.
Due to the obvious difference in the arrangement of atoms in materials, the physical and chemical properties of crystals and amorphous materials are greatly different. For example, a crystal has a fixed melting point, which melts immediately when the temperature reaches a certain temperature; However, glass and other amorphous materials have no fixed melting point, and there is a large temperature range from softening to melting.