Crystals syllabus

Crystals Syllabus Crystalline solids are based upon a simple pattern of arrangements of atoms that is repeated many times throughout the molecules The atoms or the groups of atoms are arranged in a regular order The order is so regular that knowing the arrangement at one point the arrangement at other site can be predicted This is called long range order Crystals are anisotropic in nature i.e they have different physical properties in different directions They are incompressible have a definite melting point so they pass rather sharply from solid to liquid state The Crystalline solids can be divided into two categories single crystalline(the regular order extended over the whole crystal) and polycrystalline (the regular order extend over a small range) Crystal lattice or Space lattice • A lattice is a regular array of points showing how molecules, atoms and ions are arranged in space Unit Cell • The smallest three dimensional portion of a complete space lattice which when repeated over and again in different directions produces the complete space lattice Crystal structure= lattice + basis • The lattice is an orderly arrangement of point in space while the basis consists of simplest arrangement of atoms which is repeated at every points to build up the crystal structure Types of unit cells • If we take into consideration, the symmetry of the axial distances (a,b,c) and also the axial angles between the edges (α,β,γ) the various crystals can be divided into seven systems These are called crystal habits NO Crystals Axial distance Axial angles Examples Cubic a=b=c α=β=γ=90◦ NaCl, KCl, Diamond Tetragonal a=b not equal to c α=β=γ=90◦ White tin, SnO2 Orthorhombic All distances are unequal α=β=γ=90◦ Rhombic sulphur, KNO3, BaSO4 Rhombohedral a=b=c α=β=γ not equal to 90◦ Calcite, quartz, NaNO3 Hexagonal a=b=c α=β=90◦ γ=120◦ ZnO, CdS Monoclinic All distances are unequal α=β=90◦ but γ is different Monoclinic sulphur, Na2SO4.10H2O Triclinic All distances are unequal All angles are different and not equal to 90◦ Blue Vitriol, K2Cr2O7 • Unit cells can also be further classified as Primitive or Simple unit cell : The unit cells at which lattice points are present only at the corners of the unit cell • Non primitive unit cell : The unit cell in which the lattice points are not only present at the corners of the unit cell but also at some other places are called non primitive unit cells It is divided into following categories • A) Face centred • B) End face centred • C) Body centred • Body-centered cubic (I) Body centred Face centred Crystal structure of Sodium Chloride Sodium Chloride Crystal structure • In NaCl each ion is surrounded by six ions of opposite charge • The surrounding atoms are located at the vertices of a regular octahedron • This is also called rocksalt crystal structure • It can be represented as a FCC lattice with two atoms basis or as two interpenetrating FCC lattices Crystal Structure of Zinc Sulphide (ZnS) • ZnS exists in two main crystalline forms, and this dualism is often a salient example of polymorphism • In both polymorphism, the coordination geometry at Zn and S are tetrahedral • The more stable cubic form is known also as zinc blende or sphalerite The hexagonal form is known as the mineral wurtzite • If the sulfide ions originally adopt a hexagonal closest-packed structure, the ZnS crystal is wurtzite If the sulfide ions originally adopt a cubic closest-packed structure, the ZnS crystal is zinc blende Zinc Blende or Sphalerite Wurzite NaCl crystal structure  ... also the axial angles between the edges (α,β,γ) the various crystals can be divided into seven systems These are called crystal habits NO Crystals Axial distance Axial angles Examples Cubic a=b=c