Itself in the direction perpendicular to the plane of the face, as in laying

brick wall. On the validity of this assumption, it would seem,

facts show the existence of planar faces of crystals. Figure 6

shows a model of the unfinished surface of the crystal. It is clear that the deposition of new

atom is most likely at the point and the surface, where it will be kept

three neighbors, while elsewhere in the surface of a face he will

held fewer neighbors. When will the construction of the fourth

series, will begin construction of the fifth and so on, until the end of the entire plane.

After that, the crystal growth is difficult, since the formation of the new

layer - the event is less likely. Wherever the "ass" atom on a completed

plane, everywhere he will be associated with a small number of atoms in the crystal.

The likelihood that the weak link is broken by thermal motion,

large, so that the atom can not be fixed on the chip and enters

solution or melt. With this mechanism of development of atomic planes

the growth rate should be very small. In the experiment with the same

crystal growth from the vapor supersaturation of only 1% was found

the growth rate to 101,000 times the calculated theoretically! In

Apparently, a large discrepancy between theory and experiment in physics were observed.

The explanation for this discrepancy between theory and practice was found only

recently, in 1949, the ease with which begins construction

new atomic plane, it was possible to explain the fact that real

crystals have many structural defects.

Describing the structure of crystals, we have used them ideal

models. Unlike real crystals from the ideal is that

real crystals do not have a regular crystal lattice, and have

a number of violations in the atom, called the defects. Knowledge

conditions for the formation of defects and their solutions plays an important role

using crystals in practice.

Schemes of defects in the crystals are shown in Figure 7.

The most common defects in the ideal crystal lattice occur

by substitution of its own alien atom (Fig. 7a), the introduction of

interstitial atoms (Fig. 7, b) the lack of an atom in one of the nodes

crystal lattice (ris.7.v).

Special role in the process of crystal growth play its imperfections

structure, called dislocations (offsets). The simplest forms

dislocations are edge and screw. Edge dislocation is formed

where you left off the "extra" nuclear half-plane (fig. 8).

In the case of helical

(Dislocations atomic planes form a network that resembles a spiral

ladder. The number of dislocations in a crystal can be very large,

reaching 108 - 109 cm-3. Crystals without dislocations exist.

The constant presence of an open stair screw dislocation creates

favorable conditions for the growth of a crystal, it is no need to start building

any new series or a new plane. The atoms that are attached to the steps,

increasing it, and due to this, it starts moving on the surface

face. But this movement is not moving parallel to the step

Me, because its end is fixed. It is not difficult to understand that if the atoms

stacked with a constant velocity along the length of the step, it is on

As the start to bend and take the form of a spiral. Constant increase

step new layers of atoms will lead to the fact that on the crystal face

formed spiral turret (Fig. 9). The central part of it as a

screwed into the space ahead of its movement the lower rungs

stairs that will eventually be built up completely and disappear

becoming a complete atomic layer.

Photos taken with an electron microscope,

confirmed the reality of the spiral mechanism of crystal growth. If present

many closely spaced dislocations, the crystal growth steps have

height of many atomic layers and can be seen even in a conventional microscope.

Crystal nucleation is facilitated by the presence of a solution or melt

tiny foreign bodies - dust and other contaminants. Obviously, in this

If no crystal nuclei are formed by combining at random

collisions of atoms or molecules, and the deposition of atoms on

solid foreign bodies, dust particles, are almost always present in

melt or gas. For example, the embryos are suspended in snowflakes

Airborne dust, often tiny quartz sand.

Irregular shape of the dust particles, which begins the birth of the crystal,

contributes to the emergence of dislocations and a sharp increase in the rate

crystal growth.

The emergence of new ways of layers and the growth rate of crystal faces

various substances vary. Some crystals grow in the form of plates,

others - in the form of needles. This is due to many reasons. One of them -

difference for molecules to form. The difference of the growth rates of faces

crystals of many substances due to the dependence on the direction of the value

forces of the particles forming the crystal. Sticking probability of molecules in

the direction of the big powers, of course, is greater than in

the direction of the smaller forces. This is the case in crystals

lamellar structure (mica, graphite), in which growth is

mainly along planes where strong ties.

In

directions perpendicular to these planes, the growth rate is much

below.

But not only the shape of the molecules, and a marked difference of their interaction forces

in different directions determine the shape of the growing crystal. if

crystals grow at high supersaturated vapor or solution, it is often

unusual form for the substance branched, tree-like form,

called dendrites. The reason is that the top of the crystals

in contact with a supersaturated vapor or sodium than their faces.

Outpacing the growth of the side faces, vertices are introduced into the

unused solution or vapor, which contributes to their further

rapid growth, etc, etc.

An example is the formation of dendritic snowflake patterns on the ice

glass. With slow growth of ice crystals take their normal form

hexagonal prisms. Dendrites are formed by rapid cooling of melts

salts and metals. In nature, often found in the form of dendrites

silver, copper, and gold.