Main article: Crystal.
Crystal - physical term means solids which consist of particles located in the space in accordance with a straight periodic structure.
Even in ancient times people have noticed some minerals because of their unusual forms of polyhedra which were strikingly different from the others forms of the surrounding natural materials. Some of these minerals such as quartz was called κρισταλλος - ice, due to their intrinsic transparency. Over time, the term came to refer to a broader class of minerals and now it means all natural bodies with their natural form of polyhedra.
Types of compounds
To understand why crystals are so distinguished from the variety of others substances and materials, let`s look at the structure of any matter at the molecular or atomic level. Typically, such research is carried out by using X-ray diffraction method.
You can see that some substances, such as honey, glass, various types of rubber in any approximations in the mutual arrangement of particles have no structure, all of them are arranged randomly.
While heating such substances, their particles begin moving from their previous random places to the further ones. The temperature oscillation`s amplitude increases, the process of softening of matter begins. The distance between the particles is gradually increased, but it is difficult to specify precisely whether the substance passed into the liquid state or whether it is still a solid. This boundary is too blurred. Due to this behavior these substances are called amorphous - without a specific shape, structureless.
At the same time it is possible to find other substances such as ice, metals, salts which heating leads to an increase of the temperature only up to a certain point and then starts solid state transition to a liquid retaining a temperature as constant. When the temperature increasing begins again while its further heating, it can be noted that the entire body has passed in the liquid state in the melt and nowhere fragments remained as solid.
Some of you remembering school science course can ask, if the body transmit energy for heating, and its temperature was constant at the same time, then where does that energy go away?
To answer this question we should look into the structure of matter again and see that all particles in such compounds are arranged in order along the endless rows stretched throughout the all volume of the substance like a periodic lattice. The nodes of the lattices are particles of matter that makes very small temperature oscillations, keeping in their places due to the electromagnetic forces of attraction and repulsion.
If the substance is gradually heated, it can be observed that a particle at a certain oscillation rate can no longer be held by these forces and begin to leave their places. Some particles speed can be much higher than the average one, but after a short time they stumble with slow particles, giving them a part of its speed and helping to leave the lattice. That is why the temperature of such bodies while melting remains constant - the transmitted energy goes to the gradual destruction of material lattice. After the completion of the process the average speed of the particles will start to rise again which means the transition into the liquid state. Historically, these lattices and substances called crystals.
It is worth noting that since the particles in the lattice are in a quantum-mechanical trap caused by the interaction with neighboring particles, the crystal lattice becomes a state with a lower energy than freely moving particles have. This explains the effects that is opposite to melting - crystallization of the substance from the melt or the solution. The transition from the free particle state to the periodic structure of the crystal produces heat, while temperature remains the constant.
Kinds of crystals
The crystals also have their own classification. In fact, there are a set of classifications - by the lattice`s cell form, by types of cell symmetries, by types of lattice particles and their bonds. But some the spatial classification is more important to further study.
Let`s look at the structure of matter at the level of groups of molecules or atoms again.
Some crystals at nanometer or millimeter scales still have three-dimensional particles lattice. Even at such scales crystal shows the same structure properties. Such crystals have a polyhedron shape, do not consist of individual parts and are called monocrystalline - single crystal of the material in one piece.
At the same time we can see that some crystals are composed of small regular lattices that on a large scale are scattered throughout the volume randomly, as if the crystal consists of mixed pieces of single lattices. Such crystal shows the polyhedral form only at small its parts but the whole crystal is staying shapeless. You can break it to the single parts by a small force. Such crystals are called polycrystalline - a lot of small, fused crystals.
Mono and polycrystallines shows lots of same properties, defined by their elementary lattices and chemical composition - melting point, density, ability to enter into chemical reactions and to conduct electricity, etc.
At the same time, due to the significant structural differences some of their properties are specific to each kinds.
The special properties of crystals are primarily concerned with anisotropy - the ability or crystal to demonstrate a variety of qualitative and quantitative properties depending on the direction of thir measurement.
For example the measurement of the thermal conductivity of the calcium sulfate crystal shows one value recorded in the longitudinal direction and quite different recorded in cross one. Another example is the effect of birefringence. If you paint, for example, a line on the paper and put on top a single crystal of calcium carbonate or some other substance, the image passing through the crystal to the observer will split to two ones. One image is real and pass to the eye of the observer as slightly refracted from the straight direction, when the other one passes through the another direction with other angle and is visible for observer too because of a different refractive index value in that direction.
Also due to this property single crystals have their certain shape - in different directions of the crystal growth passes with different speed. It is possible to carry out the experiment of growing crystal on the spherical seed, which will result in a significant increase of all the edges in one direction and almost the absence of them in other. It leads to growth of crystal with a certain form.
This property is opposite for the anisotropy and concerns to polycrystals.
Isotropy is ability of polycrystallines to show the same properties value in all directions of measurement.
As example the electrical resistance of metals measured by passing of current or mechanical impact strength are always same in different directions.
It is because of the fine-grained metal structure that disperses the applied voltage or the impact ot the whole crystal.
It looks like paradox that so small difference in the structure changes the properties to the opposite ones. But due to it we have electronics, architecture, and many other areas of life and science.
The common property of crystalline materials called a polymorphism is the ability of the same substance to form different lattices at different or identical conditions.
This property leads to the existence of allotropic modifications of chemical elements then the same atoms and molecules at different temperatures or pressures are built in different crystalline lattice that can fundamentally differ both in physical and chemical properties.
This feature allows certain properties to appear completely different in crystals had the same chemical composition but different structure. So, orthorhombic sulfur forms a crystal like bipyramids while monoclinic shows the form of rods or prisms, the diamond can firm and does not conduct electricity while the graphite easily crumbles and uses as conductor, the white phosphorus can be oxidized by the air at room temperature while the red is chemically stable.
Isomorphism (perfect isomorphism) - the ability of certain substances with similar crystal lattices to form mixed crystals in any concentration range.
This property allows you to grow crystal-in-crystal of alum as the crystals of one alum in a saturated solution of another alum will not dissolve and will behave as if it is a solution of their own substance.
Which property is associated not only with the same arrangement of the particles in the lattice, but with close radii of ions of these compounds, making them almost identical lattice.
Sometimes you can found term imperfect isomorphism. It means that the crystal lattice is similar, but the ionic radii differ considerably. In this case, the overall yield crystals substance only in a certain narrow temperature range.
Isodimorphism (forced isomorphism) - the ability of some substances to form mixed crystals with the various lattices, depending on their ratio. Inherent to substances that have an isomorphism at some temperature ranges and making the transition to another lattice type at another ranges, but can keep it if they are in the form of mixture.
Due to this property copper(II) and iron(II) sulfate at various concentrations can form crystals with different forms and stability, while having entirely different lattices.
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