Crystallography PDF - Second Year Chemistry

Crystallography Prepared by: Dr. wafaa soliman For second year chemistry branch  The structures of common materials The great majority of the 92 stable elements are metallic, and of these, the majority (68 in all) have one of the three simple structures we have examined already:...

Crystallography Prepared by: Dr. wafaa soliman For second year chemistry branch  The structures of common materials The great majority of the 92 stable elements are metallic, and of these, the majority (68 in all) have one of the three simple structures we have examined already: body-centered cubic (BCC), face-centered cubic (FCC), and hexagonal close packed (HCP).  The Body-centered cubic (BCC) structure -The BCC structure is described by a non-primitive, cubic, unit cell with one lattice point at each corner and one at the center of the unit. -Of the metallic elements, 21 have this structure (most are rare earths). BCC metals have the following characteristics. They are ductile when hot, allowing them to be rolled, or otherwise shaped by deformation processing. They are tough, resistant to crack propagation. They become brittle at low temperatures. Their strength depends on temperature. They can generally be hardened with interstitial solutes.  The Face-centered cubic (FCC) structure -The FCC structure is described by a non-primitive, cubic, unit cell with one lattice point at each corner and one at the center of each face; the basis is a single atom located at each lattice point. -Among the metallic elements, 17 have the FCC structure. FCC metals have the following characteristics:.They are very ductile when pure, allowing them to be rolled, or shaped by deformation processing. They are generally tough, resistant to crack propagation. They retain their ductility and toughness to absolute zero, something few other structures allow.  Hexagonal Close Packed (HCP) -Hexagonal close packed (HCP) refers to layers of spheres packed so that spheres in alternating layers overlie one another. Hexagonal close packed is a slip system, which is close-packed structure. -The HCP structure is very common for elemental metals, including: Beryllium, Cadmium, Magnesium, Titanium, Zinc, Zirconium.. The HCP structure is elastic.  Coordination In crystal chemistry, the immediate neighborhood of each atom and the forces which bind it to its neighbors play a leading role in the explanation of the overall geometry of the crystal. The number of nearest neighbors of a central atom or ion is called its coordination Number. or the number of atoms that surrounds an atom in the center of a solid.  Coordination number The coordination number of BCC crystal is 8. The body centered atom is in contact with all the eight corner atoms. Each corner atom is shared by eight unit cells and hence, each of these atoms is in touch with eight body centered atoms.  Coordination number In the FCC lattice each atom is in contact with 12 neighbor atoms. FCC coordination number Z = 12. For example, the face centered atom in the front face is in contact with four corner atoms and four other face-centered atoms behind it (two sides, top and bottom) and is also touching four face-centered atoms of the unit cell in front of it.  Coordination number In Hexagonal lattice Z = 12. The center atom of the top face is in touch with six corner atoms, three atoms of the mid layer and other three atoms of the mid layer of the unit cell above it.  The number of lattice points per cell Atom inside cell is sharing with 1 atom within the cell Atoms on faces are sharing with ½ within the cell Atoms on edges are sharing with 1/4 within the cell Atoms in corners are sharing with ⅛ within the cell The number of lattice points per cell is therefore given by where NI = number of interior points, NF = number of points on faces,and Nc = number of points on corners.  Atomic packing factor Atomic packing factor (APF) or packing efficiency indicates how closely atoms are packed in a unit cell and is given by the ratio of volume of atoms in the unit cell and volume of the unit cell. N Number of atoms N In unit cell  N Number of atoms N In unit cell R= a 2 R = √3a 4 R = √2a 4  Close-Packed Structure FCC and hexagonal crystal structures are most highly packed with packing efficiency of 74% (APF= 0.74). Such structures can be described in terms of close-packed atomic planes. In FCC, {111} planes are close-packed and the basal plane (0001) is the close-packed one in hexagonal close-packed (HCP) system. Therefore, both of these structures can be generated by stacking of these planes.  Structure-Property correlation - Aluminium (Al) is ductile while iron (Fe) and magnesium (Mg) are not. - This can be explained from their crystal structures. Al is FCC where as Fe is BCC and Mg is HCP. Slip is sliding of crystal planes over one another. Slip occurs on most densely packed planes in the most closely packed directions lying on that plane. Close-packed planes are also planes with greatest interplanar spacing and this allows slip to take place easily on these planes. The slip plane and the direction together are called a Slip system Plastic deformation in metals takes place mainly by slip. Slip in more number of slip systems allows greater plastic deformation before fracture imparting ductility to FCC Materials.