Crystalline in-va, in which the adhesion between the particles is due to preim. ionic bonds. Because between ionic and polar covalent bonds there is a continuous transition, there is no sharp boundary between I. to. and covalent crystals. Ionic crystals include crystals, in which the bonds between atoms are max. polar; in the main these are alkali salts and alkaline earth. metals. I. to. are distinguished by high melting points, which usually means. band gap, have ionic conductivity at high temperatures and next specific. optical sv-in (eg, transparency in the near IR spectrum). They m. b. built from both monatomic and polyatomic ions. An example of I. to. the first type - crystals of alkali halides and alkaline earth. metals; anions are arranged according to the law of closest spherical packing or dense spherical laying (see Fig. dense packing), cations occupy the corresponding voids. Naib. typical structures of this type are NaCl, CsCl, CaF 2 . ICs of the second type are built from monatomic cations of the same metals and finite or infinite anionic fragments. Final anions (acid residues) - NO 3 -, SO 4 2 -, CO 3 2 -, etc. Acid residues can be combined into endless chains, layers or form a three-dimensional framework, cations are located in the cavities of which, for example, in a crystalline structures silicates. For I. to. you can calculate the energy of the crystal. structure U (see table), approximately equal to the enthalpy of sublimation; The results are in good agreement with experiment. data. According to the Born-Meyer equation, for a crystal consisting of formally singly charged ions:
U= -A/R + Be - R/r - C/R 6 -D/R 8 + E 0
(R- the shortest interionic distance, BUT - Madelung constant depending on the geometry of the structure, В r - parameters describing the repulsion between particles, and characterize acc. dipole-dipole and dipole-quadrupole interaction. ions, E 0 - zero point energy, e - electron charge). As the cation grows larger, the contribution of dipole–dipole interactions increases.
U VALUES FOR SOME CRYSTAL STRUCTURES
For I. to. the electrostatic rule is carried out. Pauling valency: max. stable structures of crystals are those in which the sum of the "valence forces" of each anion Sz / K (z - the degree of oxidation, or formal charge, of the cation, K is its coordination. number) is exactly or approximately equal to the negative. charge of the anion. Thus, in the MgAl 2 O 4 spinel structure, each O 2 - ion is surrounded by three Al 3+ ions with K = 6 and one Mg 2+ ion with K = 4; Sz/K= 3.3/6 + 1.2/4 = 2. This rule is also valid for structures with means. covalent component of the bond. Naib. complete information about the chem. bonds in I. to. give x-ray data on the distributions of the electron density. r(r), where r is the radius vector. So, in NaCl crystals, the function r(r) has a minimum equal to 70 e/nm 3 ; the effective charge of each ion (in absolute value) is close to 0.9 e. Therefore, the ions are separated in space and are held in the crystal. structure in the main electrostatic forces. The electronic distribution in each of the ions is almost spherically symmetrical, and only at the periphery does the deformation of the electron density occur (especially for the anion). The Na + ion is somewhat expanded, and the Cl ion is compressed along the communication line compared to free. ions. Similar effects have been found in other halides. The presence around each ion of an area with a minimum. r(r) made it possible to introduce the concept of the separation radius R section (distance from the center of the ion to the region r(r) is a minimum along the communication line). As a rule, Rraed for cations is higher, and for anions it is lower than the classical values. ionic radii (with the exception of Ag halides). For example, Rraed for Na in NaCl is 0.121 nm, for Mg 2+ in MgO 0.092 nm (the corresponding classical ionic radii are 0.098 and 0.074 nm). Lit.: see at Art. Crystals. P. M. Zorkiy.
- - supramolecular systems of membranes of a living cell and its organelles, which are of a lipoprotein nature and provide electorate. passage of diff. ions through the membrane. Naib, channels for Na+, K+, Ca2+ ions are common...
- - molecular structures embedded in biol. membranes and carrying out the transfer of ions towards a higher electrochemical. potential...
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- - crystalline inclusions in the nucleus, cytoplasm or vacuoles of the cell, usually consisting of calcium oxalate, less often of calcium carbonate or sulfate, silica, proteins and carotenoids ...
Plant anatomy and morphology
- - , solids with a three-dimensional periodicity. at. structure and, under equilibrium conditions of formation, having natural. the shape of regular symmetrical polyhedra...
Physical Encyclopedia
- - consist of two oppositely charged ions held electrostatically. forces, dispersion, ion-dipole or some other interactions ...
Chemical Encyclopedia
- - see Atomic radii...
Chemical Encyclopedia
- - tv. bodies, atoms or molecules to-rykh form an ordered periodic. structure. To. have the symmetry of the atomic structure, the corresponding symmetry of the external. shape, as well as the anisotropy of physical. properties...
Natural science. encyclopedic Dictionary
- - ionic devices are the same as gas-discharge devices ...
Encyclopedia of technology
- - theories based on the assumption that the cause of excitation is a change in the concentration of ions inside and outside the cell ...
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- - the same as gas discharge devices ...
- - see Solid electrolytes ...
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- - crystals in which the adhesion of particles is due mainly to ionic chemical bonds. I. to. can consist of both monatomic and polyatomic ions ...
- - gas discharge devices, Electrovacuum devices, the operation of which is based on the use various kinds electrical discharges in gas or metal vapors...
- - conditional characteristics ions used for approximate estimation of internuclear distances in ionic crystals...
Great Soviet Encyclopedia
- - characteristics of the distances between the nuclei of cations and anions in ionic crystals...
- - substances that have high ionic conductivity in the solid state, comparable to the conductivity of liquid electrolytes and molten salts. These include Ag2S, AgI, AgBr, CuCl, RbAg4I5 and some solid solutions...
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What is ionic polarization
Ionic polarization consists in the displacement of ions in an external electric field and the deformation of the electron shells in this case. Consider a crystal of the $M^+X^-$ type. The crystal lattice of such a crystal can be considered as two cubic lattices, one of which is built from $M^+$ ions, the other from - $X^-$ and they are inserted one into the other. Let us direct an external uniform electric field ($\overrightarrow(E)$) along the Z axis. The ionic lattices will shift in opposite directions on segments $\pm z$. If we assume that $m_(\pm )(\omega )^2_0$ is a quasi-elastic force that returns an ion with mass $m_(\pm )$ to the equilibrium position, then the force ( $F_(upr)$), which is equal to:
In this case, the electric force ($F_e$), which acts on ions of the same lattice, is equal to:
Equilibrium conditions
In this case, the equilibrium conditions will take the form:
For positive ions:
For negative ions:
In this case, the total relative displacement of ions is equal to:
Ionic polarization is:
where $V_0$ is the volume of one molecule.
If we take, for example, the $NaCl$ structure, in which each ion is surrounded by six ions of the opposite sign, which are located at a distance a from it, then we get:
and, therefore, using (5) and (6), we get that:
Ionic polarization is established in a very short time approximately $(10)^(-13)sec.$ It does not lead to energy dissipation, does not cause dielectric losses. When withdrawing external field, electron shells return to their previous state.
The ionic lattice polarization is described by formula (9). In most cases, such polarization is anisotropic.
where $\left\langle \overrightarrow(p)\right\rangle $ is the average value of the dipole moments of ions, which are equal in absolute value but differently directed, $\overrightarrow(p_i)$ are the dipole moments of individual ions. In isotropic dielectrics, the average dipole moments coincide in direction with the intensity of the external electric field.
Local field strength for crystals
The local field strength ($\overrightarrow(E")\ or\ sometimes\ \overrightarrow(E_(lok))\ $) for cubic crystals can be expressed by the formulas:
where $\overrightarrow(E)$ is the average macroscopic field in the dielectric. Or:
If equation (10) is applicable for cubic crystals to calculate the local field, then the Clausius-Mossotti formula can be applied to such crystals:
where $\beta $ is the polarizability of the molecule, $n$ is the concentration of molecules.
The relationship between the polarizability ($\beta $) of a molecule and the dielectric susceptibility ($\varkappa$) for cubic crystals can be given by the expression:
Example 1
Task: The dielectric constant of the crystal is equal to $\varepsilon =2.8$. How many times is the local strength ($\overrightarrow(E")$) of the cubic system field greater than the strength of the average macroscopic field in the dielectric ($E$)?
We take as a basis the formula for calculating the local field strength, namely:
\[\overrightarrow(E")=\frac(\varepsilon +2)(3)\overrightarrow(E)\left(1.1\right).\]
Therefore, for the desired ratio of intensities, we can write that:
\[\frac(E")(E)=\frac(\frac(\varepsilon +2)(3)E)(E)=\frac(\varepsilon +2)(3)\left(1.2\right) .\]
Let's do the calculations:
\[\frac(E")(E)=\frac(2,8+2)(3)=1,6.\]
Answer: 1.6 times.
Example 2
Task: Determine the polarizability of carbon atoms in diamond ($\beta $) if the permittivity of diamond is $\varepsilon =5.6$ and its density is $(\rho )_m=3.5\cdot (10)^3\ frac(kg)(m^3.)$
As a basis for solving the problem, we take the Clausius-Mossotti equation:
\[\frac(\varepsilon -1)(\varepsilon +2)=\frac(n\beta )(3)\left(2.1\right).\]
where the particle concentration $n$ can be expressed as:
where $(\rho )_m$ is the mass density of the substance, $\mu =14\cdot (10)^(-3)\frac(kg)(mol)$ -- molar mass carbon, $N_A=6.02\cdot (10)^(23)mol^(-1)$ -- Avogadro's constant.
Then expression (2.1) will take the form:
\[\frac(\varepsilon -1)(\varepsilon +2)=\frac(\beta )(3)\frac((\rho )_mN_A)(\mu )\ \left(2.3\right).\]
From expression (2.3) we express the polarizability $\beta $, we get:
\[\ \beta =\frac(3\mu (\varepsilon -1))((\rho )_mN_A(\varepsilon +2))\left(2.4\right).\]
Substitute the available numerical values, carry out the calculations:
\[\beta =\frac(3\cdot 14\cdot (10)^(-3)(5.6-1))(3.5\cdot (10)^3\cdot 6.02\cdot (10 )^(23)(5,6+2))=\frac(193,2\cdot (10)^(-3))(160,132\cdot (10)^(26))=1,2\cdot ( 10)^(-29)m^3\]
Answer: $\beta =1,2\cdot (10)^(-29)m^3$.
In complex crystals consisting of elements of different valencies, the formation of an ionic type of bond is possible. Such crystals are called ionic.
When the atoms approach each other and the valence energy bands overlap, the electrons are redistributed between the elements. An electropositive element loses valence electrons, turning into a positive ion, and an electronegative element acquires it, thereby completing its valence band to a stable configuration, like in inert gases. Thus, ions are located at the nodes of an ionic crystal.
The representative of this group is an oxide crystal whose lattice consists of negatively charged oxygen ions and positively charged iron ions.
The redistribution of valence electrons in an ionic bond occurs between the atoms of one molecule (one atom of iron and one atom of oxygen).
For covalent crystals, the coordination number K, and the possible type of lattice, are determined by the valence of the element. For ionic crystals, the coordination number is determined by the ratio of the radii of the metallic and non-metallic ions, since each ion tends to attract as many ions of the opposite sign as possible. The ions in the lattice fit like balls of different diameters.
The radius of a non-metal ion is greater than the radius of a metal ion, and therefore metal ions fill the pores in the crystal lattice formed by non-metal ions. In ionic crystals, the coordination number
determines the number of ions of opposite sign that surround the given ion.
The ratios of the metal radius to the nonmetal radius given below and the corresponding coordination numbers follow from the packing geometry of balls of different diameters.
For the coordination number will be 6, since the indicated ratio is 0.54. On fig. 1.14 shows the crystal lattice. Oxygen ions form an fcc lattice, iron ions occupy pores in it. Each iron ion is surrounded by six oxygen ions, and, conversely, each oxygen ion is surrounded by six iron ions. In this regard, in ionic crystals, it is impossible to isolate a pair of ions that could be considered a molecule. Upon evaporation, such a crystal breaks up into molecules.
When heated, the ratio of ionic radii can change, since the ionic radius of a nonmetal grows more rapidly than the radius of a metal ion. This leads to a change in the type of crystal structure, i.e., to polymorphism. For example, in an oxide, when heated, the spinel crystal lattice changes to a rhombohedral lattice (see Sec. 14.2),
Rice. 1.14. Crystal lattice a - scheme; b - spatial image
The binding energy of an ionic crystal is close in magnitude to the binding energy of covalent crystals and exceeds the binding energy of metallic and even more so molecular crystals. In this regard, ionic crystals have a high melting and evaporation temperature, a high modulus of elasticity, and low coefficients of compressibility and linear expansion.
The filling of energy bands due to the redistribution of electrons makes ionic crystals semiconductors or dielectrics.
An ideal ionic crystal consists of positively and negatively charged spherical ions. If not all, then at least some of the alkali halide compounds, i.e., are most consistent with this idea. salts formed by one of the alkali metals (lithium, sodium, potassium, rubidium, cesium) and one of the halogens (fluorine, chlorine, bromine, iodine). There is evidence that the crystals of these salts are indeed formed by positive metal ions and negatively charged halogen ions. The most direct of them is the data of X-ray diffraction analysis, on the basis of which the electronic charge distribution is calculated (cm. rice. 9 for the case of NaCl).(22.74 Kb)The fact that such solids are composed of ions rather than atoms can be explained as follows. First of all, all alkali metal atoms have one outer valence electron, while outer shell halogen atoms contains seven valence electrons. When a valence electron passes from an alkali metal atom to a halogen atom, two ions are formed, each of which has a stable electronic configuration characteristic of atoms of inert gases. Even more important is the energy gain due to the Coulomb attraction between positive and negative ions. Consider sodium chloride (NaCl) as an example. To tear off the outer (valence) electron from the Na atom, you need to spend 5.14 eV (ionization energy). When this electron is attached to the Cl atom, there is an energy gain of 3.61 eV (electron affinity energy). Thus, the energy required for the transition of a valence electron from Na to Cl is (
5,14 - 3.61) eV = 1.53 eV. The Coulomb energy of attraction between two emerging Na ions+ and Cl- at a distance between them (in a crystal) equal to 2.18, is 5.1 eV. This value more than compensates for the total energy of the electron transition and leads to a decrease in the total energy of the system of ions in comparison with a similar system of free atoms. This is the main reason that alkali halide compounds are composed of ions, not atoms.Calculating the energy of ionic crystals is actually more complicated than it might seem from the above discussion. But at least for alkali halide crystals, there is good agreement between the theoretical and experimental values of the binding energy. The ionic bonds are quite strong, as indicated, for example, by the high melting point of 1074 K for NaCl.
Thanks to high degree stability of the electronic structure, ionic crystals fall into the category of dielectrics. Since positive and negative ions interact with electromagnetic waves, ionic crystals exhibit strong optical absorption in the infrared region of the spectrum. (The frequency of the oscillating external electric field in this region of the spectrum is close to the natural frequency of transverse lattice waves, in which the positive and negative ions of the crystal move in opposite directions.) In the visible region of the spectrum, the oscillation frequencies are too high for massive ions to have time to respond to the action of such waves. That's why light waves pass through the crystal without interaction, i.e. such crystals are transparent. At even higher frequencies in the ultraviolet region of the spectrum the field quanta can have sufficient energy to excite valence electrons, which ensures the transition of valence electrons of negative ions to the unoccupied states of positive ions. This leads to strong optical absorption.
covalent crystals. The best known covalent crystals are diamond, silicon and germanium. Each atom in such crystals is surrounded by four neighboring atoms located at the vertices of a regular tetrahedron. Free atoms of each of these elements have four valence electrons, and this is enough to form four paired electronic bonds (between this atom and its four nearest neighbors). Thus, two electrons are collectivized by two atoms that form a bond and are located in space along the line connecting the atoms. This is almost the same bond as between two hydrogen atoms in the hydrogen molecule H 2 . In diamond, these bonds are very strong, and since they have a strictly defined direction relative to each other, diamond is an extremely hard material. The strength of the covalent bond of an electron with a crystal is characterized by the so-called energy gap the minimum energy that must be transferred to an electron so that it can freely move in a crystal and create electricity. For diamond, silicon, and germanium, the width of this gap is 5.4, 1.17, and 0.744 eV, respectively. Therefore, diamond is a good dielectric; the energy of thermal vibrations in it at room temperature is too small to release valence electrons. In silicon, and especially in germanium, due to the relatively small width of the energy gap, thermal excitation of a certain number of valence electrons at room temperature is possible. Thus, they conduct current, but since their conductivity is much less than that of metals, silicon and germanium are classified as semiconductors.With ionic (electrostatic) nature of the bond between atoms. I. to. can consist of both monatomic and polyatomic ions. Examples of I. to. the first type - crystals of alkali halide and alkaline earth metals, formed positively charged. metal ions and negatively charged. halogen ions (NaCl, CsCl, CaF2). Examples of I. to. the second type - nitrates, sulfates, phosphates, and other metal salts, where negative. ions of acidic residues consist of several. atoms. Silicates are also referred to as I. to., in which silicon-oxygen radicals SiO4 form chains, layers or a three-dimensional framework, inside the radicals the atoms are linked by a covalent bond (see INTERATOMAL INTERACTION.
Physical Encyclopedic Dictionary. - M.: Soviet Encyclopedia
. . 1983 .IONIC CRYSTALS
Crystals with an ionic (electrostatic) bond between atoms. I. to. can consist of both monatomic and polyatomic ions. Examples of I. to. the first type - crystals of halides of alkali and alkaline earth metals, formed by positively charged metal ions and negatively charged halogen priests (NaCl, CsCl, CaF 2). Examples of I. to. the second type - carbonates, sulfates, phosphates, and other metal salts, where negative. ions of acidic residues, eg. CO 3 2-, SO 4 2-, consist of several. atoms. Formal ions, eg. Na + , Mg 2+ , O 2- , even in the most typical I. to., in fact, it turns out to be more than the real eff. charge, which is determined by radiography., Spectral and other methods. So, for example, in NaCl eff. the charge is for Na approx. +0.9 her - elementary electric. charge), and for Cl, respectively -0.9 e. For MgF 2 , CaCl 2 estimate eff. charges of anions leads to values of approx. -0.7 e, and for cations - from +1.2 e up to +1.4 e. In silicates and oxides, "divalent" O 2- actually has a charge of -0.9 to -1.1 e. Thus, in fact, in pl. And. to. communication has ionic and covalent character. the transparency of I. to. the higher, the higher the proportion of the covalent component of the bond. To describe the structure of I. to. developed detailed systems crystal chem. radii (see atomic radius).Lit.: Modern, vol. 2, M., 1979; Wells A., Structural inorganic chemistry, per. from English, vol. 1, M., 1987. B. K. Weinstein.
Physical encyclopedia. In 5 volumes. - M.: Soviet Encyclopedia
. Editor-in-Chief A. M. Prokhorov. 1988 .See what "IONIC CRYSTALS" is in other dictionaries:
IONIC CRYSTALS, crystals with an ionic (electrostatic) bond (see IONIC COMMUNICATION) between atoms. At the nodes of the crystal lattice of ionic crystals, ions of the opposite sign are alternately located, it is impossible to distinguish individual molecules in them, ... ... encyclopedic Dictionary
Crystal structure of sodium chloride (rock salt). Each atom has six nearest neighbors, as in the geometry of an octahedron. This mechanism is known as cubic close packing (CDP). Light blue \u003d Na + Dark green \u003d Cl Ionic crystals ... ... Wikipedia
Crystals in which the adhesion of particles is due predominantly to ionic chemical bonds (see Ionic bond). I. to. can consist of both monatomic and polyatomic ions. Examples of I. to. the first type of halide crystals ... ... Great Soviet Encyclopedia
IONIC CRYSTALS- crystals with a predominantly ionic (electrostatic) nature of the bond between atoms ... Paleomagnetology, petromagnetology and geology. Dictionary reference.
Crystalline in VA, in which the adhesion between the particles is due to preim. ionic bonds. Since there is a continuous transition between ionic and polar covalent bonds, there is no sharp boundary between I. to. and covalent crystals. To ionic ... ... Chemical Encyclopedia
- (solid electrolytes) substances that have high ionic conductivity s in the solid state, comparable to the conductivity of liquid electrolytes and molten salts (10 1 10 3 Ohm 1 cm 1). I. s. can be divided into 2 types. 1) Ionic crystals capable of ... ... Physical Encyclopedia
- (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
- (from the Greek. krystallos crystal; originally ice), solids with a three-dimensional periodicity. atomic (or molecular) structure and, under certain conditions, education, having natural. the shape of regular symmetrical polyhedra (Fig. ... ... Chemical Encyclopedia
- (from the Greek krystallos, letters. ice; rock crystal) solid bodies that have an ordered mutual arrangement particles of atoms, ions, molecules that form them. In an ideal crystal, particles are arranged strictly periodically in three dimensions, forming the so-called ... ... Big encyclopedic polytechnic dictionary
branch of physics that studies structure and properties solids. Scientific data on the microstructure of solids and on the physical and chemical properties their constituent atoms are necessary for the development of new materials and technical devices. Physics ... ... Collier Encyclopedia
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- Mechanics of electromagnetic continuums, Mozhen Zh.. A book by a famous French specialist, combining the advantages study guide and introduction to the current field of modern mechanics. It describes the properties of electromagnetic ...