crystal
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What is a crystal in physics?
In physics, a crystal refers to a solid material where atoms are arranged in a definite, repeating pattern, showing internal symmetry that is often reflected in the crystal's external shape.
Crystals form through a process called crystallization, starting with nucleation, where particles cluster to create a stable nucleus, followed by accretion, where more particles join this cluster in a specific arrangement. These arrangements are due to electrical forces called chemical bonds, such as ionic, covalent, or metallic bonds.
The term crystal is generally used to describe any solid with an ordered internal arrangement, regardless of its external appearance.
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How are the atoms arranged in a crystal?
Crystals are solid materials where atoms are arranged in a defined, repeating pattern that reflects internal symmetry. These patterns form a crystal lattice, which can be divided into identical blocks called unit cells.
The orderly arrangements of atoms in crystals can be described using crystallographic axes. These axes intersect at the edges of the unit cells, and their lengths are known as lattice constants.
Crystal structures are determined through scattering experiments. A beam of particles is directed at a crystal, and the scattering pattern reveals the atomic arrangement. X-rays and neutrons are commonly used in these experiments.
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What are some common examples of crystals?
Crystals are solid materials where atoms arrange in a definite pattern, exhibiting an orderly internal arrangement and external symmetry. They can form from single elements or multiple elements.
Examples of crystals include:
* Rock crystal A transparent form of quartz valued for its clarity.
* Halite Also known as rock salt, it is an ionically bonded crystal.
* Diamond A crystal of carbon where each carbon atom is covalently bonded to four neighbours in a tetrahedral arrangement.
* Ruby A synthetic crystal grown in laboratories.5 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
What is a crystal lattice?
A crystal lattice is the orderly, repeating arrangement of atoms, molecules, or ions in a crystal, forming a space lattice. Connecting these points with lines forms a crystal lattice, divided into identical blocks called unit cells. The intersecting edges of a unit cell define the crystallographic axes, with their lengths known as lattice constants. These constants and the angles between them determine the crystal system.
French scientist Auguste Bravais demonstrated that only 14 types of unit cells are compatible with the orderly arrangements of atoms found in crystals, known as Bravais lattices. Crystal structure is responsible for many of the properties of ceramics.
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How do crystals grow or form in nature?
Crystals form through a process that starts with nucleation, where atoms, ions, or molecules cluster together to create a stable solid seed. This can occur on a surface (heterogeneous nucleation) or uniformly within a medium (homogeneous nucleation), with the former being more common.
Following nucleation, the crystal grows as more particles join the initial cluster, arranging themselves in a characteristic crystalline pattern within a solution, liquid, or vapor. Temperature is critical; for instance, slowly cooling a gas to a liquid and then to a solid can produce a polycrystalline solid.
Minerals can form through various geological processes, including crystallization from magma, sedimentation, metamorphism, and precipitation from hydrothermal solutions. If a growing crystal has space, it will develop a well-defined crystal form.
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How does the structure of a crystal affect its physical properties, like strength or how it reflects light?
The structure of a crystal significantly influences its physical properties. Symmetry within a crystal affects its shape and physical attributes. The orderly arrangement of atoms determines elements of symmetry like rotation, translation, reflection, and inversion, which impact the crystal's characteristics.
Crystal imperfections, such as vacancies, interstitials, and impurities, can alter a crystal's properties. For instance, impurities can affect the color and electrical conductivity of crystals. The arrangement of atoms also influences how crystals interact with light, leading to phenomena like double refraction, where light splits into ordinary and extraordinary rays. The optical properties of crystals, such as refractive indices, are related to their crystallographic axes and affect how they interact with polarized light. Additionally, dislocations within a crystal make it mechanically harder.
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crystal, any solid material in which the component atoms are arranged in a definite pattern and whose surface regularity reflects its internal symmetry.
Classification
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 soft, and it is unclear if the butter should still be considered a solid. Many crystals behave like butter in that they are hard at low temperatures but soft at higher temperatures. They are called solids at all temperatures below their melting point. A possible definition of a solid is an object that retains its shape if left undisturbed. The pertinent issue is how long the object keeps its shape. A highly viscous fluid retains its shape for an hour but not a year. A solid must keep its shape longer than that.
Basic units of solids
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.
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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.
Long- and short-range order
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.
Most solid materials found in nature exist in polycrystalline form rather than as a single crystal. They are actually composed of millions of grains (small crystals) packed together to fill all space. Each individual grain has a different orientation than its neighbours. Although long-range order exists within one grain, at the boundary between grains, the ordering changes direction. A typical piece of iron or copper (Cu) is polycrystalline. Single crystals of metals are soft and malleable, while polycrystalline metals are harder and stronger and are more useful industrially. Most polycrystalline materials can be made into large single crystals after extended heat treatment. In the past blacksmiths would heat a piece of metal to make it malleable: heat makes a few grains grow large by incorporating smaller ones. The smiths would bend the softened metal into shape and then pound it awhile; the pounding would make it polycrystalline again, increasing its strength.
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Categories of crystals
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 classified as a single type of solid, while others have intermediate behaviour. Cadmium sulfide (CdS) can be prepared in pure form and is an excellent insulator; when impurities are added to cadmium sulfide, it becomes an interesting semiconductor. Bismuth (Bi) appears to be a metal, but the number of electrons available for electrical conduction is similar to that of semiconductors. In fact, bismuth is called a semimetal. Molecular solids are usually crystals formed from molecules or polymers. They can be insulating, semiconducting, or metallic, depending on the type of molecules in the crystal. New molecules are continuously being synthesized, and many are made into crystals. The number of different crystals is enormous.