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Diffracting methods




Lecture № 4.

Theme: a crystal condition of silicate materials. Methods of study of structure of crystal substances. The basic rules of construction ionic-covalent of structures.

1. Silicate in crystal condition.

2. Methods of study of structure of crystal substances.

3. Basic rules of construction ionic-covalent of structures.

ДТА - differential thermal analysis

ТГ - thermogravimetric the analysis

To diffracting to methods of research of structure concern roentgenography, electronography and neutronography. The methods are based on use of radiations with length of a wave, commensurable with distance between structural elements of crystals. Passing through a crystal, beams дифрагируют, arising diffracting the picture strictly corresponds(meets) to structure of researched substance.

Method diffraction of x-ray radiation.

The development X-ray diffraction analysis began from famous experience of m. Laye (1912), shown, that a bunch(beam) of x-ray radiation, passing

Through a crystal, tests diffraction, and the symmetry, distribution diffraction, of maxima corresponds(meets) to symmetry

Crystal. Diffraction, maxima arise in all directions adequate(answering) the basic law X-ray diffraction analysis to the equation Vulf-Bregg

Diffraction the methods can conditionally be divided(shared) into two groups: 1) corner of fall of a beam on a crystal constant, and length of radiation varies; 2) length of a wave constant, and the corner of fall varies.

The method Laye, consisting concerns to methods of the first group that polychromatic the x-ray radiation is directed on a motionless monocrystal, behind which the film settles down. From set of lengths of waves available in polychromatic radiation, always there will be such wave, which satisfies to conditions of the equation Vulf-Bregg. The method Laye enables to reveal symmetry of a crystal. The methods of rotation of a monocrystal and polycrystalline sample concern to methods of the second group. In a method of rotation of a monocrystal

monochromatic the beam is directed on a monocrystal rotating around of an axis, normal to a direction of a beam. Thus the various planes of a crystal get in a rule(situation) appropriate to conditions diffraction, that results in education appropriate diffraction of a picture. By measurement of integrated intensity and definition of a set of structural amplitudes it is possible to decipher structure of a crystal.

At study of polycrystalline materials the sample is shined(covered) монохроматическим by radiation. In set of arbitrary focused crystals always there will be such, which orientation answers the equation Вульфа-Брэгга. The reflected beam is registered by a photoway (fig. 2) either ионизационными or сцинтилляционными by counters, the signal through system of amplifiers and пересчетных of devices moves on потенциометр, writing down a curve of distribution of intensity (fig. 3). On an arrangement дифракционных of maxima judge geometry of a lattice, and on their intensity - about distribution of electronic density, т. е. About probability of a presence(finding) электронов in this or that point of a crystal (fig. 4). The distribution of electronic density enables to define(determine) not only rule(situation) of atoms in a lattice, but also type of chemical communication(connection). The high-temperature prefixes to дифрактометрам allow to register polymorphic transformations at heating, to watch(keep up) for твердофазовыми by reactions.




Рентгенография gives also opportunity to study defects in crystals.

Output(exit) of a beam; 4 - area of small corners 9

Fig. 2. Shooting o рентгенограммы of polycrystalline samples by a method of photoregistration:

Fig. 3. Рентгенограмма of quartz received on installation with сцинтилляционным by a method of registration

Method дифракции электронов (электронография). The method is based that at interaction with an electrostatic field of atoms there is a dispersion of a bunch(beam) электронов. As against x-ray, the electronic radiation can penetrate only on small depth therefore researched samples should look like thin пленок. With the help электронографии it is possible, besides definition межплоскостных of distances in a crystal, to study a rule(situation) of easy atoms in a lattice, that cannot be made through x-ray radiation poorly dissipating by easy atoms.



Method дифракции of neutrons. For reception of a bunch(beam) of neutrons is necessary nuclear реактор, therefore given method is used rather seldom. At an output(exit) from реактора the bunch(beam) is considerably weakened, therefore it is necessary to use a wide bunch(beam) and accordingly to increase the size of a sample. Advantage of a method is the opportunity of definition of a spatial rule(situation) of atoms of hydrogen, that it is impossible to make by others дифракционными by methods.

Fig. 4. Distribution of electronic density (о) and structure (б) of a crystal with ковалентной by communication(connection) (diamond)

Спектроскопические methods

Спектроскопия studies spectra of electromagnetic radiation which is let out, absorbed or рассеиваемого by researched substance. The electromagnetic radiation is characterized either power, or wave parameters. On ranges излучаемых of lengths of waves distinguish scale - спектроскопию, x-ray, optical and радиоспектроскопию. Optical спектроскопия, in turn, is subdivided on спектроскопию seen, infra-red and ultra-violet radiation. To each kind of electromagnetic radiation there corresponds(meets) any certain nuclear or molecular process.

Scale - спектроскопия. Most short-wave? The radiation is characterized by wave number 1010 см-1.

Рентгеноспектроскопия. The x-ray radiation is characterized by wave numbers 109... 107 см-1. It results from bombardment of substance электронами of high energy or at a rigid x-ray irradiation. Thus occurs вырывание электронов from internal electronic layers, to the released(exempted) place pass электроны from layers, farther from a nucleus, that is accompanied испусканием of quantums of characteristic x-ray radiation. The frequency of x-ray radiation of an element is linearly connected to his(its) nuclear number(room). Рентгеноспектральный the analysis is used for qualitative and quantitative definition of chemical structure of substance.

Distinguish fluorescent рентгеноспектральный the analysis, in which for excitation of a x-ray spectrum the x-ray radiation is used, and the microx-ray analysis, in which x-ray spectrum is generated by a bunch(beam) электронов. The fluorescent x-ray analysis is widely applied in factory laboratories, he enables quickly to receive the exact data on chemical structure of controllable object and to use them for automatic control of technological process. Микрорентгеноспектральный the analysis spend through an electronic probe, which enables to investigate structure of substance in a point, to define(determine) character of microinclusions and to give them a qualitative estimation.

Optical спектроскопия. The method is used for the characteristic of molecular processes. In a molecule there are transitions external электронов - ultra-violet and seen radiation (106... 104 см-1), fluctuation of atoms in molecules - near infra-red radiation (103 см-1) and rotation of molecules - distant infra-red radiation (102 см-1). The radiation of electronic transitions with the help of seen and ultra-violet radiations enables to define(determine) the power characteristics of molecules - energy of excitation, ionization and chemical communication(connection).

The oscillatory spectra are investigated with the help of infra-red radiation. On them it is possible to establish a spatial structure of groupings and molecules, to characterize a nature of chemical communication(connection) and its(her) polarity. The oscillatory spectrum of a molecule defines(determines) mainly weights of varying atoms and their groupings

The fluctuations of atoms in a molecule can occur along the communication line (валентные of fluctuation) and under a direct corner to the communication line (деформационные of fluctuation). Валентные of fluctuation characterize rigidity of chemical communication(connection), деформационные - rigidity валентных of corners.

The fluctuations of group of atoms depend on coordination number of the central ion and from a type coordination многогранника. In process of complication of complexes the shift of maxima of absorption in short-wave area is observed. So, for example, at island силикатов maxima of absorption lay in more длинноволновой of area, than at chained, tape, layered силикатов (fig. 5).

The intensity of absorption is connected to concentration of substance, therefore Ик-spectra can be used also for quantitative definition of the contents of the given substance in a mix.

Fig. 5. Ик-spectra island (1), chained (2), tape (3) and layered

(4) силикатов

Радиоспектроскопия. Through the longest waves (radiowaves), which are characterized by wave number (10... 15) -3 см-1, it is possible to register spin transitions of nucleuses and электронов.

Термография

Термография is one of most widely used methods of the физико-chemical analysis. The majority of chemical and physical processes proceeding in the given substance or system, is accompanied by absorption or allocation of heat, besides during heating the linear sizes, weight, electrical conductivity and some other parameters change. The thermal analysis is widely applied at construction of the diagrams of a condition of substance. The essence of the thermal analysis consists in definition of temperatures, at which the physical condition or chemical structure of substance (system) changes. At the thermal analysis are registered change of energy (differential thermal analysis), weight (термогравиметрический the analysis), linear sizes (дилатометрия), electrical conductivity. Most often are used the differential thermal analysis (ДТА) and термогравиметрический the analysis (ТГ). Recently wide circulation has received дериватография, including simultaneous removal of curves ДТА, ТГ and differential loss of weight (ДТГ). On эндо- and экзотермическим the effects on curves ДТА can judge what processes proceed in system. The absorption of heat at heating (эндотермические processes) is connected to processes of decomposition of substance with allocation of a gas phase, decomposition of substance without allocation of a gas phase, энантиотропного of polymorphic transformation, плавления of substance. The allocation of heat (экзотермические processes) is caused by reactions accompanying with absorption of a gas phase (oxidation), polymorphic transformations монотропного of character, transition of unstable phases in steady (transition коллоидов and glasses in a crystal condition), кристаллизацией расплава; by reactions of education of substances in a firm phase.

Electronic микроскопия

Modern electronic микроскопы give useful increase in 300 000 times and have resolution (2... 5) х10-! 0 м, that allows the researchers to observe objects, not distinct in usual light микроскопе. The method electronic микроскопии has allowed to establish, that many substances considered earlier аморфными, consist of smallest crystals. The methods of research through electronic микроскопа are divided on direct and indirect.

The basic rules of construction ионно-ковалентных of structures.

Structure of crystals with properties of atoms, making them, for the first time has connected In. M. Гольдшмидт, which gave the special meaning(importance) to the size ионных of radiuses.

Л. To. Полинг has formulated some rules of construction of structures ионных of crystals.

First of these rules is specification of a rule Гольдшмидта about communication(connection) of coordination of ions from them ионными in radiuses and says, that everyone катион is surrounded анионами, taking place in tops coordination полиэдра (многогранника) - fig. 6.

Fig. 6. Coordination многогранники:

A - гантель (КЧ - 2); - triangle (КЧ -3); in - тетраэдр (КЧ - 4);

г - октаэдр (КЧ-6); д - cube (КЧ-8); е - кубооктаэдр (КЧ -12)

Distance centre to centre катиона and next аниона is defined(determined) by the sum ионных of radiuses, and coordination number (КЧ) - their attitude(relation) (гк: rа).

The second rule - rule of electrostatic valency - says, that in steady ионной the sum of forces of electrostatic communications(connections) connecting анион with environmental it(him) катионами, is equal to structure to a charge аниона; thus as force of communication(connection) 5 the attitude(relation) of a charge катиона to number environmental it(him) анионов is called. The force of communication(connection) катиона is various at change of his(its) coordination number.

The third rule speaks that the presence in structure of general(common) edges and is especial of sides two next полиэдров reduces stability of structure. This rule is especially essential for многозарядных катионов with the large charges and low coordination numbers, such as Si+4.

Fig. 7. Various ways of connection кремнекислородных тетраэдров:

а - tops (steady system); - edges; in - sides (unstable systems)

The fourth rule says, that if in structure there are some kinds катионов, катионы with small coordination numbers, but with higher charge aspire to such packing, that their coordination полиэдры would have a minimum quantity of general(common) tops.

The fifth rule is a rule of profitability: the chemically identical ions usually have identical coordination полиэдры and coordination numbers in structure.

Last two corrected have no such universality, as previous.

According to a principle of a minimum of potential energy each atom aspires to cooperate with the maximum large number of other atoms. It results in education of most dense packings.

Analyzing structures combined from various atoms, Н. In. Белов has shown, that the principle of most dense packing can be enclosed to them. Using systems ионных of radiuses, it is possible to consider(examine) geometrical opportunities of "settling" of emptiness in packing анионов катионами. These opportunities are defined(determined) by limits of parities(ratio) ионных of radiuses appropriate тетраэдрической or октаэдрической to coordination. However in ионных structures the sizes катионов very much often appear more, than it is necessary, from only geometrical parities(ratio), and they as though move apart анионы.

Fig. 8. Most dense spherical packings:

а - cubic; гексагональная

Questions:

2. Name rules of construction ионно-ковалентных of structures.

1. What methods of study of structure of crystal substances you know?





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