All matter is composed of atoms and their clusters called molecules. The arrangement of atoms and molecules characterize a substance and assign its unique identity. Superficially it is difficult to appreciate such arrangements but if you were to have a peek down to the atomic and molecular levels you will be amazed to see that many of the commonly occurring materials display a highly symmetric and orderly arrangement of atoms and molecules. X-ray diffraction or XRD is tool that helps you gain such an insight into structure of matter at the atomic level. An understanding of the science of crystallography will be necessary to fully exploit the potential of the XRD technique.
What is Crystallography?
Crystallography is a science dealing with study of crystals. It is an established fact that pure compounds have a definite arrangement of atoms which are responsible for their characteristic properties irrespective of the source of the material. Examples are common salt which has a cubic arrangement of sodium and chlorine atoms, diamond a pure form of carbon has tetrahedral atomic arrangement of carbon atoms whereas graphite has hexagonal ring structures.
It is obvious that crystals can exist in solid state only. Gases or liquids do not exhibit crystallinity. However, solids can exist both in crystalline or non-crystalline states. The present article discusses some of the crystalline states and characteristic features of materials that are common in nature.
Crystals comprise of well organized array of atoms, molecules or ions. A unit cell or a crystal lattice is the smallest structure of the compound or element which repeats itself by translation throughout the crystal. It can be said that a lattice is theoretically an infinite array of atoms, molecules or ions which repeats itself to constitute a crystal. A lattice made up of same atoms as in case of elements is called monoatomic and if it has more than one type of atoms it is called a polyatomic lattice. There are different types of lattices which are classified according to the geometrical arrangements of atoms which will be taken in a subsequent article.
Types of crystals and their properties
Single crystals are large enough in size and their outer boundaries display distinct geometrical shapes. Some of these are found naturally but for others the shapes can be cut artificially to make them more attractive. Typical examples are gemstones which can be cut to different shapes to improve their shapes and light reflection properties.
The atoms constituting ionic crystals have different electro- negatives and are held together in their positions by electrostatic forces. Such crystals are generally hard and have high melting points
Covalent crystals are characterized by sharing of electrons between atoms of the crystal. Strong bonding makes the crystals extremely hard and give them high melting points. A typical example of such crystals is diamond whose hardness is difficult to match.
Metallic crystals are constituted of metal atoms positioned on the lattice sites. The outer shell electrons of metal atoms are free to move around throughout the lattice. This property makes such crystals good conductors of heat and electricity and also show high melting points.
Molecular crystals comprise of individual molecules at the lattice points. Such molecules are held together by weak non-covalent forces such as van der waals forces or hydrogen bonding. Such interactions render molecular crystals with softness and low melting points. Examples of molecular crystals are proteins which exhibit crystalline features.
Polycrystalline materials are clusters of several crystallites or grains having different sizes and orientations. The orientations can be random in nature and influenced by their growth and processing during formation.
Allotropes or Polymorphs
Allotropes or polymorphs are different forms of an element which are made up of the atoms bound together in different geometrical arrangements. A common example is diamond which can be present in a tetragonal arrangement in diamond but in hexagonal ring arrangement in graphite. Such polymorphs show different physical properties.
Amorphous or non-crystalline solids do not exhibit long-range order characteristic of a particular crystalline material. Such materials often exhibit only a short-range order. Examples are glass, polymer films and gels
Anisotropy refers to the difference in properties when measured along different axial directions in a crystal. Such physical properties include absorbance of light, reflective index, conductivity, tensile strength, etc. Anisotropy generally results from distortion and elongation of grains in some direction during the formation of crystals.