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- Each of a crystal’s millions of individual structural units (unit cells) contains all the substance’s atoms, molecules, or ions in the same proportions as in its chemical formula (see formula weight). The cells are repeated in all directions to form a geometric pattern, manifested by the number and orientation of external planes (crystal faces).
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- Cubic. The cubic crystal lattice family is the most fundamental of all the seven lattice families. It refers to the three main types of cubic crystals: face-centered cubic (FCC), body-centered cubic (BCC), and simple cubic (SC).
- Tetragonal. The tetragonal lattice structure can be distinguished by its four-sided unit cell and axes of different lengths perpendicular to each other.
- Orthorhombic. Orthorhombic crystals are known for their rectangular prism shape, with three unequal axes intersecting at right angles. These axes intersect at right angles, akin to the dimensions of a cuboid, giving orthorhombic crystals their distinct rectangular prism-like shape.
- Rhombohedral. The defining characteristic of the rhombohedral crystal lattice family lies in the shape of its unit cell, a rhombohedron—a six-faced geometric figure where all sides are of equal length but angles are not necessarily right angles.
Aug 28, 2022 · A crystal structure is defined as the particular repeating arrangement of atoms (molecules or ions) throughout a crystal. Structure refers to the internal arrangement of particles and not the external appearance of the crystal.
The crystal structure consists of the same group of atoms, the basis, positioned around each and every lattice point. This group of atoms therefore repeats indefinitely in three dimensions according to the arrangement of one of the Bravais lattices.
- Overview
- Classification
- Basic units of solids
- Long- and short-range order
- Categories of crystals
crystal, any solid material in which the component atoms are arranged in a definite pattern and whose surface regularity reflects its internal symmetry.
The definition of a solid appears obvious; a solid is generally thought of as being hard and firm. Upon inspection, however, the definition becomes less straightforward. A cube of butter, for example, is hard after being stored in a refrigerator and is clearly a solid. After remaining on the kitchen counter for a day, the same cube becomes quite so...
The basic units of solids are either atoms or atoms that have combined into molecules. The electrons of an atom move in orbits that form a shell structure around the nucleus. The shells are filled in a systematic order, with each shell accommodating only a small number of electrons. Different atoms have different numbers of electrons, which are distributed in a characteristic electronic structure of filled and partially filled shells. The arrangement of an atom’s electrons determines its chemical properties. The properties of solids are usually predictable from the properties of their constituent atoms and molecules, and the different shell structures of atoms are therefore responsible for the diversity of solids.
All occupied shells of the argon (Ar) atom, for example, are filled, resulting in a spherical atomic shape. In solid argon the atoms are arranged according to the closest packing of these spheres. The iron (Fe) atom, in contrast, has one electron shell that is only partially filled, giving the atom a net magnetic moment. Thus, crystalline iron is a magnet. The covalent bond between two carbon (C) atoms is the strongest bond found in nature. This strong bond is responsible for making diamond the hardest solid.
A solid is crystalline if it has long-range order. Once the positions of an atom and its neighbours are known at one point, the place of each atom is known precisely throughout the crystal. Most liquids lack long-range order, although many have short-range order. Short range is defined as the first- or second-nearest neighbours of an atom. In many liquids the first-neighbour atoms are arranged in the same structure as in the corresponding solid phase. At distances that are many atoms away, however, the positions of the atoms become uncorrelated. These fluids, such as water, have short-range order but lack long-range order. Certain liquids may have short-range order in one direction and long-range order in another direction; these special substances are called liquid crystals. Solid crystals have both short-range order and long-range order.
Solids that have short-range order but lack long-range order are called amorphous. Almost any material can be made amorphous by rapid solidification from the melt (molten state). This condition is unstable, and the solid will crystallize in time. If the timescale for crystallization is years, then the amorphous state appears stable. Glasses are an example of amorphous solids. In crystalline silicon (Si) each atom is tetrahedrally bonded to four neighbours. In amorphous silicon (a-Si) the same short-range order exists, but the bond directions become changed at distances farther away from any atom. Amorphous silicon is a type of glass. Quasicrystals are another type of solid that lack long-range order.
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Crystals are classified in general categories, such as insulators, metals, semiconductors, and molecular solids. A single crystal of an insulator is usually transparent and resembles a piece of glass. Metals are shiny unless they have rusted. Semiconductors are sometimes shiny and sometimes transparent but are never rusty. Many crystals can be clas...
4 days ago · The most interesting crystal structures are those of elements that are neither metallic, covalent, nor diatomic. Although boron (B) and sulfur (S) have several different crystal structures, each has one arrangement in which it is usually found.
Aug 23, 2021 · In a crystal structure, the centers of atoms, ions, or molecules lie on the lattice points. Atoms are assumed to be spherical to explain the bonding and structures of metallic crystals. These spherical particles can be packed into different arrangements.
Crystals are generally formed when a liquid solidifies, a vapour becomes supersaturated (see saturation), or a liquid solution can no longer retain dissolved material, which is then precipitated. Metals, alloys, mineral s, and semiconductor s are all crystalline, at least microscopically.
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