Crystal Structure (PDF)

MODULE 2 – Unit 1 The Chemistry of Engineering Materials Basic Concepts of Crystal Structure Crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Crystallography is the experiment...

MODULE 2 – Unit 1 The Chemistry of Engineering Materials Basic Concepts of Crystal Structure Crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids A crystalline solid: Brighter atoms are atomic resolution image strontium and darker of strontium titanate. ones are titanium. Strontium titanate is an oxide of strontium and titanium with the chemical formula SrTiO3. A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns that repeat along the principal directions of three-dimensional space in matter.A representation of a three-dimensional Cartesian coordinate system with the x-axis pointing towards the observer.  The smallest group of particles in the material that constitutes the repeating pattern is the unit cell of the structure. The unit cell completely defines the symmetry and structure of the entire crystal lattice, which is built up by repetitive translation of the unit cell along its principal axes. The repeating patterns are said to be located at the points of the Bravais lattice. Simple Cubic Lattice or Simple Cubic Unit Coordination number = 6 5 3 1 2 4 6 1/8 atom at each of 8 corners Consider this atom. Why is the Packing Efficiency 52%? Body-centered Cubic Lattice Face-centered Cubic Lattice Summary SIMPLE CUBIC BODY-CENTERED CUBIC FACE-CENTERED CUBIC Number of atoms per unit 1 2 4 cell Relation between side of unit cell (l) and atomic radius (r) Packing Efficiency (%) 52.4 68 74 % of Empty Space 47.6 32.0 26.0 For body-centered cubic unit cell:  Why consider three significant figures? From the problem: DEFECTS IN CRYSTALS - Crystal defect, imperfection in the regular geometrical arrangement of the atoms in a crystalline solid. These imperfections result from deformation of the solid, rapid cooling from high temperature, or high-energy radiation (X-rays or neutrons) striking the solid. Located at single points, along lines, or on whole surfaces in the solid, these defects influence its mechanical, electrical, and optical behaviour.  POLYMORPHISM In materials science, polymorphism is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements. e.g. glycine, which is able to form monoclinic and hexagonal crystals. Calcite and aragonite, both forms of calcium carbonate.