Classification of Solids

Solids, on the basis of their structures, may be classified as:

  1. Inorganic solids: (a) Metallic solids  (b) Ionic solids (c) Covalent solids
  2. Organic (or molecular) solids
  3. Complex structure solids

Examples of such solids are as under:

Metallic solids: Na, Cu, Zn; compounds such as borides, hydrides, nitrides, carbides etc. formed by transition elements.

Ionic solids: NaCl, Na2SO4, MgAl204, calcite (CaCO3), magnesia (Mg0), Zincblende (or sphalerite) ZnS having FCC structure, and wurtzite ZnS having HCP structure.

Covalent solids: SiC, diamond, quartz, carborundum, sialons (Si3N4, Al203 alloys).

Complex solids: UO2, a ceramic which is nuclear fuel.

We will discuss them in brief now.

Metallic Solids

Metallic solids find wide use owing to their favorable properties of strength, ductility, workability, conductivity and versatility etc. In their formation, each atom attracts as many atoms as it can. The result is a closely packed structure.

Metallic solids have high density, strong bonds and lower potential energy. The hexagonally closed packed (HCP) and face centered cube (FCC) are closed packed solid structures.

The HCP and FCC are mainly similar but have an important difference in their stacking arrangement. In HCP, the pattern of stacking of atoms is ….ABABAB…….whereas in FCC, it is of ……..ABCABCABCABC…… type.

The difference between packing arrangements in HCP and FCC is illustrated in Figures 1. a-b. The top and middle layers are the same in both configurations but in FCC structure the bottom layer is rotated at 60° relative to HCP structure.

Hexagonally Closed Packed Structure (HCP)

This is also known as closed packed hexagonal (CPH) structure. Arrangement of atomic packing and sequential piling of atoms in it is of …ABAB…. form as already discussed. HCP structure is different from hexagonal structure. It is denser than the hexagonal one.

A HCP unit cell is shown in Fig. 2. There are total seventeen atoms in it. Six atoms are placed on each of the bottom and top hexagonal corners, one each on bottom and top hexagonal faces, and three atoms on vertical alternate planes.

The atoms are closed packed, but are shown separated for clarity. The atoms in planes A are placed at lattice points but the atoms in plane B are not. The geometrical details of HCP unit cell are given as follows:

Here c is the distance between top and bottom hexagonal planes A and A.

Diamond Cubic (DC) Structure

Carbon exists in two forms viz. diamond and graphite. Both have quite different characteristics and properties. Diamond has (sp3) hybrid covalent bond. Each of its atoms has four bonds. The bonds are directional in nature. The bonds are primary in nature and extend in a three-dimensional network. The directional bond angle in diamond is 109.5°. The structure of diamond is better known as diamond cubic (DC).

Unit cell of a DC is shown in Fig. 3. It contains a total of 18 atoms. Of these, 8 are placed on corners of the cube, one on each of the six faces and four completely inside. The two inside atoms are placed at three-fourth (3/4) and other two at one-fourth (1/4) distance above the base when the height of unit cell is unity i.e. 1.

These four atoms are placed on body diagonal, and are not the lattice points. Diamond is the hardest known solid. Its hardness on Moh’s scale is 10. Other geometrical details are given as below:

Besides diamond, the DC structure also exists in silicon, germanium, grey tin etc.

Graphite Structure

Graphite is another form of carbon in which covalent bond is of (sp2) hybrid type. Graphite has hexagonal honey bee form of sheet structure as shown in Fig. 4. There are three bonds per atom in it with bond angle of 120°. Its planer configuration is primary bonded while the secondary bond extends in the direction of thickness. The weak secondary bond of Van der Waals type makes it soft.

Graphite is used in pencil making with a blend of clay. It is also used as a solid lubricant or in a graphited oil. It has directional property. Its thermal and electrical conductivities are much higher along the direction of the sheet as compared along its perpendicular direction.

When the sheet is aligned in fiber form, its strength and modulus increases considerably. Graphite fibers are used in fabrication of fibrous composites. Graphite can be used as dry powder, as a spray, as a paste, as grease, or as liquid dispersion.

In one form or the other, graphite is used in I.C. engines, machine tools, air compressor etc. Graphite is very suitable for use at high temperature and pressure because of its low coefficient of friction. It is opaque, and grey black in color. It is also used in dry cells, and as moderator in nuclear reactor.

As a remarkable use of graphite, it can be converted to synthetic diamond at 1600°C by a pressure of 50000 to 60000 atmospheres.

Structures of Sodium Chloride and Cesium Chloride

Sodium chloride (NaCl) is an ionic solid having BCC structure. In electrical engineering applications, it is use in glazing of dielectrics. Besides, it is used in washing soda, soap and preparation of freezing mixture. The cation in this structure is sodium (Na) and the anion is chlorine (Cl).

Sodium forms positive and chloride forms negative ions in the structure of sodium chloride. The ions are arranged alternately. Each ion is surrounded by equal number of ions of the opposite sign. The exact number of surrounding ions is decided by the relative sizes of the ions.

The ionic radius of Cl is 1.81 oA while that of Na+ is 0.98 oA. Only 6 Cl ions can be surrounded by one Na+ ion. The central Cl ion is surrounded by 6 Na+ ions at the centers of the cube faces. A close inspection reveals that two FCC structures are superimposed. The scheme is shown in Fig. 5a.

Structure of cesium chloride (CsCl) is similar to NaCl with the exception that ions are nearly of the same size which is not the case with NaCl. Hence 8 Cl ions can be packed around each Cs+ ion to form a BCC structure.

CsCl is not strictly BCC as its centre and corner atoms are different. Structure of a unit cell of CsCl is shown in Fig. 5b. In it, chlorine atoms are placed on eight corners and cesium atom is placed at the centre.

Structures of other simple ionic solids such as MgO, BaO, SrO, CaO, CaF2 (calcium fluoride), NaF, NaBr and Nal etc. are of NaCI type. Halides such as LiCl, LiBr, LiI adopt NaCI type structure having coordination number = 6.

Voids

The closed packing of atoms described above is redrawn in Fig. 6a showing the top view. The vacant space available between atoms marked V is called void. These voids are two different kinds of voids. These are:

  1. Tetrahedral voids (Fig. 6b), and
  2. Octahedral voids (Fig. 6c).

The atoms of other elements fit into these spaces during formation of an alloy. The maximum size of foreign atom that fits into tetrahedral void without distorting the parent atoms is 0.0255r where r is the radius of closed packed parent atom.

The largest size of a foreign atom that may fit into an octahedral void is 0.414r. Carbon atoms occupy octahedral voids in FCC form.

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