Basic concepts of Solid-state Chemistry
Study, synthesis, structure and properties of solid phase material are called as Solid State Chemistry. This can also be defined as the branch of chemistry which deals with the preparation, physical, chemical and structural properties of elements and compounds in the solid state. Sometimes it is also referred as material chemistry. Basically it is the study of synthesis, structure and properties of solid phase materials, particularly of non molecular solids particularly, non-molecular solids.
Modern day technology has made wide progress in solid state inorganic chemistry mainly because of demands of industry with a good academic research. .A solid can be defined as a substance which has a definite shape and size. The main reasons for a solid for its existence are intermolecular forces and low thermal energy. In a solid state the constituent particles are mainly closely packed with very small intermolecular distances. The constituent particles occupy a fixed position with a slight oscillation. Apart from these they have few more properties like rigidity, high density, a definite shape and more over they are incompressible.
A general classification is done on the basis of arrangement of constituent particles.
1-Crystalline Compounds-A solid is called in a crystalline form when the constituent ions, atoms or molecules are arranged in a definite geometric pattern. Examples- Stones, Gems, Wood, Charcoal.
2-Amorphous- In compounds like silica, glass and pitch the constituent particles have no fixed arrangement or a definite pattern.
3-Isotropy-In the amorphous solids there is no regular arrangement of particles thus the properties like thermal conductivity, electrical conductivity are identical in all directions. This property is called isotropy; amorphous solids are isotropic in nature.
4-Anisotropy-Anisotropy is a property where due to regular arrangement of constituent particles, the different particles fall in different ways of a crystalline solid. The values of standard properties like electrical conductivity and thermal expansion not remain same in all the directions.
The classification can also be done in another way which is based on the binding forces:
1-Molecular-Dry ice is a example of molecular binding .These are soft, with high melting point and are poor conductors.
2-Ionic- Ionic compounds are very hard and brittle with very high melting points, high heat of fusion .Example-NaCl, ZnS..These compounds are poor conductors.
3-Covalent-Example-Quartz,diamond ,silicon Compounds like these are very hard .They have high melting points. They exhibit poor thermal and electrical conductance.
4-Metallic solids-All the metals fall in this category, their state can vary from soft to very hard. They exhibit low to very high melting points. Metals are excellent thermal and electrical conductors.
Importance of solid state chemistry
The main reason for the vast development of this branch of chemistry is to solve many problems in the application of materials from microelectronics, information, communication, energy conservation and storage, electronic, packaging, domestic and advanced ceramics to mineral technology. The discovery of zeolites, borophosphates or borophosphosilicates as packaging materials.
After the tremendous progress in the field of solid state chemistry ,inorganic chemistry has become a very important field of chemistry because most high technological material have an inorganic origin.Some minerals that are found in nature possess conducting (sulfides),magnetic(ferrites)or ion exchange (zeolites) properties.
Many technological applications like integrated circuits, solar energy panels, convertors, fertilizers, molecular sieves are the big discoveries of solid state chemistry. These discoveries are directly related to basic properties like electrical and ionic conduction, solar energy conservation, photoelectric effects, optical, magnetism and mechanical processes. Many transition metals are being used in super conductors like, tin, yatrium, barium, and niobium. The other nonmetals which are widely used as semiconductors are silicon, galium, arsenic. We all know metallic conductors Cu,Ag,Fe.Many metals are used as alloys for example titanium in aeronautical industry, and we all know the wide use of stainless steel. Cement is a combination of Ca and SiO. .Alkali earth metal lithium is used in lithium -ion batteries. Chromium and iron are used as Ferro magnets.
The property of electrical conductivity is shown by many solids, these solids can be classified into three types, and this classification is basically depends on the conductivity they show:
Metallic conductors belong to the former category and electrolytes to the later. With the movement of electrons or ions electricity is conducted in a conductor. Metallic conductors use their valence electrons for the conduction .Electrolytes in the solution form conduct electricity with the movement of ions.
Conductance of electricity is done in metals in both the states ,solid as well as molten state, the free flow of electrons always depends on the applied electric field ,as well as the available valence electrons per atom. Molecular orbital's are formed when atomic orbital's of more or less same energy combine, these molecular orbital's when they get combined results in the formation of a band , the band formed in this way, combine if they are partially filled or overlaps with a higher energy unoccupied conduction band ,conductivity is done.
Few compounds ,called insulators ,they have a large gap of energy between the lowest occupied orbital and the available unoccupied higher energy orbital, because of this huge gap electrons cannot jump to the higher one resulting in less conductivity.
Some substances like silicon and germanium are called semiconductors ,these atoms have a very small gap of energy between the valence and the conduction band, because of this small gap the energy released is very small, and of no use. To overcome this defect Doping is done, a definite amount of impurity is added to the original compound.
This doping is done in two ways-1-Electron rich impurities 2-Electron deficit impurities Since there is a increase in conductivity with a electron rich impurity .These type of elements are called n-type conductors. The increase in conductivity is done with the extra fifth electron which is made available when elements of group 14 like germanium or silicon are doped with group 15 elements ,the four valence electrons form four covalent bonds
The second type of conductors are p-type conductors ,they are created by doping with Group 13 elements like boron and aluminum or gallium which contain three electrons in the valence shell .These valence shell electrons can be used for doping with silicon or germanium ,there is electron vacancy or electron hole for the fourth electron. Common example is silicon for boron.
Both the types are used in making electronic components.Diode, transistors, are the major examples of these semiconductors. To convert light energy into electrical energy solar cells are used which is perfect example of a photodiode.
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