The main properties of crystals. Unique properties of crystals

Date: 10.10.2019

Send your good work in the knowledge base is simple. Use the form below

Students, graduate students, young scientists who use the knowledge base in their studies and work will be very grateful to you.

Posted on http://www.allbest.ru/

Generalproperties of crystals

Introduction

Crystals are solids having a natural foreform The correct symmetric polyhedra based on their internal structure is, that is, on one of several certain regular locations of the component of the particle substance.

The basis of solid physics is the idea of \u200b\u200bthe crystallinity of the substance. All the theories of the physical properties of crystalline solids are based on the presentation of the perfect periodicity of crystalline lattices. Using this presentation and the provisions arising from it about symmetry and the anisotropy of crystals, physics developed the theory of the electronic structure of solids. This theory allows you to give a strict classification of solids, defining their type and macroscopic properties. However, it allows you to classify only well-known, studied substances and does not allow to predetermine the composition and structure of new complex substanceswho would have the specified properties complex. This last task is particularly important for practice, as its decision would make it possible to create materials on request for each specific case. With appropriate external conditions, the properties of crystalline substances are determined by their chemical composition and type of crystal lattice. The study of the dependence of the properties of the substance from its chemical composition and the crystal structure is usually divided into the following sections 1) the general study of the crystals and the crystalline state of the substance 2) constructing the theory of chemical bonds and its application to the study of various classes of crystalline substances 3) study of the general patterns of changing the structure of crystalline substances when changing their chemical composition 4) the establishment of rules allowing to predetermine chemical composition and the structure of substances possessing a specific complex of physical properties.

Maintenanceproperties of crystals - anisotropy, homogeneity, the ability to self-proclaimize and the presence of a constant melting point.

1. Anisotropy

crystal anisotropy self-discharge

Anisotropy - it is expressed in that physical properties Crystals are unequal in different directions. TO physical quantities These parameters can be attributed - strength, hardness, thermal conductivity, speed of light propagation, electrical conductivity. A characteristic example of a substance with pronounced anisotropy is mica. Crystal plates of mica - easily split only in planes. In the transverse directions, the plates of this mineral are much more difficult.

An example of anisotropy-is the crystal of Mineral Dysteshen. In the longitudinal direction, a dyspenin hardness is 4.5, in transverse - 6. Mineral Dysten (Al 2 O), characterized by a sharply different hardness in unequal directions. Along the lengthening of the Dysteshen crystals, the knife blade is easily scratched, in the direction of perpendicular elongation, the knife does not leave any traces.

Fig. 1 Crystal Dystenna

Mineral Cordieritis (Mg 2 Al 3). Mineral, aluminosilicate magnesium and iron. Cordierite Crystal in three different directions seems differently painted. If from such a crystal cut the cube with edges, then you can notice the following. Perpendicular to these areas, then the cube is diagonally (from the top to the top there is a grayish-blue color, in the direction of the vertical - indigo-blue color, and in the direction of the cube - yellow.

Fig. 2 cube cut from Cordierite.

Crystal crash saltwhich has a cube shape. From such a crystal, you can cut rods in different directions. Three of them perpendicular to the edges of the cube, parallel to the diagonal

Each example is exceptional in its characterity. But by accurate studies, scientists came to this conclusion that all crystals in one respect or another have anisotropy. Also, solid amorphous formations can be homogeneous and even anisotropic (anisotropy, for example, it can be observed during stretching or squeezing of brakes), but amorphous bodies cannot themselves take a multifaceted form under any conditions.

Fig. 3 Identification of the anisotropy of thermal conductivity on quartz (a) and its absence on glass (b)

As an example (Fig. 1), the anisotropic properties of crystalline substances must first be mentioned about the mechanical anisotropy, which is as follows. All crystalline substances are not solved equally along various directions (mica, plaster, graphite, etc.). Amorphous substances - in all directions split the same, because the amorphism is characterized by isotropy (equivorion) - physical properties in all directions are the same.

The anisotropy of thermal conductivity is easy to just follow the next simple experience. On the edge of the quartz crystal to apply a layer of colored wax and bring the needle to the center the needle to the center of the alcohol. The formed wax circle around the needle will take the form of an ellipse on the verge of prisms or the shape of an irregular triangle on one of the edges of the crystal head. On the isotropic substance, for example, glass - the form of melt wax will always be the right circle.

Anisotropy is also manifested in the fact that when interacting with a crystal of a solvent, speed chemical reactions Different in different directions. As a result, each crystal when dissolved in the end acquires its characteristic forms.

Ultimately, the cause of the anisotroposis of crystals is that with the ordered arrangement of ions, molecules or atoms of the interaction force between them and interatomic distances (as well as some not associated values \u200b\u200bof them, for example, electrical conductivity or polarizability) are unequal in different directions. The cause of anisotropy of the molecular crystal may also be asymmetry of its molecules, I would like to note that all amino acids, except for the simplest - glycine, asymmetrical.

Any particle of the crystal has a strictly defined chemical composition. This property of crystalline substances is used to obtain chemically pure substances. For example, when freezing sea water It becomes fresh and suitable for drinking. Now guess, the sea ice is fresh or salty?

2. Uniformity

Homogeneity - it is expressed in the fact that any elementary volumes crystalline substanceEqually oriented in space, absolutely the same in all its properties: have the same color, mass, hardness, etc. Thus, every crystal is homogeneous, but at the same time an anisotropic body. The body is considered homogeneous in which at the final distances from any of its point there are other equivalent to it not only in physically, but also geometric. In other words, there are in the same environment as the original, since the placement of material particles in the crystalline space "controls" a spatial grille, we can assume that the edge of the crystal is a materialized flat nodular grille, and the rib is a materialized nodal row. As a rule, well-developed edges of the crystal are determined by nodal grids with the largest nodes' location. The point in which three and more faces converge is called a vertex of the crystal.

The homogeneity is inherent not only for crystalline bodies. Solid amorphous education may also be homogeneous. But amorphous bodies cannot take a multifaceted form by themselves.

Developments are underway, which can increase the coefficient of crystals uniformity.

This invention is patented by our Russian scientists. The invention relates to the sugar industry, in particular to the receipt of attelligers. The invention provides an increase in the coefficient of homogeneity of the crystals in the approach, and also contributes to an increase in the growth rate of crystals at the final stage of increasing due to the gradual growth of the submission coefficient.

The disadvantages of the known method are the low coefficient of uniformity of crystals in the uthylene of the first crystallization, a significant duration of the development of the Utfel.

The technical result of the invention is to increase the coefficient of the homogeneity of crystals in the utefel of the first crystallization and the intensification of the process of obtaining the development of the Utfel.

3. Ability to self-strength

The ability to self-dine is expressed in that any debris or flushing the ball in the corresponding medium corresponding to its growth over time is covered with graphs characteristic of this crystal. This feature is associated with the crystal structure. The glass ball, for example, does not possess such a feature.

The mechanical properties of crystals include properties associated with such mechanical effects on them, like a blow, compression, tensile, and other - (sphey, plastic deformation, break, hardness, fragility).

Self-standing ability, i.e. Under certain conditions, take a natural multifaceted form. This also manifests itself its correct inner structure. It is this property that distinguishes the crystalline substance from amorphous. An illustration is an example. Two rolls of quartz and glass of the ball are lowered into a solution of silica. As a result, the quartz ball will cover the edges, and the glass will remain round.

Crystals of the same mineral can have different shape, the value and number of faces, but the corners between the appropriate edges will always be permanent (Fig. 4 A-d) is the law of constant corners in crystals. At the same time, the size and shape of the facets in various crystals of the same substance, the distance between them and even their number may vary, but the angles between the corresponding grains in all crystals of the same substance remain constant under the same pressure and temperature conditions. The corners between the glands of crystals are measured using a goniometer (Coromet). The law of constancy of face angles is explained by the fact that all crystals of the same substance are identical inland structure. Have the same structure.

According to this law, the crystals of a certain substance are characterized by their specific angles. Therefore, the measurement of the angles can be proved by the belonging of the crystal under study to one or another substance.

In ideally formed crystals, there is a symmetry, which in natural crystals is extremely rare due to the leading growth of the faces (Fig. 4 d).

Fig. 4 The law of constant corners in crystals (A - D) and the growth of the advanced faces of 1.3 and 5 growing on the wall of the cavity of the crystal (E)

Sphey is called such a property of crystals in which to split or split according to certain crystallographic directions, as a result, smooth smooth planes are formed, called spreadiness planes.

Spound planes are oriented in parallel with valid or possible edges of crystals. This property is entirely dependent on the inner structure of minerals and is manifested in those directions in which the clutch forces between the material particles of crystal lattices are the smallest.

You can allocate, depending on the degree of perfection, several types of scope:

Very perfect - mineral is easily split into separate thin plates or leaves, split it in another direction is very difficult (mica, plaster, talc, chlorite).

Fig. 5 chlorite (Mg, Fe) 3 (Si, Al) 4 O 10 (OH) 2 · (MG, FE) 3 (OH) 6)

Perfect - mineral relatively easily splits predominantly on the planes of the scope, and the punched pieces often resemble individual crystals (calcite, galenite, ilitis, fluorite).

Fig. 6 calcite

The average - when splitting, they are formed both the plane of the scope and uneven bends in random directions (pyroxes, field spasters).

Fig. 7 Field Ploves ((K, Na, Ca, Sometimes Ba) (Al 2 Si 2 or Alsi 3) O 8))

Imperfect - minerals split at arbitrary directions with the formation of uneven surfaces of the breakfast, separate planes of spoundation are found with difficulty (native sulfur, pyrite, apatite, olivine).

Fig. 8 Apatite crystals (CA 5 3 (F, CL, he))

In some minerals, only uneven surfaces are formed when splitting, in this case, they are talking about very imperfect spyality or the absence of it (quartz).

Fig. 9 quartz (SiO 2)

Spoundism can manifest in one, two, three, rarely more directions. For more detailed characteristics It is indicated by the direction in which the spoundism passes, for example, according to Rombohedra - at Calcite, in Cuba - at Galita and Galenita, in octahedra - in fluorite.

The plane of the scope needs to be distinguished from the edges of the crystals: the plane, as a rule, has a stronger shine, form a number of parallel to each other planes and, unlike the edges of the crystals on which we cannot observe hatching.

Thus, the scope can be traced one by one (mica), two (field spasters), three (calcite, religion), four (fluorite) and six (sphalerite) directions. The degree of perfection of spoundament depends on the structure of the crystal lattice of each mineral, since the gap in some planes (flat grids) of this lattice due to weaker bonds is much easier than in other destinations. In the case of the same clutch forces between the particles of the crystal, the scope is missing (quartz).

Flee - the ability of minerals to split not on the planes of the scope, but on the complex uneven surface

Separateness - the property of some minerals to split with the formation of parallel, although most often not very even flat planes that are not caused by the structure of the crystal lattice, which is sometimes taken for spidity. Unlike scope, separate is the property of only some individual specimens of this mineral, and not a mineral species as a whole. The main difference separately from the sphey is that the resulting pumps cannot be split further into smaller fragments with smooth parallel chips.

Symmetry - The most common pattern associated with the structure and properties of the crystalline substance. It is one of the generalizing fundamental concepts of physics and natural science in general. "Symmetry has a property geometric figures Repeat your parts, or, expressing more precisely, the property of them in various positions to come into combination with the initial position. " For the convenience of study, the models of crystals transmitting the forms of ideal crystals are used. To describe the symmetry of crystals, it is necessary to determine the elements of symmetry. Thus, this object is symmetric, which can be combined with itself with certain transformations: rotations or (and) reflections (Figure 10).

1. The plane of symmetry is an imaginary plane that divides the crystal into two equal parts, and one of the parts is like a mirror reflection of the other. There may be several symmetry planes in the crystal. The symmetry plane is indicated by the Latin letter R.

2. The axis of symmetry is a line, when rotating around which, 360 ° Crystal N-O, the number of times repeats its initial position in space. Denoted by the letter L. N - determines the order of the axis of symmetry, which in nature can only be 2, 3, 4 and 6th order, i.e. L2, L3, L4 and L6. The axes of the fifth and above the sixth order in the crystals are not, and the axes of the first order are not taken into account.

3. The Symmetry Center is an imaginary point located inside the crystal in which they intersect and divide the line connecting the corresponding points on the surface of the crystal1. The symmetry center is indicated by the letter S.

All varieties found in the nature of crystalline forms are combined into seven syngami (systems): 1) cubic; 2) hexagonal; 3) tetragonal (square); 4) trigonal; 5) rhombic; 6) monoclinal and 7) triclinic.

4. Permanent melting point

Melting - transition of a substance from a solid state into liquid.

It is expressed in the fact that when the crystalline body is heated, the temperature rises to a certain limit; With the further heating, the substance begins to melt, and the temperature remains constant for some time, since everything heat goes to the destruction of the crystal lattice. The reason for this phenomenon is considered that the main part of the heater's energy supplied to the solid is to reduce the bonds between the particles of the substance, i.e. to the destruction of the crystal lattice. This increases the energy of interaction between particles. The molten substance has a big margin internal energythan in solid state. The remaining part of the heat of melting is spent on the performance of work on changing the volume of the body when it is melted. The temperature at which melting begins is called the melting point.

When melting, the volume of most crystalline bodies increases (by 3-6%), and during hardening decreases. But, there are substances that, when melting, the volume decreases, and during harvesting - increases.

These include, for example, water and cast iron, silicon and some others. That is why ice floats on the surface of the water, and solid cast iron - in its own melt.

Amorphous substances, in contrast to crystalline, do not have a clearly pronounced melting point (amber, resin, glass).

Fig. 12 amber

The amount of heat required for melting substance is equal to the product of the specific heat of melting on the mass of this substance.

The specific heat of melting shows which heat treatment is necessary for the complete conversion of 1 kg of a substance from a solid state into a liquid, taken at the melting pace.

The unit of specific heat melting in si serves as 1J / kg.

In the process of melting, the temperature of the crystal remains constant. This temperature is called melting point. Each substance has its own melting point.

The melting point for this substance depends on the atmospheric pressure.

In crystalline bodies, at a melting point, a substance can be observed simultaneously in solid and liquid states. On the cooling curves (or heating) of crystalline and amorphous substances, it can be seen that in the first case there are two sharp beggars that correspond to the beginning and the end of crystallization; In the case of cooling the amorphous substance, we have a smooth curve. On this basis, it is easy to distinguish crystalline substances from amorphous.

Bibliography

1. Handbook of Chemist 21 "Chemistry and Chemical Technology" p. 10 (http://chem21.info/info/1737099/)

2. Handbook of geology (http://www.geolib.net/crystallography-svoystva-kristallov.html)

3. "Urf named after the first president of Russia B.N. Yeltsin ", section Geometric Crystallography (http://media.ls.urfu.ru/154/489/1317/)

4. Chapter 1. Crystallography with the basics of crystal chemistry and mineralogy (http://kafgeo.igpu.ru/web-text-books/geology/r1-1.htm)

5. Application: 2008147470/13, 01.12.2008; IPC C13F1 / 02 (2006.01) C13F1 / 00 (2006.01). Patent holder (s): state educational institution Higher vocational education Voronezh State Technological Academy (RU) (http://bd.patent.su/2371000-2371999/pat/servl/servlet939d.html)

6. Tula State Pedagogical University Him L.N. Tolstoy Department of Ecology Golyn F.A. "The concept of minerals as crystalline substances" (http://tsput.ru/res/geogr/geology/lec2.html)

7. Computer learning course "General Geology" course of lectures. Lecture 3 (http://igd.sfu-kras.ru/sites/igd.institute.sfu-kras.ru/files/kurs-geologia/%D0% BB% D0% B5% D0% BA% D1% 86% D0% B8% D0% B8 /% D0% BB% D0% B5% D0% BA% D1% 86% D0% B8% D1% 8F_3.htm)

8. Class of physics (http://class-fizika.narod.ru/8_11.htm)

Similar documents

Crystalline and amorphous state of solid bodies, causes of point and linear defects. The birth and growth of crystals. Artificial receipt precious stones, solid solutions and liquid crystals. Optical properties of cholesteric liquid crystals.

abstract, added 04/26/2010

Liquid crystals as a phase state in which some substances are passing under certain conditions, their main physical properties and factors affecting them. History of research, types, the use of liquid crystals in the production of monitors.

examination, added 06.12.2013

Features and properties of the liquid crystal state of the substance. Structure of smectic liquid crystals, properties of their modifications. Segroelectric characteristics. The study of the oxicoidal structure of the smectics C * by the method of molecular dynamics.

abstract, added 12/18/2013

The history of the development of the idea of \u200b\u200bliquid crystals. Liquid crystals, their types and basic properties. The optical activity of liquid crystals and their structural properties. Frederix effect. Physical principle of devices on the LCD. Optical microphone.

tutorial, added 12/14/2010

Consideration of the history of the discovery and directions for the use of liquid crystals; Their classification for adjacent, nematic and cholesteric. The study of optical, diamagnetic, dielectric and acousto-optic properties of liquid crystal substances.

coursework, added 06/18/2012

Determination of liquid crystals, their essence, opening history, properties, features, classification and direction of use. Characteristics of classes of thermotropic liquid crystals. Broadcast degrees of freedom of column phases or "liquid threads".

abstract, added 12/28/2009

Crystals are real solid bodies. Thermodynamics of point defects in crystals, their migration, sources and drains. Investigation of dislocation, linear defect of the crystal structure solid tel. Two-dimensional and three-dimensional defects. Amorphous solid bodies.

report, added 07.01.2015

presentation, added 09/29/2013

The concept and main features of the condensed state of the substance characteristic of the processes. Crystal and amorphous bodies. The essence and features of the anisotropy of crystals. Distinctive features polycrystals and polymers. Thermal properties and structure of crystals.

course of lectures, added 02/21/2009

Evaluation of viscous temperature properties (oils). The dependence of the pressure of the outbreak of pressure. Dispersion, optical activity. Laboratory methods Distillation of oil and petroleum products. The heat of melting and sublimation. Specific and molecular refraction.

The fact of the geometrically natural arrangement of material particles in crystal structures, finally established using X-rays, is based on all modern crystallography. But the theory of the lattice of the structure of crystals was created long before the X-rayanalysis. The greatest crystallographs of Auguste Brave, L. Zheke, E.S. Fedorov, A.Senflis, and others gave the mathematical development of this theory. The use of x-ray rays confirmed empirically by the correctness of their speculative constructions.

The theory of the crystal structure until 1912 was based on some features of the crystalline state captured by experimentally. These are the most important properties of crystals include:

1. Studito. This is a fixed arrangement of each other in relation to a friend. In amorphous substance there are fragments of crystals, but over time, these fragments are destroyed. A hundreds of years in the windows, for example, changes and they "flow" are occurring.

2. Roodiness or homogeneity. According to an experimental data, a homogeneous is called such a body, which in all its volume detects the same properties. The homogeneity of the crystals is established when studying its properties by parallel directions. The crystal body with the same structure in all its sites should be uniformity. It does not take into account the extraneous pollution, the inclusion and imperfections of real crystals associated with external influences.

3. Anicalotropy - (translated "AN" -n, "Izos" -vnodno, "Stroofos" -diections, i.e. non-residents). Anisotropic is called such a homogeneous body, which, with the same properties in parallel directions, has in the general case with unequal properties in parallel directions. Due to the lattice of the structure, the same atoms (ions, molecules) should be located strictly equally, forming the same intervals among themselves. Therefore, the properties of crystals must be the same in such directions. According to non-parallel directions, the particles in the general case will take apart from each other at different distances, as a result of which the properties in such directions should be different.

For example, mica. The crystal plates of this mineral are easily cleaved only in planes parallel to its lamelty. In the transverse directions to split the salivary plates is much more difficult.

Another example of anisotropy is dysten mineral (Al 2 O), characterized by a sharply different hardness in unequal directions. Along the lengthening of the Dysteshen crystals, the knife blade is easily scratched, in the direction of perpendicular elongation, the knife does not leave any traces.

Fig.1. Crystal Dystenna

Mineral Cordieritis (Mg 2 Al 3). Cordierite Crystal in three different directions seems differently painted. If such a crystal cut cut cubes with edges. Perpendicular to these areas, they diagonally diagonally (from the top to the top there is a grayish-blue color, in the direction across the cube - yellow, and in the direction of the vertical - indigo-blue color.

Fig.2. Cube carved from Cordieritis.

Crystal salt, which has a cube shape. From such a crystal, you can cut rods in different directions. Three of them perpendicular to the edges of the cube, parallel to the diagonal. It turned out that various efforts are needed for the break of these rods: a tearing force for the first rod (vertical along the axis) is expressed 570 g / mm 2, for the second (horizontal diagonal) - 1150 g / mm 2 and for the third (diagonal from the top to the vertex ) - 2150 g / mm 2. (Fig. 3)

The above examples are exceptional in their own characterity. But by way of accurate studies, it was possible to conclude that all crystals in one or another had anisotropy.

Solid amorphous formations can also be homogeneous and even anisotropic (anisotropy, for example, can be observed during stretching or squeezing of stalk). But under no circumstances, the amorphous bodies can themselves take a multifaceted form.

The main properties of crystals are anisotropy, homogeneity, the ability to self-proclaimize and the presence of a constant melting point is determined by their inner structure.

Fig. 1. An example of anisotropy - Crystal Mineral Dystenna. In the longitudinal direction, its hardness is 4.5, in the transverse - 6. © Parent Géry

This property is called still non-loss. It is expressed in the fact that the physical properties of crystals (hardness, strength, thermal conductivity, electrical conductivity, speed of light propagation) are not the same directions. The particles forming the crystal structure on non-parallel directions will take apart from each other at different distances, as a result of which the properties of the crystalline substance should be different in such directions. A characteristic example of a substance with pronounced anisotropy is mica. Crystal plates of this mineral are easily cleaved only in planes parallel to its lamellariness. In the transverse directions, the plates of mica are much harder.

Anisotropy manifests itself in the fact that when exposed to a crystal of a solvent, the rate of chemical reactions is different in different directions. As a result, each crystal during dissolution acquires its characteristic forms that bear the name of etching figures.

Amorphous substances are characterized by isotropy (equation) - physical properties in all directions are manifested the same.

Uniformity

It is expressed in that any elementary volumes of the crystalline substance equally oriented in space, absolutely the same in all its properties: have the same color, mass, hardness, etc. Thus, every crystal is homogeneous, but at the same time an anisotropic body.

The homogeneity is inherent not only for crystalline bodies. Solid amorphous education may also be homogeneous. But amorphous bodies cannot take a multifaceted form by themselves.

Ability to self-strength

The crystals of the same substance may differ from each other with their size, the number of faces, ribs and the shape of the faces. It depends on the conditions of crystal formation. With uneven growth, crystals are obtained flattened, elongated, etc. The angles remain unchanged between the respective edges of the growing crystal. This feature of crystals is known as the law of constancy of face angles. At the same time, the size and shape of the facets in various crystals of the same substance, the distance between them and even their number may vary, but the angles between the corresponding grains in all crystals of the same substance remain constant under the same pressure and temperature conditions.

The law of constancy of the face angles was set at the end of the XVII century by the Danish scientist wall (1699) on iron shine crystals and mining crystal, subsequently, this law was confirmed by M.V. Lomonosov (1749) and French scientists Roma de Lill (1783). The law of constancy of face angles was called the first law of crystallography.

The law of constancy of face angles is explained by the fact that all crystals of the same substance are identical on the inner structure, i.e. Have the same structure.

To measure the crystals of dugran corners were invented special instruments - Goniometers.

Permanent melting point

Amorphous substances, in contrast to crystalline, do not have a clear melting point. On the cooling curves (or heating) of crystalline and amorphous substances, it can be seen that in the first case there are two sharp beggars that correspond to the beginning and the end of crystallization; In the case of cooling the amorphous substance, we have a smooth curve. On this basis, it is easy to distinguish crystalline substances from amorphous.

Solid bodies are separated on amorphous bodies and crystals. The difference between the first of the first is that the crystals atoms are arranged according to some law, thereby forming three-dimensional periodic laying, which is called the crystal lattice.

It is noteworthy that the name of the crystals comes from the Greek words to "stick" and "cold", and at the time of Homer, this word was called mountain crystal, which was then considered " frozen ice" First, this term was called only fellow transparent formations. But later, the crystals were also called non-transparent and non-faceted bodies of natural origin.

Crystal structure and grille

The perfect crystal is represented in the form of periodically repeated identical structures - the so-called elementary crystal cells. In the general case, the form of such a cell is a row-agole parallelepiped.

Such concepts as a crystal lattice and crystal structure should be distinguished. The first is a mathematical abstraction depicting a regular location of some points in space. While the crystal structure is a real physical object, a crystal, in which a certain group of atoms or molecules is associated with each point of the crystal lattice.

Crystal Pomegranate Structure - Rhombo and Dodecahedron

The main factor determining the electromagnetic and mechanical properties of the crystal is the structure of the elementary cell and atoms (molecules) associated with it.

Anisotropy of crystals

The main property of crystals, distinguishing them from amorphous bodies - is anisotropy. This means that the properties of the crystal are different, depending on the direction. For example, an inelastic (irreversible) deformation is carried out only by certain planes of the crystal, and in a certain direction. Due to the anisotropy of the crystals, they react differently to the deformation depending on its direction.

However, there are crystals that do not possess anisotropy.

Types of crystals

Crystals are divided into single crystals and polycrystals. Monocrystals call substances whose crystal structure applies to the whole body. Such bodies are homogeneous and have continuous crystal lattice. Usually, such a crystal has a pronounced cut. Examples of natural single crystal are stone salt monocrystals, diamond and topaz, as well as quartz.

Many substances have a crystalline structure, although usually do not have a form characteristic of crystals. These substances include, for example, metals. Studies show that such substances consist of large number Very small single crystals - crystalline grains or crystallites. A substance consisting of many such multi-oriented single crystals is called polycrystalline. Polycrystals often do not have cuts, and their properties depend on the average size of crystalline grains, their mutual location, as well as the structure of the intergranulated border. Polycrystals include substances such as metals and alloys, ceramics and minerals, as well as others.

The main properties of crystals

The crystals grow multifaceted, since their growth rates in different directions are different. If they were the same, it would be the only form - the ball.

Not only growth rate, but also almost all of their properties are different in different directions, i.e. Crystal is inherent anisotropy ("An" - not, "bottom" - the same, "Tropos" is a property), non-uniformity in the directions.

For example, calcite when heated in the longitudinal direction is stretched (a \u003d 24.9 · 10 -6 o C -1), and in the transverse - compressing (a \u003d -5.6 · 10 -6 o C -1). There is also a direction in which thermal expansion and compression compensate each other (the direction of zero expansion). If you cut a plate perpendicular to this direction, then when heated, the thickness will not be changed, and it can be used to make parts in accurate engineering.

Graphite expansion along vertical axis 14 times more than in directions transverse to this axis.

Especially visual anisotropy of the mechanical properties of crystals. Crystals with a layered structure - mica, graphite, talc, gypsum - in the direction of the layers are completely easily split into thin leaves, split them in other directions is incomparably more difficult. Salt is divided into small cubes, Spanish spat - on rhombohedra (spike phenomenon).

In crystals there is also anisotropy of optical properties, thermal conductivity, electrical conductivity, elasticity, etc.

IN polycrystalconsisting of a randomly oriented random single-crystal grains, there is no anisotropy of properties.

Once again it is necessary to emphasize that amorphous substances are also isotropsy.

In some crystalline substances, an isotropy may appear. For example, the spread of light in crystals cubic Singonia It occurs at the same speed in different directions. It can be said that such crystals are optically isotropic, although the anisotropy of mechanical properties may be observed in these crystals.

Uniformity - Property physical body Be the same in all volumes. The homogeneity of the crystalline substance is expressed in the fact that any sections of the crystal the same form And equally oriented, characterized by the same properties.

Self-converted ability - The ability of the crystal in favorable conditions to take a multifaceted form. Describes the law of constancy of the angles of the wall.

Plograge and directurality . The surface of the crystal is limited to planes or faces, which, intersecting, form straight lines - ribs. The intersection points of the ribs form vertices.

Ribs, ribs, tops, as well as bike angles (straight, stupid, sharp) are elements of external crystal limitations. Two angled angles (these are two intersecting planes), as mentioned above, for this type of substance are constant.

The Euler formula establishes the relationship between the elements of the restriction (only simple closed forms):

R + B \u003d p + 2,

G - the number of faces

In - the number of vertices,

P - Number of ribs.

For example, for cube 6 + 8 \u003d 12 + 2

The edges of the crystals correspond to the rows of the lattice, the face - flat grids.

Symmetry of crystals .

"The crystals shook their symmetry," wrote the great Russian crystallograph E.S. Fedorov.

Symmetry is a regular repeatability of equal shapes or equal parts of the same figure. "Symmetry" - with Greek. "Commodity" of respective points in space.

If the geometrical object in three-dimensional space is rotated, is shifted or reflected and, at the same time, it is accurately combined with himself (transformed into himself), i.e. It remained invariant to the conversion attached to it, the object is symmetrical, and the transformation is symmetric.

There may be cases of alignment:

1. Combining equal triangles (or other figures) occurs by turning them clockwise to 180 o and overlay one to another. Such figures are compatible and equal. An example is the same gloves (left or right).