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Crystal Structure Revealed: Why is the hexagonal crystal family so unique?
In crystallography, the hexagonal crystal family is one of six crystal families, covering two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). Although often confused, the trigonal crystal system is not equivalent to the rhombohedral lattice system, and this is particularly important in the hexagonal crystal family. This article will explore the uniqueness of the hexagonal crystal family and learn about its lattice structures, crystal systems, and their applications in multi-element structures.
Basic structure of the hexagonal crystal family
The hexagonal crystal family consists of 12 point groups, and at least one space group has a hexagonal lattice as the basis. There are a total of 52 associated space groups whose dimensions are defined by the Bravais lattice as either hexagonal or rhombohedral. These structures are characterized by their symmetries, which are particularly well-specified in crystalline systems.
The hexagonal crystal family is unique not only in its structure, but also in its symmetry and potential for applications in materials science.
Introduction to lattice system
The hexagonal crystal family consists of two lattice systems: hexagonal and rhombohedral. Each lattice system consists of a Bravais lattice. In the hexagonal crystal system, the crystal is usually represented by a crystal with two equal axes (a and a) and an inclusion angle (γ) of 120° and a height (c) perpendicular to the two basal axes. This structure makes the hexagonal lattice more convenient for practical applications.
Hexagonal and Trigonal Crystal Systems
The two major crystal systems in the hexagonal crystal family include trigonal and hexagonal. The trigonal crystal system contains five point groups with a single three-fold axis of rotation, while the hexagonal crystal system contains seven point groups with a single six-fold axis of rotation. For example, the five point groups of a trigonal crystal system correspond to its space group, while the seven point groups of a hexagonal crystal system have 27 space groups designated as hexagonal lattice systems.
The trigonal crystal system is the only one with multiple lattice systems associated, which highlights its complexity in the crystal structure.
Hexagonal close-packed structure
Hexagonal close packing (hcp) is one of the two types of atomic packing with the highest density. It differs from face-centered cubic (fcc) in that it is not a Bravais lattice but a lattice point made up of two atoms. This feature brings great application potential to materials science, especially in the research of metals and alloys.
Multi-element structure
Compounds based on the structure of the hexagonal crystal family are relatively common in materials science. One example is the Wurtzite structure, which represents the B4 structure in crystallography and has various applications, including use in semiconductors. The Wurtzite structure can exhibit non-centrosymmetric characteristics, so it has excellent properties such as piezoelectricity and thermoelectricity.
One of the striking features of the Wurtzite structure is its lack of inversion symmetry, which makes its properties distinct from other structures.
Conclusion
The uniqueness of the hexagonal crystal family is reflected in its complex structure, changeable crystal system and strong application potential. This family of crystals shows endless possibilities, both in basic science and in applications. In the future, how will scientists use these unique structures to promote technological progress?
