Main article: Crystallographic defect.
Crystal defect - any interrupt of ideal periodicity of the crystal structure
Ideal vs real crystal
The theoretical model of the crystal structure implies the perfect periodicity in the whole body of the crystal and the presence of particles only of its substance.
However, this is not the case in the real crystal as there are always a variety of interrupts of periodicity and symmetry, the impurity appearing, transfecting matter to the space between the lattice nodes, etc.
Any attempt to create an ideal crystal is doomed to failure. One of the most important roles is played by the second law of thermodynamics. According to it a closed system free randomly moving particles <=> crystal structure entropy (disorder measure) can`t decrease itself. As well as the formation of structure is a system ordering process, the structure can only be formed with defects. If you try to circumvent the law by finding a way to manually move the particles to the desired location, then because even small differences in the temperature of different parts particles will randomly change its position with time, leading to defect forming.
That is why such a mathematical abstraction as ideal crystal can`t exist in reality.
Kinds of defects
You can find below a classification of defects according to their dimensions.
Transformation and interaction of defects
Since the lattice nodes are never been in a static position, they always do thermal oscillations, so they often move between the lattice nodes. This means that defects are also moved after them.
Due to this effect defects can move across the all crystal structure and also interact complicatedly.
For example, a vacancy may move closer to the crystal surface, resulting to their "disappearing", or may merge to several neighboring areas and create a tilt boundary, or may simultaneously carry out both variants forming the crack.
The interaction of certain types of defects may lead to their neutralization, such as interstitial particle and a vacancy merging will give an ideal lattice, as well as the merging screw dislocations set and twist boundary with reversed rotation direction.
Influence of defects
The presence of any defects leads to an increase of the full lattice energy comparing with the ideal one. Such changes can have very unusual consequences at the macro level.
Dislocations and disclinations increase the brittleness of the material when exposed to a strong short-term, but at the same time give it a hardness and ability to withstand prolonged stress. The presence of macro impurities has opposite effect.
Interstitial impurities and vacancies strongly affect the electric and magnetic permeability of the material, the polarization properties, cause the appearance of color, segneto-, piezo- and pyroelectric phenomena. For example, putting some impurity substances into a silicon crystal makes it a semiconductor instead of dielectric, adding impurity into a transparent corundum crystals giving colored rubies and sapphires.
Eliminating of defects
For giving a close to ideal crystalline structure a lot of methods to get rid of the defects are used.
Most of them are based on increasing the speed of movement defects while heating. One of these methods, a zone melting method uses a melted dot on crystal surface that is slowly moving in the whole volume of the crystal. It helps to get rid of a lot of different types of defects and give you even more purified material than before.
Another method called annealing is used for metals. It is a heating of the material to a point close to its melting point with following slow cooling step (less than a degree for several hours). This allows you to eliminate vacancies, interstitials and dislocations from the end product, increasing its plasticity.
Many fields of industry require the presence of certain types of defects.
For example, in metallurgy mechanical processing of products by forging or rolling increases the number of dislocations and disclinations. This gives the material strength in directions are required, but also lowers their plasticity.
To add the impurity atoms and ions a lot of different methods are used, like a direct introduction of impurities to the melt, coating, exposure to ionizing radiation.
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