Crystals are solids whose faces are regular polygons. It has a typical property that the angles between
the edges of the faces and between the faces themselves are constant. In the case of some crystals like Iceland spar or quartz, a certain direction called the crystal or optic axis is found.
Monocrystal is a large single crystal of regular shape and having anisotropy. Its mechanical strength vary in different direction. The elasticity of monocrystal also vary in different directions. Monocrystals also display an anisotropy of thermal properties by having different coefficients of linear expansion and thermal conductivity in different directions.
By taking special measures, monocrystals can be obtained from a melt of a metal. For example, if iron is melted and then cooled, the body obtained does not possess anisotropy. When the iron surface is finely ground, acid etched and observed under microscope, the substance consists of separate grains of microscopic dimension. Each such grain is a crystal of irregular shape because its growth being interfered by adjacent crystals. The consequent granular
structure is
polycrystalline. Because the crystal axes of all the grains are randomly oriented, anisotropy is not exhibited over appreciable distances. So the polycrystal is isotropic. Examples are rock, sand, metals, salts etc. Solids like glass not having a granular structure are
amorphous solids.
A crystal is made up of unit cells of regular geometric shape , that form a regular
lattice. Here we could distinguish a certain cell of minimum size that can be transposed parallel to itself, each time a distance equal to the length of its edge , to form a large monocrystal. This being the unit cell, the length of its edge is equal to the lattice constant of the crystal. Each vertex of the cell is a lattice point, a st. line passing through the points is a lattice line and a plane called the lattice plane passes through the points. Fedorov established that in nature there can be only 230 possible different kinds of spatial symmetry or
Fedorov groups, which can be divided into
7 crystal systems or 32 classes. This classification was affirmed via X-ray structural analysis.
There is no single crystal that does not belong to one of the Fedorov groups.
Though usually the centers of atoms, ions or molecules are located at the crystal lattice points, not all atoms are at lattice points. The lattice spacing is very small, typically about 3-7 angstrom units.
There is a long range order in the packing of atoms or other particles of which the unit cell of the crystal consist. However a crystal does not have ideal structure , so long-range order would not be observed over unlimited distances. This results in a defect in the regular structure in a crystal and is called a
dislocation. In an edge dislocation, an extra lattice plane forms in some part of the crystal. So the crystal is divided into two blocks and on their interface the dislocation core or center lies. A screw dislocation forms when a lattice line of one of the blocks seem to slip one lattice constant above or below its normal direction. The maximum distortion being along the axis of dislocation.
Defects in the packing and block structure of crystals may also occur when a lattice point is occupied by a foreign atom or is vacant or a foreign atom occupy the spaces or interstices between the lattice points and the lines.
In a crystal, at a sufficiently high temperature, a process is quite probable in which the foreign atom and an atom of the base substance of the crystal change places, leading to movement of the foreign atom through the crystal. The more is the number of defects in the lattice, more probable is this
diffusion. This diffusion is also accelerated with
temperature increase of the crystal.
X –ray structural analysis show that the crystals of
ice have an exceptionally loose structure . The structure of ice is not a closely packed one, the lattice having large “holes”. When ice is melted and the crystals break down into separate molecules, the substance becomes more dense.
Lattices can be generally represented as the
packing of spheres, but there are exceptions .A body centered cubic lattice is found in all alkali metals and in the atom of iron at high temperature. The atoms of carbon can form two types of space lattices as in the case of diamond and graphite. In
diamond 4 atoms are located at the vertices of a regular tetrahedron with a 5th atom located at its center. Covalent forces are directed from the central atom to the vertices of the tetrahedron. In
graphite, each atom is bound to 4 neighbors but the forces and the directions of the bonds differ. An atom here has a strong bond with 3 other atoms that lie together with it in the same layer at 120 degree angle, and a weaker bond with a 4th atom lying in the adjacent layer. So the physical properties of diamond and graphite also differ much.
Researches have shown that at pressures of the order of 60000 atmospheres and a temperature above 1500 degree Celsius, the
atoms in the lattice of graphite are brought closer and rearranged so that this crystal lattice may be transformed into the lattice of diamond.