Monocrystal and Polycrystalline
Monocrystal is a crystal in which the ordered arrangements of the atoms or molecules extends throughout the piece of solid, irrespective of its size.
Polycrystal is a crystalline solid in which each piece of the solid has a number of monocrystals with developed faces joined together.
The polycrystal ceramic made from PbO, ZnO and TiO are used in gas lighters and telephone receivers.
Liquid Crystals
Some organic crystalline solid. when heated acquire fluidity but retain their anisotropic properties. They axe called liquid crystals.
Some liquid crystals like cyan biphenyl can change the plane of polarization of light and such Liquid Crystal Displays( LCD) are used in watches and micro calculators.
Crystal Lattice
A crystal is made up of a three- dimensional array of points such that each point is surrounded by the height bouring Points in an identical way. Such an array of points is known as bravais lattice or space lattice.
Unit cell is the smallest unit of the crystal lattice, repetition of which in three dimensions gives rise to crystal lattice.
The length of three sides of a unit cell are called Primitives or lattice constant represented by a, b, c. The angle between three crystallographic axis are called interfacial angles represented by
α, β and γ. The primitives and interfacial angles constitute the lattice parameters of a unit cell.
[ The cubic crystal may be of the form, simple cubic( sc) lattice, the body centred cubic( bee) lattice, the face centred Cubic( fcc) lattice.]