Crystal definition. What is a crystal? What is a crystal stone

The content of the article

CRYSTALS- substances in which the smallest particles (atoms, ions or molecules) are "packed" in a certain order. As a result, during the growth of crystals, flat faces spontaneously appear on their surface, and the crystals themselves take on a variety of geometric shapes. Everyone who has visited the museum of mineralogy or the exhibition of minerals, could not help but admire the grace and beauty of the forms that "inanimate" substances take.

And who has not admired snowflakes, the variety of which is truly endless! Back in the 17th century. famous astronomer Johannes Kepler wrote a treatise About hexagonal snowflakes and three centuries later, albums were published containing collections of enlarged photographs of thousands of snowflakes, and none of them repeats the other.

The origin of the word "crystal" is interesting (it sounds almost the same in all European languages). Many centuries ago, among the eternal snows in the Alps, on the territory of modern Switzerland, they found very beautiful, completely colorless crystals, very reminiscent of pure ice. The ancient naturalists called them so - "crystallos", in Greek - ice; This word comes from the Greek "krios" - cold, frost. It was believed that ice, being in the mountains for a long time, in severe frost, petrifies and loses its ability to melt. One of the most authoritative ancient philosophers, Aristotle, wrote that "crystallos is born from water when it completely loses heat." The Roman poet Claudian in 390 described the same thing in verse:

In the fierce alpine winter, ice turns to stone.

The sun is not able to melt such a stone.

A similar conclusion was made in ancient times in China and Japan - ice and rock crystal were designated there by the same word. And even in the 19th century. poets often combined these images together:

Barely transparent ice, fading over the lake,

He covered motionless jets with a crystal.

A.S. Pushkin. To Ovid

A special place among crystals is occupied by precious stones, which have attracted human attention since ancient times. People have learned how to artificially obtain a lot of precious stones. For example, bearings for watches and other precision instruments have long been made from artificial rubies. They also artificially produce beautiful crystals that do not exist in nature at all. For example, cubic zirkonia - their name comes from the abbreviation FIAN - Physical Institute of the Academy of Sciences, where they were first obtained. Cubic Zirconia ZrO 2 crystals are cubic zirconia crystals that look very similar to diamonds.

The structure of crystals.

Depending on the structure, crystals are divided into ionic, covalent, molecular and metallic. Ionic crystals are built from alternating cations and anions, which are held in a certain order by electrostatic attraction and repulsion forces. Electrostatic forces are non-directional: each ion can hold around itself as many ions of the opposite sign as it fits. But at the same time, the forces of attraction and repulsion must be balanced and the overall electrical neutrality of the crystal must be preserved. All this, taking into account the size of the ions, leads to different crystal structures. So, when Na + ions (their radius is 0.1 nm) and Cl - (radius 0.18 nm) interact, octahedral coordination occurs: each ion holds six ions of the opposite sign located at the vertices of the octahedron. In this case, all cations and anions form the simplest cubic crystal lattice, in which the cube vertices are alternately occupied by Na + and Cl - ions. Crystals of KCl, BaO, CaO, and a number of other substances are similarly arranged.

Cs + ions (radius 0.165 nm) are close in size to Cl - ions, and cubic coordination occurs: each ion is surrounded by eight ions of the opposite sign, located at the vertices of the cube. In this case, a body-centered crystal lattice is formed: in the center of each cube formed by eight cations, one anion is located, and vice versa. (It is interesting that at 445° C CsCl transforms into a simple cubic lattice of the NaCl type.) The crystal lattices of CaF 2 (fluorite) and many other ionic compounds are more complex. In some ionic crystals, complex polyatomic anions can be combined into chains, layers, or form a three-dimensional framework, in the cavities of which cations are located. So, for example, silicates are arranged. Ionic crystals form most salts of inorganic and organic acids, oxides, hydroxides, salts. In ionic crystals, the bonds between ions are strong; therefore, such crystals have high melting points (801 ° C for NaCl, 2627 ° C for CaO).

In covalent crystals (they are also called atomic) at the nodes of the crystal lattice there are atoms, identical or different, which are connected by covalent bonds. These bonds are strong and directed at certain angles. A typical example is a diamond; in his crystal, each carbon atom is bonded to four other atoms located at the vertices of the tetrahedron. Covalent crystals form boron, silicon, germanium, arsenic, ZnS, SiO 2 , ReO 3 , TiO 2 , CuNCS. Since there is no sharp boundary between polar covalent and ionic bonds, the same is true for ionic and covalent crystals. Thus, the charge on the aluminum atom in Al 2 O 3 is not +3, but only +0.4, which indicates a large contribution of the covalent structure. At the same time, in cobalt aluminate CoAl 2 O 4 the charge on aluminum atoms increases to +2.8, which means the predominance of ionic forces. Covalent crystals are generally hard and refractory.

Molecular crystals are built from isolated molecules between which relatively weak attractive forces act. As a result, such crystals have much lower melting and boiling points, and their hardness is low. So, crystals of noble gases (they are built from isolated atoms) melt even at very low temperatures. From inorganic compounds, molecular crystals form many non-metals (noble gases, hydrogen, nitrogen, white phosphorus, oxygen, sulfur, halogens), compounds whose molecules are formed only by covalent bonds (H 2 O, HCl, NH 3, CO 2, etc.) . This type of crystals is also characteristic of almost all organic compounds. The strength of molecular crystals depends on the size and complexity of the molecules. Thus, helium crystals (atomic radius 0.12 nm) melt at –271.4°C (under a pressure of 30 atm), and xenon crystals (radius 0.22 nm) melt at –111.8°C; fluorine crystals melt at –219.6°C, and iodine at +113.6°C; methane CH 4 - at -182.5 ° C, and triacontane C 30 H 62 - at + 65.8 ° C.

Metal crystals form pure metals and their alloys. Such crystals can be seen on the fracture of metals, as well as on the surface of galvanized sheet. The crystal lattice of metals is formed by cations, which are connected by mobile electrons ("electron gas"). This structure determines the electrical conductivity, malleability, high reflectivity (brilliance) of crystals. The structure of metal crystals is formed as a result of different packing of atoms-balls. Alkali metals, chromium, molybdenum, tungsten, etc. form a body-centered cubic lattice; copper, silver, gold, aluminum, nickel, etc. - a face-centered cubic lattice (in addition to 8 atoms at the vertices of the cube, there are 6 more located in the center of the faces); beryllium, magnesium, calcium, zinc, etc. - the so-called hexagonal dense lattice (it has 12 atoms located at the vertices of a rectangular hexagonal prism, 2 atoms - at the center of the two bases of the prism and 3 more atoms - at the vertices of the triangle in the center of the prism).

All crystalline compounds can be divided into mono- and polycrystalline. A monocrystal is a monolith with a single undisturbed crystal lattice. Large natural single crystals are very rare. Most crystalline bodies are polycrystalline, that is, they consist of many small crystals, sometimes visible only under high magnification.

Crystal growth.

Many prominent scientists who made a great contribution to the development of chemistry, mineralogy, and other sciences began their first experiments precisely with the growth of crystals. In addition to purely external effects, these experiments make us think about how crystals are arranged and how they are formed, why different substances give crystals of different shapes, and some do not form crystals at all, what needs to be done to make the crystals large and beautiful.

Here is a simple model that explains the essence of crystallization. Imagine that parquet is being laid in a large hall. It is easiest to work with square-shaped tiles - no matter how you turn such a tile, it will still fit into place, and the work will go quickly. That is why compounds consisting of atoms (metals, noble gases) or small symmetrical molecules crystallize easily. Such compounds, as a rule, do not form non-crystalline (amorphous) substances.

It is more difficult to lay parquet from rectangular boards, especially if they have grooves and protrusions on the sides - then each board can be laid in its place in one single way. It is especially difficult to lay out a parquet pattern from planks of complex shape.

If the parquet floorer is in a hurry, then the tiles will arrive at the installation site too quickly. It is clear that the correct pattern will not work now: if the tile is warped at least in one place, then everything will go crooked, voids will appear (like in the old Tetris computer game, in which the “glass” is filled with details too quickly). Nothing good will come of it even if a dozen craftsmen start laying parquet in a large hall at once, each from his own place. Even if they work slowly, it is extremely doubtful that the adjacent sections will be well joined, and in general, the view of the room will turn out to be very unsightly: in different places the tiles are located in different directions, and holes gape between separate sections of even parquet.

Approximately the same processes occur during the growth of crystals, only the difficulty here is also in the fact that the particles must fit not in a plane, but in a volume. But after all, there is no “parquet floor” here - who puts the particles of matter in their place? It turns out that they fit themselves, because they continuously make thermal movements and “look for” the most suitable place for themselves, where it will be most “convenient” for them. In this case, "convenience" also implies the most energetically favorable location. Once in such a place on the surface of a growing crystal, a particle of matter can remain there and after a while be already inside the crystal, under new accrued layers of matter. But another thing is also possible - the particle will again leave the surface into the solution and again begin to “seek” where it is more convenient for it to settle down.

Each crystalline substance has a certain external form of a crystal peculiar to it. For example, for sodium chloride this shape is a cube, for potassium alum it is an octahedron. And even if at first such a crystal had an irregular shape, it will still sooner or later turn into a cube or an octahedron. Moreover, if a crystal with the correct shape is deliberately spoiled, for example, its vertices are beaten off, edges and faces are damaged, then with further growth such a crystal will begin to “heal” its damage on its own. This happens because the “correct” crystal faces grow faster, the “wrong” ones grow more slowly. To verify this, the following experiment was carried out: a ball was carved from a salt crystal, and then it was placed in a saturated NaCl solution; after a while, the ball itself gradually turned into a cube! Rice. 6 Crystal forms of some minerals

If the crystallization process is not too fast, and the particles have a convenient shape for stacking and high mobility, they easily find their place. If, however, the mobility of particles with low symmetry is sharply reduced, then they “freeze” at random, forming a transparent mass similar to glass. This state of matter is called the glassy state. An example is ordinary window glass. If the glass is kept very hot for a long time, when the particles in it are sufficiently mobile, silicate crystals will begin to grow in it. Such glass loses its transparency. Not only silicates can be glassy. So, with slow cooling of ethyl alcohol, it crystallizes at a temperature of -113.3 ° C, forming a white snow-like mass. But if cooling is carried out very quickly (lower a thin ampoule with alcohol into liquid nitrogen at a temperature of -196 ° C), the alcohol will solidify so quickly that its molecules will not have time to build a regular crystal. The result is transparent glass. The same happens with silicate glass (for example, window glass). With very rapid cooling (millions of degrees per second), even metals can be obtained in a non-crystalline glassy state.

It is difficult to crystallize substances with an "uncomfortable" form of molecules. Such substances include, for example, proteins and other biopolymers. But ordinary glycerin, which has a melting point of + 18 ° C, easily supercools when cooled, gradually solidifying into a glassy mass. The fact is that already at room temperature glycerin is very viscous, and when cooled it becomes very thick. At the same time, it is very difficult for asymmetric glycerol molecules to line up in a strict order and form a crystal lattice.

Methods for growing crystals.

Crystallization can be carried out in different ways. One of them is the cooling of a saturated hot solution. At each temperature, no more than a certain amount of a substance can dissolve in a given amount of solvent (for example, in water). For example, 200 g of potassium alum can dissolve in 100 g of water at 90°C. Such a solution is called saturated. We will now cool the solution. With decreasing temperature, the solubility of most substances decreases. So, at 80 ° C, no more than 130 g of alum can be dissolved in 100 g of water. Where will the remaining 70 g go? If the cooling is carried out quickly, the excess substance will simply precipitate. If this precipitate is dried and examined with a strong magnifying glass, then many small crystals can be seen.

When the solution is cooled, particles of a substance (molecules, ions), which can no longer be in a dissolved state, stick together with each other, forming tiny embryonic crystals. The formation of nuclei is facilitated by impurities in the solution, such as dust, the smallest irregularities on the walls of the vessel (chemists sometimes specially rub a glass rod on the inner walls of the glass to help crystallize the substance). If the solution is cooled slowly, few nuclei are formed, and, gradually overgrowing from all sides, they turn into beautiful crystals of the correct shape. With rapid cooling, many nuclei are formed, and particles from the solution will “pour” onto the surface of growing crystals, like peas from a torn bag; of course, correct crystals will not be obtained in this case, because the particles in solution may simply not have time to “settle” on the surface of the crystal in their place. In addition, many rapidly growing crystals interfere with each other just like several parquet floors working in the same room. Foreign solid impurities in the solution can also play the role of crystallization centers, so the purer the solution, the more likely it is that there will be few crystallization centers.

Having cooled a solution of alum saturated at 90°C to room temperature, we will already have 190 g in the sediment, because at 20°C only 10 g of alum dissolves in 100 g of water. Will this result in one large crystal of the correct shape weighing 190 g? Unfortunately, no: even in a very pure solution, a single crystal is unlikely to start growing: a mass of crystals can form on the surface of the cooling solution, where the temperature is slightly lower than in the volume, as well as on the walls and bottom of the vessel.

The method of growing crystals by gradual cooling of a saturated solution is not applicable to substances whose solubility depends little on temperature. Such substances include, for example, sodium and aluminum chlorides, calcium acetate.

Another method for obtaining crystals is the gradual removal of water from a saturated solution. The "extra" substance crystallizes. And in this case, the slower the water evaporates, the better the crystals are obtained.

The third method is the growth of crystals from molten substances by slowly cooling the liquid. When using all methods, the best results are obtained if a seed is used - a small crystal of the correct shape, which is placed in a solution or melt. In this way, for example, ruby ​​crystals are obtained. Growth of crystals of precious stones is carried out very slowly, sometimes for years. If, however, to accelerate crystallization, then instead of one crystal, a mass of small ones will turn out.

Crystals can also grow when vapors condense - this is how snowflakes and patterns on cold glass are obtained. When metals are displaced from solutions of their salts with the help of more active metals, crystals are also formed. For example, if an iron nail is lowered into a solution of copper sulfate, it will be covered with a red layer of copper. But the resulting copper crystals are so small that they can only be seen under a microscope. On the surface of the nail, copper is released very quickly, and therefore its crystals are too small. But if the process is slowed down, the crystals will turn out to be large. To do this, copper sulfate should be covered with a thick layer of table salt, put a circle of filter paper on it, and on top - an iron plate with a slightly smaller diameter. It remains to pour a saturated solution of table salt into the vessel. Copper sulfate will begin to slowly dissolve in brine (the solubility in it is less than in pure water). Copper ions (in the form of complex anions CuCl 4 2– green) will very slowly, over many days, diffuse upwards; the process can be observed by the movement of the colored border.

Having reached the iron plate, copper ions are reduced to neutral atoms. But since this process is very slow, the copper atoms line up in beautiful shiny crystals of metallic copper. Sometimes these crystals form branches - dendrites. By changing the conditions of the experiment (temperature, the size of vitriol crystals, the thickness of the salt layer, etc.), it is possible to change the conditions for copper crystallization.

supercooled solutions.

Sometimes a saturated solution does not crystallize on cooling. Such a solution, which contains in a certain amount of solvent more solute than it is "supposed" at a given temperature, is called a supersaturated solution. A supersaturated solution cannot be obtained even by very long mixing of the crystals with a solvent; it can only be formed by cooling a hot saturated solution. Therefore, such solutions are also called supercooled. Something in them interferes with the onset of crystallization, for example, the solution is too viscous, or large nuclei are required for the growth of crystals, which are not present in the solution.

Solutions of sodium thiosulfate Na 2 S 2 O 3 are easily supercooled. 5H 2 O. If you carefully heat the crystals of this substance to about 56 ° C, they will "melt". In fact, this is not melting, but the dissolution of sodium thiosulfate in the "own" water of crystallization. With increasing temperature, the solubility of sodium thiosulfate, like most other substances, increases, and at 56 ° C, its water of crystallization is sufficient to dissolve all the salt present. If now carefully, avoiding sharp shocks, cool the vessel, crystals will not form and the substance will remain liquid. But if a ready-made embryo, a small crystal of the same substance, is introduced into a supercooled solution, then rapid crystallization will begin. It is interesting that it is caused by a crystal of only this substance, and the solution can be completely indifferent to an outsider. Therefore, if you touch a small crystal of thiosulfate to the surface of the solution, a real miracle will happen: a crystallization front will run from the crystal, which will quickly reach the bottom of the vessel. So after a few seconds, the liquid will completely “harden”. The vessel can even be turned upside down - not a single drop will spill out of it! Solid thiosulfate can be melted again in hot water and repeated all over again.

If a test tube with a supercooled solution of thiosulfate is placed in ice water, the crystals will grow more slowly, and they themselves will be larger. Crystallization of a supersaturated solution is accompanied by its heating - this is the release of thermal energy obtained by the crystalline hydrate during its melting.

Sodium thiosulfate is not the only substance that forms a supercooled solution in which rapid crystallization can be induced. For example, sodium acetate CH 3 COONa has a similar property (it is easy to obtain by the action of acetic acid on soda). With sodium acetate, experienced lecturers demonstrate such a “miracle”: they slowly pour a supersaturated solution of this salt onto a small slide of acetate in a saucer, which, in contact with the crystals, immediately crystallizes, forming a column of solid salt!

Crystals are widely used in science and technology: semiconductors, prisms and lenses for optical devices, solid-state lasers, piezoelectrics, ferroelectrics, optical and electro-optical crystals, ferromagnets and ferrites, single crystals of high purity metals...

X-ray diffraction studies of crystals made it possible to establish the structure of many molecules, including biologically active ones - proteins, nucleic acids.

Faceted crystals of precious stones, including those grown artificially, are used as jewelry.

Ilya Leenson

Natural crystals... They are also called beautiful, rare stones or solids. We imagine a crystal stone as a large, bright, transparent or colorless polyhedron with perfect shiny edges. In life, we often meet such solid substances in the form of grains of irregular shape, grains of sand, fragments. But their properties are the same as those of perfect large crystals. Plunge with us into the magical world of natural crystal stones, get acquainted with their structure, shapes, types. Well, let's go...

Mystery of crystals

The world of crystals is beautiful and mysterious. Multi-colored pebbles have attracted and attracted us with their beauty since childhood. We feel their mystery on an intuitive level and admire their natural beauty. People have always wanted to know as much as possible about natural solids, about the properties of crystals, the formation of their forms, growth and structure.

The world of these stones is so unusual that you want to look inside them. What will we see there? A picture of endlessly stretching, strictly ordered rows of atoms, molecules and ions will open before your eyes. All of them strictly obey the laws that rule in the world of crystal stones.

Crystalline substances are very widespread in nature, because all rocks are composed of them. The entire earth's crust is made up of rocks. It turns out that these unusual substances can even be grown at home yourself. It is important to note that "crystal" in ancient Greek meant "ice" or "rock crystal".

What is a crystal stone?

What do school textbooks say about crystals? They say that these are solid bodies that are formed under the influence of natural or laboratory conditions and have the appearance of polyhedra. The geometric structure of these bodies is infallibly strict. The surface of crystalline figures is made up of perfect planes - faces that intersect along straight lines, which are called edges. Vertices appear at the intersection points of edges.

The solid state of matter is the crystal. It has a certain shape, a specific number of faces, depending on the arrangement of atoms. So, solids, in which molecules, atoms, ions are arranged in a strict pattern in the form of nodes of spatial lattices.

We most often associate crystals with rare and beautiful gemstones. And this is not in vain, diamonds are also crystals. But not all solids are rare and beautiful. After all, particles of salt and sugar are also crystals. There are hundreds of substances around us in the form of them. One of these bodies is considered to be frozen water (ice or snowflakes).

Formation of various forms of crystals

In nature, minerals are formed as a result of rock-forming processes. Solutions of minerals in the form of hot and molten rocks lie deep underground. When these hot rocks are pushed to the surface of the earth, they cool down. Substances cool very slowly. Minerals form crystals in the form of solids. For example, minerals of quartz, feldspar and mica are present in granite.

Each crystal contains a million individual elements (monocrystals). A cell of a crystal lattice can be represented as a square with atoms at the corners. These can be oxygen atoms or other elements. It is known that crystals can react to various energies, remember people's attitude towards them. That is why they are used for healing and cleansing. Crystals can be of various shapes. Depending on this, they are divided into 6 large types.

Different types and types of natural solids

Crystal sizes can also be different. All solids are divided into ideal and real. Ideal bodies are those with smooth edges, strict long-range order, a certain symmetry of the crystal lattice, and other parameters. Real crystals include those that are found in real life. They may contain impurities that lower the symmetry of the crystal lattice, the smoothness of the faces, and the optical properties. Both types of stones are united by the rule for the arrangement of atoms in the above lattice.

According to another criterion for dividing them, they are divided into natural and artificial. For the growth of natural crystals, natural conditions are needed. Artificial solids are grown in the laboratory or at home.

According to the aesthetic and economic criterion, they are divided into precious and non-precious stones. Precious minerals are rare and beautiful. These include emerald, diamond, amethyst, ruby, sapphire and others.

The structure and forms of accumulations of solids

One-peak crystals are hexagonal stones with a pyramidal top. The base of such generator minerals is wider. There are crystals with two peaks - Yin and Yang. They are used in meditation to balance the material and spiritual principles.

Minerals, in which 2 of the 6 faces on the side are wider than all the others, are called lamellar. They are used for telepathic healing.

Crystals formed as a result of impacts or cracks, which then decompose into 7 shades, are called iridescent. They relieve depression and frustration.

Minerals with various inclusions of other elements are called ghost crystals. First, they stop growing, then other materials settle on them, and then growth around them resumes again. Thus, the contours of a mineral that has stopped growing are visible, so it appears ghostly. Such crystals are used to attract crops in garden plots.

Unusual Druzes

The Druze are a very beautiful sight. This is a collection of many crystals on one base. They have positive and negative polarity. They clean the air and recharge the atmosphere. Druses of quartz, emerald, topaz are found in nature. They bring peace and harmony to a person.

Druse is also called intergrown crystals. Most often, garnets, pyrites, and fluorites are subject to this phenomenon. They are often exhibited as exhibits in museums.

Small intergrown crystals are called a brush, large minerals are called a flower. A very beautiful variety of druze are geodes. They grow on the walls. Druses can be very small and large. These are very valuable finds. Druses of agate, selenite, amethyst, citrine, morion are highly valued.

How do crystals store information and knowledge?

Scientists have found that there are triangles on the edges of the crystals, indicating the presence of knowledge in them. This information can only be accessed by a specific person. If such a person appears, then the stones will give him their true insides.

Crystals are capable of transmitting vibrations, awakening the higher powers of consciousness, and balancing spiritual forces. Therefore, they are often used in meditation. Previous civilizations stored information in stones. For example, rock crystal was considered the gem of the gods. Crystals were revered as living beings. Even "cosmos" originally meant "precious stone".

Gems

It is important to note that precious crystals in their raw form are not all that beautiful. They are also called stones or minerals. They are called precious because they are very beautiful in cutting and are used in jewelry. Many are familiar with amethysts, diamonds, sapphires, rubies.

Diamond is considered the hardest stone. A fragile crystal of a grassy green color - an emerald. Ruby is a red variety of corundum. Deposits of this crystal exist on almost all continents. What is considered his undeniable ideal? Burmese rubies. Ruby deposits in the Russian Federation are located in the Chelyabinsk and Sverdlovsk regions.

What other expensive minerals are there? Transparent precious crystals of various colors - from pale blue to dark blue - are sapphires. Although it is a rare mineral, it is valued below ruby.

An expensive variety of quartz is the beautiful gemstone amethyst. Once it was inserted by the high priest Aaron into the number of 12 stones of his pectoral. Amethyst has a beautiful purple or lilac tint.

Russian diamonds

So, the hardest crystal - diamond - is mined from kimberlite pipes formed as a result of eruptions of underground volcanoes. The crystal lattice of this stone is formed under the influence of high temperature and high pressure of carbon.

Diamond mining in Russia began in Yakutia only in the middle of the last century. Today, the Russian Federation is already among the leaders in the extraction of these precious stones. Billions of rubles are allocated annually for diamond mining in Russia. It is worth noting that there are several carats of diamonds per ton of kimberlite pipes.

Natural crystals have always aroused the curiosity of people. Their color, brilliance and shape affected the human sense of beauty, and people decorated themselves and their homes with them. For a long time, superstitions have been associated with crystals; as amulets, they were supposed not only to protect their owners from evil spirits, but also to endow them with supernatural powers. Later, when the same minerals began to be cut and polished like precious stones, many superstitions were preserved in talismans "for good luck" and "one's own stones" corresponding to the month of birth. All natural gemstones, except for opal, are crystalline, and many of them, such as diamond, ruby, sapphire, and emerald, come across as beautifully cut crystals. Crystal jewelry is just as popular today as it was in past centuries.

Crystal (from the Greek krystallos - "transparent ice") was originally called transparent quartz (rock crystal), found in the Alps. Rock crystal was mistaken for ice, hardened from the cold to such an extent that it no longer melts. Initially, the main feature of the crystal was seen in its transparency. Later, they began to make glass that was not inferior in brilliance and transparency to natural substances. Objects made of such glass were also called "crystal". Even today glass of special transparency is called crystal.

An amazing feature of rock crystal and many other transparent minerals is their smooth flat faces. At the end of the 17th century it was noticed that there is a certain symmetry in their arrangement. It has also been found that some opaque minerals also have a natural regular cut and that the shape of the cut is characteristic of a particular mineral. There was a conjecture that the form may be associated with the internal structure. In the end, all solids that have a natural flat facet came to be called crystals.

A significant milestone in the history of crystallography was a book written in 1784 by the Frenchman R. Gayuy. He suggested that crystals arise as a result of the correct stacking of tiny identical particles, which he called "molecular blocks." Hayuy showed how smooth flat faces of calcite can be obtained by stacking such "bricks". He explained the differences in the form of different substances by the difference, both in the form of "bricks" and in the way they were laid.

Bearing in mind the possibility of direct study of the internal structure, many involved in crystallography began to use the term "crystal" in relation to all solids with an ordered internal structure.

All that is needed is favorable conditions, they believed, in order for the internal

orderliness manifested itself in the form of a regular external cut. Some scientists prefer to call solids with no externally manifested internal order "crystalline", and by "crystals" to understand, as it used to be, solids with a natural cut.

They are arranged regularly, forming a three-dimensionally periodic spatial arrangement - a crystal lattice.

Crystals are solids that have a natural external shape of regular symmetrical polyhedra based on their internal structure, that is, on one of several certain regular arrangements that make up the substance of particles (atoms, molecules, ions).

Crystal structure

The crystal structure, being individual for each substance, refers to the basic physical and chemical properties of this substance.

Crystal cell

The particles that make up this solid form a crystal lattice. If the crystal lattices are stereometrically (spatially) the same or similar (have the same symmetry), then the geometric difference between them lies, in particular, in different distances between the particles occupying the lattice nodes. The distances between particles themselves are called lattice parameters. The lattice parameters, as well as the angles of geometric polyhedra, are determined by physical methods of structural analysis, for example, methods of X-ray structural analysis.

Often solids form (depending on conditions) more than one form of crystal lattice; such forms are called polymorphic modifications. For example, among simple substances, orthorhombic and monoclinic sulfur, graphite and diamond are known, which are hexagonal and cubic modifications of carbon, among complex substances - quartz, tridymite and cristobalite are various modifications of silicon dioxide.

Types of crystals

It is necessary to separate the ideal and real crystal.

Perfect Crystal

It is, in fact, a mathematical object that has a complete symmetry inherent in it, ideally smooth smooth edges.

real crystal

It always contains various defects in the internal structure of the lattice, distortions and irregularities on the faces and has a reduced symmetry of the polyhedron due to the specific growth conditions, inhomogeneity of the feeding medium, damage and deformation. A real crystal does not necessarily have crystallographic faces and a regular shape, but it retains its main property - the regular position of atoms in the crystal lattice.

Anisotropy of crystals

Many crystals have the property of anisotropy, that is, the dependence of their properties on direction, while in isotropic substances (most gases, liquids, amorphous solids) or pseudo-isotropic (polycrystals) bodies, properties do not depend on directions. The process of inelastic deformation of crystals is always carried out along well-defined slip systems, that is, only along certain crystallographic planes and only in a certain crystallographic direction. Due to the inhomogeneous and unequal development of deformation in different parts of the crystalline medium, intense interaction occurs between these parts through the evolution of microstress fields.

At the same time, there are crystals in which there is no anisotropy.

A wealth of experimental material has been accumulated in the physics of martensitic inelasticity, especially on questions of shape memory effects and plasticity of transformation. Experimentally proved the most important position of crystal physics about the predominant development of inelastic deformations almost exclusively through martensitic reactions. But the principles of constructing the physical theory of martensitic inelasticity are not clear. A similar situation takes place in the case of deformation of crystals by mechanical twinning.

Significant progress has been made in the study of the dislocation plasticity of metals. Here, not only are the basic structural and physical mechanisms for the implementation of inelastic deformation processes understood, but also effective methods for calculating phenomena have been created.

Physical sciences studying crystals

• crystallography studies ideal crystals from the standpoint of symmetry laws and compares them with real crystals.
• structural crystallography deals with the determination of the internal structure of crystals and the classification of crystal lattices.
• crystal optics studies the optical properties of crystals.
• crystal chemistry studies the laws governing the formation of crystals from various substances and in various media.

In general, the properties of real crystals are a huge scientific branch, suffice it to say that all the semiconductor properties of some crystals (on the basis of which precision electronics and, in particular, computers are created) arise precisely due to defects.

see also

Notes

Literature

• Chemistry: Ref. ed. / W. Schroeter, K.-H. Lautenschleger, H. Bibrak and others: Per. with him. - M.: Chemistry, 1989.
• General Physics Course, Book 3, I. V. Savelyev: Astrel, 2001, ISBN 5-17-004585-9.
• Crystals / M. P. Shaskolskaya, 208 pp. ill. 20 cm, 2nd ed., corrected. - M.: Nauka, 1985.
• Likhachev V. A., Malinin V. G. Structural-analytical theory of strength. - St. Petersburg: Science. - 471 p.
• Zorky P.M. Symmetry of molecules and crystal structures. M.: Publishing House of Moscow State University, 1986. - 232 p.

Links

• Mineral crystals , Forms of natural dissolution of crystals
• The definition of "Crystal" in the Big Encyclopedic Dictionary
• The only factory of its kind that produces Crystals

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See what "Crystals" are in other dictionaries:

CRYSTALS- (from the Greek krystallos, the original meaning is ice), solids with a three-dimensional periodicity. at. structure and, under equilibrium conditions of formation, having natural. the shape of regular symmetrical polyhedra (Fig. 1). K. balanced ... ... Physical Encyclopedia

CRYSTALS- (from the Greek crystallos ice), homogeneous solids that have a regular internal structure. The scheme of such a structure is the so-called spatial lattice (see figure), which must be understood as a geometric image ... ... Big Medical Encyclopedia

CRYSTALS- (from the Greek krystallos, originally ice), solids whose atoms or molecules form an ordered periodic structure (crystal lattice). Crystals have the symmetry of the atomic structure, the corresponding symmetry of the external form, ... ... Big Encyclopedic Dictionary

crystals- I Crystals (from the Greek krýstallos, originally ice, later rock crystal, crystal) are solid bodies that have the natural shape of regular polyhedra (Fig. 1). This form is a consequence of the ordered arrangement of atoms in K., ... ... Great Soviet Encyclopedia

crystals- (from the Greek krýstallos, originally ice), solids whose atoms or molecules form an ordered periodic structure (crystal lattice). Crystals have the symmetry of the atomic structure, the corresponding symmetry of the external ... ... encyclopedic Dictionary

Rice. 1: a - some synthetic single crystals and products made from them (quartz, garnet, KH 2 RO 4, potassium alum, etc., ruby ​​rods for lasers, sapphire plates); b - crystal of aspartate transaminase (length ~1 mm); (c) Ge microsingle crystal (size ~5 μm).

and determined by it ext. cut. A crystal that has grown under non-equilibrium conditions and does not have the correct cut (or has lost it as a result of processing) retains its crystallinity. structure and all St. Islands defined by it. At the macro level, i.e. when measuring sections of the crystal, significantly exceeding the distance between atoms and the size of unit cells, the crystal can be considered as a continuous homogeneous solid medium, physical., physical.-chem. and other St. Islands to-swarm have anisotropy and symmetry. Most solid materials are polycrystalline; they consist of many individual randomly oriented small crystals. grains (crystallites), for example. pl. rocks, tech. metals and alloys. Large individual homogeneous crystals with a continuous crystalline. lattice is called single crystals. Such are the crystals of minerals, for example. huge (up to hundreds of kg) crystals of quartz (rock crystal), fluorite, calcite, feldspar or relatively small crystals of beryl, diamond, etc. Crystals form and grow most often from a liquid phase - solution or melt; it is possible to obtain crystals from the gas phase or during phase transformation. in the solid phase (see Crystallization, single crystal growth). There are prom. and lab. methods of growing synthetic. crystals - analogues of nature. crystals (quartz, ruby, diamond, etc.) and decomp. tech. crystals, eg. Si, Ge, leucosapphire, garnets. Crystals are formed from such nature. in-in, like proteins, nucleic acids, as well as from viruses. Under certain conditions, synthetic crystals can be obtained. polymers. Main methods for studying crystals, their atomic structure and its defects - radiography, neutron diffraction, electron diffraction, electron microscopy; optical is also used. and spectroscopic. methods, incl. EPR, NMR, electron and Mössbauer spectroscopy, etc.

The atomic structure of crystals is described as a set of identical elementary cells repeating in space, having the shape of parallelepipeds with edges a, b, c (crystal lattice periods). The arrangement of the atomic planes of the crystal. lattice (which can correspond to the Crimea and the face of the crystal) is characterized by crystallographic. indices (or Miller indices). They are associated with cutoffs by the corresponding plane on three crystallographic axes. coordinate systems by segments, the lengths of which p 1 , p 2 and p 3 are expressed in lattice constants a, b, c. If the reciprocals of p 1, p 2 and p 3 are brought to a common denominator and then discarded, then the resulting three integers h=p 2 p 3 , k=p 1 p 3 , l=p 1 p 2 and are Miller indexes. They are written in parentheses (hkl). As a rule, a crystal has faces with small index values, for example. (100), (110), (311). The equality to zero of one or two indices means that the planes are parallel to one of the crystallographic. axes (coordinate axes). If the edge intersects negative. direction of the axis, then a minus sign is placed above the index, e.g. (121). Cell periods a, b, c and angles between edges a , b , y is measured radiographically.
Symmetry of crystals. With some geom. transformations g i the crystal is able to combine with itself, remaining invariant (unchanged). On fig. 3a shows a quartz crystal. Ext. its shape is such that by turning 120 ° around axis 3 it can be. combined with itself (compatible equality). The Na 2 SiO 3 crystal (Fig. 3.6) is transformed into itself by reflection in the plane of symmetry m (mirror equality). Transformations (operations) of the symmetry of any crystal g i - rotations, reflections, parallel transfers or combinations of these transformations - constitute a mat. groups G(g 0 , g 1 ,..., g n-1). The number n of operations that form a group G, called. group order. Groups of transformations of crystals denote G 3 m , where m is the number of dimensions, in which the object is periodic, top. index 3 means three dimensions of space, c. to-rykh these groups are defined. Crystalline the polyhedron is macroscopically non-periodic, the symmetry groups of such polyhedra (point groups) are denoted by G 3 0 . The microstructure of crystals at the atomic level is three-dimensionally periodic, i.e.

Rice. 3. Examples of crystals of different symmetry: a quartz crystal (3 - axis of symmetry of the 3rd order; 2 x, 2 y, 2 w - axes of the 2nd order); b - aqueous Na 2 SiO crystal, (m - plane of symmetry).

described as crystalline. lattice, corresponding symmetry groups G 3 3 . After the symmetry transformation, the parts of the object that were in one place coincide with the parts that are in another place. This means that a symmetrical object consists of equal - compatible and (or) mirror - parts. The symmetry of crystals is manifested not only in their structure and properties in real three-dimensional space, but also in the description of energy. spectrum of crystal electrons, when analyzing X-ray and electron diffraction in crystals in reciprocal space, etc. An example of a crystal, to-rum inherent in several. symmetry operations, -quartz crystal; it coincides with itself during rotations around the 3 axis by 120° (operation g 1), by 240° (operation g 2), and also during rotations by 180° around the axes 2 x , 2 y , 2 w (operations g 3 , g4, g5). Each operation of symmetry m. b. an element of symmetry is compared - a straight line, a plane or a point, relative to which this operation is performed. For example, axes 3, 2 x , 2 y , 2 w - axis of symmetry, plane m - plane of mirror symmetry, etc. Consistent performing two symmetry operations is also a symmetry operation. There is always an identity operation (identification) g 0 =1, which does not change anything in the crystal, geometrically corresponding to the immobility of the object or its rotation by 360° around any axis. Point Symmetry Groups. Operations of point symmetry of a crystal - rotations around the symmetry axis of order N at an angle equal to 360 o /N (Fig. 4, a), reflection in the plane of symmetry m (mirror reflection; Fig. 4.6), inversion I (symmetry about a point; Fig. 4, c) inversion rotations N (combination rotation through an angle of 360°/N with simultaneous inversion ; Fig. 4, d). Geometrically possible combinations of these operations determine one or another point symmetry group. Under point symmetry transformations, at least one point of the object remains fixed. It intersects

Rice. 4. The simplest symmetry operations: a - rotation; b - reflection; in - inversion; d - inversion turn; d - screw turn; e - sliding reflection.

all elements of symmetry are repented. The number of point symmetry groups G 0 3 is infinite. However, in crystals, due to the presence of crystalline. lattices, only operations and resp. symmetry axes up to the 6th order, except for the 5th (in crystalline


Note. Point group symmetry is more common than mine in lit. designate them with international symbols. lattice, such an axis is impossible), which are denoted by the symbols 1, 2, 3, 4. 6, as well as inversion axes (it is also the center of symmetry), 2 (it is also the plane of symmetry m), 3, 5, 6. Therefore, the number of point symmetry groups of crystals, otherwise called. crystallography by h. classes of crystals, limitedly, there are only 32 of them (see table). The international notation for point groups includes the symbols of the symmetry operations that generate them. These groups are combined according to the symmetry of the shape of the unit cell into 7 syngonies - triclinic, monoclinic, rhombic, tetragonal, trigonal, hexagonal, cubic.

Rice. 5. Simple forms (a) of crystals and some combinations of them (b).

The set of crystallographically identical faces (i.e., coinciding with each other during symmetry operations of a given group) forms the so-called. simple crystal form. In total there are 47 simple forms of crystals, but only some of them can be realized in each class. A crystal can be faceted with faces of one simple shape (Fig. 5, a), but more often with a combination of these forms (Fig. 5.5). The faceting of each crystal obeys the point symmetry group that describes it, with a uniform development of the crystal. polyhedron when it has an ideal shape (Fig. 6). Groups containing only rotations describe crystals consisting only of compatible equal parts (groups of the 1st kind; examples of such operations are given in Fig. 4, a, e). Groups containing reflections or inversion rotations describe crystals, in which there are mirror equal parts (groups of the 2nd kind; examples in Fig. 4.6, d, f). Crystals described by groups of the 1st kind, for example. quartz, wine to-you, can crystallize in two enantiomorphic forms (right and left), each of which does not contain symmetry elements of the 2nd kind (see Enantiomorphism). Mn. Holy Islands of crystals belonging to certain point symmetry groups are described by the so-called. limit point groups containing symmetry axes of infinite order: . The presence of an axis: means that


Rice. Fig. 6. Examples of faceting of crystals belonging to different point symmetry groups (classes): a - class 2 (one axis of symmetry of the 2nd order, left and right forms); b - class m (one plane of symmetry); c - class (center of symmetry); d - class 6 (one inversion axis of the 6th order); e - class 432 (axes of the 4th, 3rd and 2nd orders).

the object is aligned with itself when rotated through any, including infinitely small, angle (isotropic solids, textures). There are 7 such groups (Fig. 7). So arr., in total there are 39 point groups that describe the symmetry of the holy-in crystals.

Rice. 7. Figures illustrating limit symmetry groups.

spaces. antisymmetry groups G 3,a 0 (Shubnikov groups). If the additional variable acquires not two values, but several (the numbers 3, 4, 6, 8, ..., 48 are possible), then Belov's color symmetry arises. So, 81 point groups G 3, and 0 and 2942 groups C 3, and 3 are known. The apparatus of symmetry in the space of 4, 5 dimensions is also developed, which makes it possible to describe superperiodic, so-called. commensurate and incommensurate structures of ferroelectrics, magn. and other structures.

Rice. 10. The figure described by the point group of antisymmetry.

The structure of real crystals. Non-equilibrium conditions of crystallization lead to decomp. deviations in the shape of crystals from flat faces - to rounded faces and ribs (vicinals), the appearance of lamellar, acicular, filamentous (see Filamentous crystals), branched (dendritic), snowflake-type crystals. If a large number of crystallization centers are formed at once in the volume of the melt, then the growing crystals, meeting with each other, take the form of irregular grains. Often there are microscopy, twins, and other splices. When growing crystals, they do not necessarily strive to obtain them in the correct crystallographic. cutting, the main quality criterion is uniformity and perfection atomic structure, the absence of its defects. Some crystals, when grown, are given the shape of the required product - a pipe, a rod, a plate. Due to the violation of the equilibrium conditions of growth and the capture of impurities during crystallization, as well as under the influence of decomp. kind of external influences ideal three-dimensionally-periodic. the atomic structure of a crystal always has certain disturbances. These include point defects - vacancies, substitutions