Not all solids are crystals. There are many amorphous bodies.
Amorphous bodies do not have strict order in the arrangement of atoms. Only the closest neighbor atoms are arranged in some order. But there is no strict directionality in all directions of the same structural element, which is characteristic of crystals in amorphous bodies.
Often the same substance can be found in both crystalline and amorphous states. For example, quartz SiO2 can be in either crystalline or amorphous form (silica). The crystalline form of quartz can be schematically represented as a lattice of regular hexagons. The amorphous structure of quartz also has the form of a lattice, but irregular shape. Along with hexagons, it contains pentagons and heptagons.
In 1959 English physicist D. Bernal conducted interesting experiments: He took many small plasticine balls of the same size, rolled them in chalk powder and pressed them into a large ball. As a result, the balls were deformed into polyhedra. It turned out that in this case predominantly pentagonal faces were formed, and the polyhedra had an average of 13.3 faces. So there's some order in amorphous substances oh definitely there is.
Amorphous bodies include glass, resin, rosin, sugar candy, etc. Unlike crystalline substances, amorphous substances are isotropic, that is, their mechanical, optical, electrical and other properties do not depend on direction. Amorphous bodies do not have a fixed melting point: melting occurs in a certain temperature range. The transition of an amorphous substance from a solid to a liquid state is not accompanied by an abrupt change in properties. Physical model the amorphous state has not yet been created.
Amorphous bodies occupy an intermediate position between crystalline solids and liquids. Their atoms or molecules are arranged in relative order. Understanding the structure solids(crystalline and amorphous) allows you to create materials with specified properties.
At external influences amorphous bodies exhibit both elastic properties, like solids, and fluidity, like liquids. Thus, under short-term impacts (impacts), they behave like solid bodies and, under a strong impact, break into pieces. But at very prolonged exposure amorphous bodies flow. Let's follow a piece of resin that lies on a smooth surface. Gradually the resin spreads over it, and the higher the temperature of the resin, the faster this happens.
Amorphous bodies at low temperatures their properties resemble solids. They have almost no fluidity, but as the temperature rises they gradually soften and their properties become closer and closer to the properties of liquids. This happens because as the temperature increases, the jumps of atoms from one position to another gradually become more frequent. Amorphous bodies, unlike crystalline ones, do not have a specific body temperature.
When cooling liquid substance crystallization does not always occur. under certain conditions, a nonequilibrium solid amorphous (glassy) state can form. In a glassy state they can be simple substances(carbon, phosphorus, arsenic, sulfur, selenium), oxides (for example, boron, silicon, phosphorus), halides, chalcogenides, many organic polymers. In this state, the substance can be stable for a long period of time, for example, the age of some volcanic glasses is calculated for millions of years. Physical and Chemical properties substances in a glassy amorphous state can differ significantly from the properties crystalline substance. For example, glassy germanium dioxide is chemically more active than crystalline one. Differences in the properties of the liquid and solid amorphous state are determined by the nature thermal movement particles: in an amorphous state, particles are capable only of vibration and rotational movements, but cannot move through the substance.
Under the influence of mechanical loads or temperature changes, amorphous bodies can crystallize. Reactivity substances in the amorphous state is significantly higher than in the crystalline state. Main sign amorphous (from the Greek "amorphos" - formless) state of matter - the absence of an atomic or molecular lattice, that is, the three-dimensional periodicity of the structure characteristic of the crystalline state.
There are substances that can only exist in solid form in an amorphous state. This refers to polymers with an irregular sequence of units.
Along with crystalline solids, amorphous solids are also found. Amorphous bodies, unlike crystals, do not have a strict order in the arrangement of atoms. Only the closest atoms - neighbors - are arranged in some order. But
There is no strict repeatability in all directions of the same structural element, which is characteristic of crystals, in amorphous bodies.
Often the same substance can be found in both crystalline and amorphous states. For example, quartz can be in either crystalline or amorphous form (silica). The crystalline form of quartz can be schematically represented as a lattice of regular hexagons (Fig. 77, a). The amorphous structure of quartz also has the appearance of a lattice, but of irregular shape. Along with hexagons, it contains pentagons and heptagons (Fig. 77, b).
Properties of amorphous bodies. All amorphous bodies are isotropic: their physical properties the same in all directions. Amorphous bodies include glass, many plastics, resin, rosin, sugar candy, etc.
Under external influences, amorphous bodies exhibit both elastic properties, like solids, and fluidity, like liquids. Under short-term impacts (impacts), they behave like a solid body and, with a strong impact, break into pieces. But with very long exposure, amorphous bodies flow. For example, a piece of resin gradually spreads over a solid surface. Atoms or molecules of amorphous bodies, like liquid molecules, have certain time“sedentary life” is the time of oscillations around the equilibrium position. But unlike liquids, this time is very long. In this respect, amorphous bodies are close to crystalline ones, since jumps of atoms from one equilibrium position to another rarely occur.
At low temperatures, amorphous bodies resemble solids in their properties. They have almost no fluidity, but as the temperature rises they gradually soften and their properties become closer and closer to the properties of liquids. This happens because with increasing temperature, jumps of atoms from one position gradually become more frequent.
balance to another. There is no specific melting point for amorphous bodies, unlike crystalline ones.
Solid state physics. All properties of solids (crystalline and amorphous) can be explained on the basis of knowledge of their atomic-molecular structure and the laws of motion of molecules, atoms, ions and electrons that make up solids. Studies of the properties of solids are combined into large area modern physics- solid state physics. The development of solid state physics is stimulated mainly by the needs of technology. Approximately half of the world's physicists work in the field of solid state physics. Of course, achievements in this area are unthinkable without deep knowledge all other branches of physics.
1. How are they different? crystalline bodies from amorphous? 2. What is anisotropy? 3. Give examples of monocrystalline, polycrystalline and amorphous bodies. 4. How do edge dislocations differ from screw dislocations?
AMORPHOUS BODIES(Greek amorphos - formless) - bodies in which elementary constituent particles (atoms, ions, molecules, their complexes) are randomly located in space. To distinguish amorphous bodies from crystalline ones (see Crystals), X-ray diffraction analysis is used (see). Crystalline bodies on X-ray diffraction patterns give a clear, defined diffraction pattern in the form of rings, lines, spots, while amorphous bodies give a blurred, irregular image.
Amorphous bodies have following features: 1) in normal conditions isotropic, that is, their properties (mechanical, electrical, chemical, thermal, etc.) are the same in all directions; 2) do not have a certain melting point, and with increasing temperature, most amorphous bodies, gradually softening, turn into a liquid state. Therefore, amorphous bodies can be considered as supercooled liquids that have not had time to crystallize due to a sharp increase in viscosity (see) due to an increase in the interaction forces between individual molecules. Many substances, depending on the production methods, can be in amorphous, intermediate or crystalline states (proteins, sulfur, silica, and so on). However, there are substances that exist almost exclusively in one of these states. Thus, most metals and salts are in a crystalline state.
Amorphous bodies are widespread (glass, natural and artificial resins, rubber, and so on). Artificial polymer materials, which are also amorphous bodies, have become indispensable in technology, everyday life, and medicine (varnishes, paints, plastics for prosthetics, various polymer films).
In living nature, amorphous bodies include the cytoplasm and most structural elements cells and tissues consisting of biopolymers - long-chain macromolecules: proteins, nucleic acids, lipids, carbohydrates. Molecules of biopolymers easily interact with each other, giving aggregates (see Aggregation) or swarm-coacervates (see Coacervation). Amorphous bodies are also found in cells in the form of inclusions and reserve substances (starch, lipids).
A feature of polymers that make up the amorphous bodies of biological objects is the presence of narrow limits of physicochemical zones of reversible state, for example. When the temperature rises above the critical temperature, their structure and properties irreversibly change (protein coagulation).
Amorphous bodies, formed nearby artificial polymers, depending on temperature, can be in three states: glassy, highly elastic and liquid (viscous-fluid).
The cells of a living organism are characterized by transitions from a liquid to a highly elastic state at a constant temperature, for example, retraction of a blood clot, muscle contraction (see). IN biological systems amorphous bodies play decisive role in maintaining the cytoplasm in stationary state. The role of amorphous bodies in maintaining the shape and strength of biological objects is important: cellulose shell plant cells, shells of spores and bacteria, animal skin and so on.
Bibliography: Bresler S. E. and Yerusalimsky B. L. Physics and chemistry of macromolecules, M.-L., 1965; Kitaygorodsky A.I. X-ray structural analysis of fine-crystalline and amorphous bodies, M.-L., 1952; aka. Order and disorder in the world of atoms, M., 1966; Kobeko P. P. Amorphous substances, M.-L., 1952; Setlow R. and Pollard E. Molecular biophysics, trans. from English, M., 1964.
Unlike crystalline solids, there is no strict order in the arrangement of particles in an amorphous solid.
Although amorphous solids are capable of maintaining their shape, crystal lattice They dont have. A certain pattern is observed only for molecules and atoms located in the vicinity. This order is called close order . It is not repeated in all directions and is not stored in long distances, like crystalline bodies.
Examples of amorphous bodies are glass, amber, artificial resins, wax, paraffin, plasticine, etc.
Features of amorphous bodies
Atoms in amorphous bodies vibrate around points that are randomly located. Therefore, the structure of these bodies resembles the structure of liquids. But the particles in them are less mobile. The time they oscillate around the equilibrium position is longer than in liquids. Jumps of atoms to another position also occur much less frequently.
How do crystalline solids behave when heated? They begin to melt at a certain melting point. And for some time they are simultaneously in solid and liquid state until all the substance melts.
In amorphous bodies certain temperature no melting . When heated, they do not melt, but gradually soften.
Place a piece of plasticine near the heating device. After some time it will become soft. This does not happen instantly, but over a certain period of time.
Since the properties of amorphous bodies are similar to the properties of liquids, they are considered as supercooled liquids with very high viscosity (frozen liquids). Under normal conditions they cannot flow. But when heated, jumps of atoms in them occur more often, viscosity decreases, and amorphous bodies gradually soften. The higher the temperature, the lower the viscosity, and gradually the amorphous body becomes liquid.
Ordinary glass is a solid amorphous body. It is obtained by melting silicon oxide, soda and lime. By heating the mixture to 1400 o C, a liquid glassy mass is obtained. When cooling liquid glass does not solidify like crystalline bodies, but remains a liquid, the viscosity of which increases and the fluidity decreases. Under normal conditions, it appears to us as a solid body. But in fact it is a liquid that has enormous viscosity and fluidity, so low that it can barely be distinguished by the most ultrasensitive instruments.
The amorphous state of a substance is unstable. Over time, it gradually turns from an amorphous state into a crystalline state. This process in different substances passes with at different speeds. We see candy canes becoming covered in sugar crystals. This does not take very much time.
And for crystals to form in ordinary glass, a lot of time must pass. During crystallization, glass loses its strength, transparency, becomes cloudy, and becomes brittle.
Isotropy of amorphous bodies
In crystalline solids, physical properties vary in different directions. But in amorphous bodies they are the same in all directions. This phenomenon is called isotropy .
An amorphous body conducts electricity and heat equally in all directions and refracts light equally. Sound also travels equally in amorphous bodies in all directions.
The properties of amorphous substances are used in modern technologies. Special interest cause metal alloys that do not have crystal structure and belong to amorphous solids. They are called metal glasses . Their physical, mechanical, electrical and other properties differ from those of ordinary metals for the better.
Thus, in medicine they use amorphous alloys whose strength exceeds that of titanium. They are used to make screws or plates that connect broken bones. Unlike titanium fasteners, this material gradually disintegrates and is replaced over time by bone material.
High-strength alloys are used in the manufacture of metal-cutting tools, fittings, springs, and mechanism parts.
An amorphous alloy with high magnetic permeability has been developed in Japan. By using it in transformer cores instead of textured transformer steel sheets, eddy current losses can be reduced by 20 times.
Amorphous metals have unique properties. They are called the material of the future.
Amorphous solids, in many of their properties and mainly in their microstructure, should be considered as highly supercooled liquids with a very high viscosity coefficient. The structure of such bodies is characterized only by short-range order in the arrangement of particles. Some of these substances are not capable of crystallizing at all: wax, sealing wax, resins. Others, under a certain cooling regime, form crystalline structures, but in the case fast cooling An increase in viscosity prevents ordering in the arrangement of particles. The substance hardens before the crystallization process takes place. Such bodies are called glassy: glass, ice. The process of crystallization in such a substance can also occur after solidification (glass cloudiness). Solids are also classified as amorphous. organic matter: rubber, wood, leather, plastics, wool, cotton and silk fibers. The process of transition of such substances from the liquid phase to the solid phase is shown in Fig. – curve I.
Amorphous bodies do not have a solidification (melting) temperature. On the graph T = f(t) there is an inflection point, which is called the softening temperature. A decrease in temperature leads to a gradual increase in viscosity. This nature of the transition to solid state, causes the absence of specific heat of fusion in amorphous substances. The reverse transition, when heat is supplied, smooth softening occurs to a liquid state.
CRYSTALINE SOLIDS.
A characteristic feature of the microstructure of crystals is the spatial periodicity of their internal electric fields and the repeatability in the arrangement of crystal-forming particles - atoms, ions and molecules (long-range order). Particles alternate in in a certain order along straight lines, which are called nodal lines. In any flat section of a crystal, two intersecting systems of such lines form a set of completely identical parallelograms that tightly, without gaps, cover the section plane. In space, the intersection of three non-coplanar systems of such lines forms a spatial grid that divides the crystal into a set of completely identical parallelepipeds. The intersection points of the lines forming the crystal lattice are called nodes. The distances between nodes along a certain direction are called translations or lattice periods. A parallelepiped built on three non-coplanar translations is called a unit cell or lattice repeatability parallelepiped. The most important geometric property of crystal lattices is the symmetry in the arrangement of particles with respect to certain directions and planes. For this reason, although there are several ways to select a unit cell for a given crystal structure, it is chosen so that it matches the symmetry of the lattice.
Crystalline solids can be divided into two groups: single crystals and polycrystals. For single crystals, a single crystal lattice is observed throughout the entire body. And although external shape single crystals of the same type can be different, the angles between the corresponding faces will always be the same. A characteristic feature of single crystals is the anisotropy of mechanical, thermal, electrical, optical and other properties.
Single crystals are often found in their natural state in nature. For example, most minerals are crystal, emeralds, rubies. Currently, for production purposes, many single crystals are grown artificially from solutions and melts - rubies, germanium, silicon, gallium arsenide.
The same chemical element can form several crystal structures differing in geometry. This phenomenon is called polymorphism. For example, carbon - graphite and diamond; ice five modifications, etc.
Correct external faceting and anisotropy of properties, as a rule, do not appear for crystalline bodies. This is because crystalline solids usually consist of many randomly oriented small crystals. Such solids are called polycrystalline. This is due to the crystallization mechanism: when the conditions necessary for this process are achieved, crystallization centers simultaneously appear in many places in the initial phase. The nascent crystals are located and oriented relative to each other completely randomly. For this reason, at the end of the process, we obtain a solid in the form of a conglomerate of fused small crystals - crystallites.
From an energetic point of view, the difference between crystalline and amorphous solids is clearly visible in the process of solidification and melting. Crystalline bodies have a melting point - the temperature when a substance exists stably in two phases - solid and liquid (Fig. curve 2). The transition of a solid molecule into a liquid means that it acquires an additional three degrees of freedom of translational motion. That. unit of mass of a substance at T pl. in the liquid phase has greater internal energy than the same mass in the solid phase. In addition, the distance between particles changes. Therefore, in general, the amount of heat required to convert a unit mass of a crystalline substance into a liquid will be:
λ = (U f -U cr) + P (V f -V cr),
where λ – specific heat melting (crystallization), (U l -U cr) – difference internal energies liquid and crystalline phases, P – external pressure, (V l -V cr) – difference in specific volumes. According to the Clapeyron-Clausius equation, the melting temperature depends on pressure:
It can be seen that if (V f -V cr)> 0, then 
> 0, i.e. As pressure increases, the melting point increases. If the volume of a substance decreases during melting (V f -V cr)<
0 (вода, висмут), то рост давления приводит
к понижению Т пл.
Amorphous bodies have no heat of fusion. Heating leads to a gradual increase in the rate of thermal movement and a decrease in viscosity. There is an inflection point on the process graph (Fig.), which is conventionally called the softening temperature.
THERMAL PROPERTIES OF SOLIDS
Thermal motion in crystals due to strong interaction is limited only by vibrations of particles near the nodes of the crystal lattice. The amplitude of these oscillations usually does not reach 10 -11 m, i.e. is only 5-7% of the lattice period along the corresponding direction. The nature of these oscillations is very complex, since it is determined by the forces of interaction of the oscillating particle with all its neighbors.
An increase in temperature means an increase in the energy of particle motion. This, in turn, means an increase in the amplitude of particle vibrations and explains the expansion of crystalline solids when heated.
l t = l 0 (1 + αt 0),
Where l t and l 0 – linear dimensions of the body at temperatures t 0 and 0 0 C, α – linear expansion coefficient. For solids, α is of the order of 10 -5 – 10 -6 K -1. As a result of linear expansion, the volume of the body increases:
V t = V 0 (1 + βt 0),
here β is the coefficient of volumetric expansion. β = 3α in the case of isotropic expansion. Monocrystalline bodies, being anisotropic, have three different values of α.
Each particle that vibrates has three degrees of freedom of oscillatory motion. Considering that, in addition to kinetic energy, particles also have potential energy, energy ε = kT should be assigned to one degree of freedom of particles of solid bodies. Now for the internal energy of the mole we will have:
U μ = 3N A kT = 3RT,
and for molar heat capacity:
Those. The molar heat capacity of chemically simple crystalline bodies is the same and does not depend on temperature. This is the Dulong-Petit law.
As the experiment showed, this law is satisfied quite well, starting from room temperatures. Explanations for deviations from the Dulong-Petit law at low temperatures were given by Einstein and Debye in the quantum theory of heat capacity. It was shown that the energy per degree of freedom is not a constant value, but depends on temperature and oscillation frequency.
REAL CRYSTALS. DEFECTS IN CRYSTALS
Real crystals have a number of violations of the ideal structure, which are called crystal defects:
a) point defects –
Schottky defects (units unoccupied by particles);
Frenkel defects (displacement of particles from nodes to internodes);
impurities (introduced foreign atoms);
b) linear - edge and screw dislocations. It's local irregularly
sty in the arrangement of particles
due to the incompleteness of individual atomic planes
or due to irregularities in the sequence of their development;
c) planar – boundaries between crystallites, rows of linear dislocations.