Introduction:

Definition of crystal:

The crystal is any solid material in which the component atoms are ordered in a definite prototype and whose surface regularity reflects its internal symmetry.

Basically, a crystalline solid is a substance that is comprised of a fundamental unit which is repeated in (3-D) three dimensions in an ordered fashion. The basic 3-D repeating pattern is termed as the unit cell and can be comprised of atoms, ions or molecules. The collection of particles that make up the unit cell should meet three necessities:

1) They must point out the coordination number.

2) They must be reliable by the empirical formula.

3) They must produce the crystal structure whenever repeated in (3-D) three dimensions.

These particles can as well be taken as uniform, hard spheres which might form various structures based on how they are packed. One of the controlling features of the packing of spheres is how they are arranged in each layer. In certain layers, all of the spheres are touching and each and every sphere is surrounded via six others. This is the most proficient manner to pack the spheres and is termed as closest packing (observe figure A). Layers can as well be fashioned from spheres which don't make such efficient utilization of the available space; in these layers one sphere will be in contact by only four other spheres (observe figure B). The least efficient packing of sphere layers takes place when no contact at all exists between the spheres (observe figure C). The arrangement of the spheres in (2-D) two dimensional sheets symbolizes only one feature of the overall crystal structure; how the (2-D) two dimensional sheets are stacked should as well be considered.

Two different kinds of closest packed structures can be produced based on how close-packed layers of spheres are stacked. The hexagonal close-packed structure is built up by placing one layer of spheres in the depressions made by a first layer and then adding a third directly above the first. A hexagonal prism in produced via the repeating pattern stated by the hexagonal unit cell (observe figure D).

Fig: Packing of Spheres

In the close-packed structures holes are made up as the outcome of the spheres being positioned on top of each other; they are termed to as voids or interstices. There are two kinds of voids based on the number of spheres surrounding the void:

1) Tetrahedral voids are made up whenever one sphere in one layer is fit over three spheres in the other producing a void that is surrounded via four spheres.

2) Octahedral voids are made up whenever three spheres in one layer are fit over three spheres in another producing a void which is surrounded via six spheres.

Fig: body-centered cubic cell

The placement of layers of spheres that contain four nearest neighbors (observe figure B) directly on top of one other produces the simplest of three structures and is comprised of a simple cubic unit cell (observe the figure below).

Fig: simple cubic cell

Types of Crystals:

However there is no unique manner to categorize all the crystalline solids found in nature, even then these are categorized into four important kinds on the basis of chemical binding of the constituent atoms.

1) Ionic Crystals:

These are made up by a combination of highly electropositive ions (that is, cations) and highly electronegative ions (that is, anions). Therefore strong electrostatic force of attraction acts in the ionic crystals. Thus, a huge amount of energy is needed to separate the ions from one another. The kind of crystal lattice based on:

• The size of the ion.
• The necessity for the preservation of electrical neutrality.

Thus alternate cations and anions in equal amount are ordered in the ionic crystal example: NaCl, KF, BaSO₄ and so on.

=> General features of Ionic crystals:

The Ionic crystals are hard, encompass high melting points and are brittle. Whenever they melt, the resultant liquids conduct electricity fine. Such properties reflect the strong attractive forces between the ions of opposite charge and also the repulsions that take place whenever ions of like charge are placed close to each other. They are brittle and tend to smash into smaller crystals whenever stressed. If a crystal is hammered or stressed, ions having like charges are forced to close proximity. The crystal is then literally self destructs due to the electrostatic repulsion.

2) Covalent Crystals:

These are made up by sharing of valence electrons between the two atoms resultant in the formation of a covalent bond. The covalent bonds make bigger in two or three dimensions making a giant interlocking structure termed as network. Diamond, quartz, silicon and graphite are good illustrations of this kind.

=> Features of covalent crystals:

• The quantum mechanics is required to compute the binding energy.
• Covalent bonds are highly directional (that is, low APF and low density).
• Quite a few crystals are covalently bound (like diamond, Si, Ge and SiC).
• Covalent bonds are strong (that is, hardest materials, high melting points and insoluble).
• Each and every covalent crystal has tetrahedral (or diamond) structure.
• There is a continuous range of crystals among the ionic and covalent limits.

3) Molecular Crystals:

In these crystals, the molecules occupy the lattice points of unit cells, apart from in the solidified noble gases in which the units are atoms, where the binding is due to the van der Waals' forces and dipole-dipole forces. As van der Waals' forces are non-directional, thus structure of crystal is found out by the geometric consideration only. Solids such as H₂, O₂, CO₂, I₂, S₈, sugar and so on are famous illustrations of such crystals in which van der Waals forces are acting. Ice is the common illustration in which the dipole-dipole forces of attraction (that is, hydrogen bonding) are active. Most of the organic and inorganic crystals comprise hydrogen bonds. However, these are comparatively weaker however they play a very significant role in finding out the structure of substances example: polynucleoides, proteins and so on.

=> General features of Molecular crystals:

The molecular crystals are basically solids in which the lattice sites are occupied via the atoms - as in solid argon or krypton - or by molecules - as in solid CO₂, SO₂, or H₂O. These solids tend to be soft and encompass low melting points as the particles in the solid experience relatively weak intermolecular attractions. The crystals are soft as little effort is required to separate the particles or to move them past one other. The solid melts at low temperatures as the particles need little kinetic energy to break away from the solid. If the crystals include only individual atoms, as in solid argon or krypton, or if they are comprised of non-polar molecules, as in naphthalene, the only attractions between the molecules are the London forces. In crystals, having polar molecules like sulphur dioxide, the main forces which hold the particles altogether are dipole-dipole attractions. In crystals like water the main forces of attractions are due to the hydrogen bonding.

4) Metallic Crystals:

These are made up by a combination of atoms of electropositive elements. These atoms are bound through metallic bonds. It might be stated as:

The force which binds a metal ion to a number of electrons in its sphere of influence is termed as metallic bond.

OR

A bond that is made up between the electropositive elements.

OR

The attractive force that holds the atoms of two or more metals altogether in a metal crystal or in the alloy. We are familiar that the force of attraction between the metal ions and valency electrons is extremely strong. This force of attraction is accountable for a compact solid structure of metal.

=> General features of metallic crystals:

Metallic crystals encompass properties which are quite dissimilar from those of the other three kinds of crystals above. They conduct heat and electricity fine, and they encompass the lustre that we characteristically relate with metals. A number of different models have been developed to illustrate metals. The simplest one views the crystal as encompassing positive ions at the lattice positions that are surrounded via electrons in a cloud which spreads all through the whole solid.

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