The Solid State

Types of solids and their properties The solids have definite mass, volume and shape. They are rigid and incompressible. Intermolecular distances between constituents of solids are very small. The constituent particles occupy fixed positions and can only oscillate about their mean positions. They are classified in two categories: crystalline solids and amorphous solids. Crystalline solids have a well defined three-dimensional geometrical arrangement of constituent particles. Depending on the type of bonding in crystalline solids, they are classified as Ionic solids, molecular solids, covalent (or network) solids and metallic solids Amorphous solids do not have definite geometrical shape and are often referred as pseudo solids. Solids show electrical, magnetic and dielectric properties. On the basis of conductivity, solids are classified as: Conductors- (Conductivity ranges 104 to 107W–1m–1), Insulators- (Conductivity ranges 10–20 to 10–10W–1m–1) and Semiconductors–(Conductivity ranges 10–6 to 104 W–1m–1). The magnetic properties of substance are due to the electrons present in them. Each electron in an atom behaves like a tiny magnet. On the basis of magnetic properties, solids can be categorized as paramagnetic; diamagnetic; ferromagnetic; antiferromagnetic and ferromagnetic. Insulators do not conduct electricity because their electrons are tightly bonded to the nuclei and are unable to migrate under an applied electric field. However, under the influence of the electric field, the atoms or the ions might undergo polarisation to form dipoles. These dipoles may align in different ways and show different properties such as Piezoelectricity, Pyroelectricity, Ferroelectricity and Anti-ferroelectricity, depending on the alignment of the dipoles. Close packed structures and voids In solids the constituent particles are packed in such a way that they leave minimum empty space. The constituent particles are considered as identical hard spheres. In close packing in one dimension, the constituent particles are arranged in one row in such a way that each sphere is touching two neighboring spheres. Hence the coordination number for this type of packing is 2. The rows are stacked over each other in close packing in two dimensions. It is of two types: two-dimensional square close packing and two-dimensional hexagonal close packing. All real solids have three-dimensional structure. Close packing in three dimensions is of two types: three-dimensional close packing from two-dimensional square close-packed layers and three-dimensional close packing from two-dimensional hexagonal close packed layers. The percentage of total space filled by the articles is known as packing efficiency. For different type of arrangement of constituents, the value of packing efficiency is different. In any type of packing, some space is always present which is known as voids. Voids are of two types: tetrahedral voids and octahedral voids. Radius of tetrahedral void is 0.225 times the radius of the constituent atom. Since ionic bond is non-directional, the arrangement of ions in crystal and the coordination number depends upon the ratio of the cation radius(r+) to that of the anion radius (r–). Space lattice and unit cell A crystal lattice is an infinite regular arrangement of particles (represented by points). There are only 14 possible three-dimensional lattices, which are called Bravais Lattices. Unit cell is the smallest portion of a crystal lattice, which when repeated in different directions, forms the entire lattice. Unit cells are of two types: Primitive unit cells (having constituent particles are present only at the corners and Non- primitive unit cells (having constituent particles at the corners and at other places also). Non- primitive unit cells are further classified as face-centered unit cell, body-centered unit cell and end-centered unit cell. Atoms placed in different positions in a unit cell are shared by different number of unit cells. The number of atoms present in different type of unit cell is different. The density of a crystal depends upon the number of particles present in its unit cell. The higher the number of particles per unit cell, the greater is the density of the unit crystal. Imperfections in solids The departure from perfectly ordered arrangement of atoms in crystal is known as imperfection. It is of two types: point defect (arises due to the irregularity in the arrangement of an atom) and line defect (like edge dislocation - from inserting an extra row of atoms, distortion in lattice). In stoichiometric defects the ratio between cations and anions remains same. Vacancy Defects, Interstitial defects, Schottky defect, Frenkel Defect are the different types of Stoichiometric defects. In case of non-stoichiometric defects the ratio of cations and anions is not same as indicated by the ideal chemical formula. They are of two types–metal excess defect and metal deficiency defect. Impurity defects arise when foreign atoms are present at the lattice site in place of host atoms or at the vacant interstitial site. The process of adding impurities to a crystalline substance so as to change its properties is called doping.

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