Lecture 3 - Crystal Bonds

1 Chapter 3 - Crystal structure Contents Chapter 3 - Crystal structure ........................................................................................................... 1 What is the difference between short range and long range order? ............................................ 3 What are the 2 sorts of packing, and how do their energy differ? .............................................. 3 What are the differences between the 2 types of materials? ....................................................... 3 What is the crystal structure? ...................................................................................................... 3 Crystalline or amorphous? Metals, ceramics, glasses, polymers ................................................ 3 What are space lattices? .............................................................................................................. 3 What are unit cells? ..................................................................................................................... 3 Define metallic crystal structures ................................................................................................ 4 Define some structures ................................................................................................................ 4 Stacking sequence? ..................................................................................................................... 4 What is polymorphism/allotropy? ............................................................................................... 5 What is the rule of thumb for the densities of different material classes? .................................. 5 How do we group crystals? ......................................................................................................... 5 Name a few different crystal systems ......................................................................................... 6 How to name them! ..................................................................................................................... 7 Single crystals vs polycrystals .................................................................................................... 7 Anisotropic vs isotropic .............................................................................................................. 7 What is X-ray diffraction pattern used for? ................................................................................ 8 Amorphous materials .................................................................................................................. 8 Definitions: ..................................................................................................................................... 9 Close packed plane ..................................................................................................................... 9 Coordination number .................................................................................................................. 9 Atomic packing ........................................................................................................................... 9 Linear atomic density .................................................................................................................. 9 Planar atomic densities ............................................................................................................... 9 Grain boundary ........................................................................................................................... 9 Polycrystalline ............................................................................................................................. 9 Math: ............................................................................................................................................. 10 Atomic packing factor ............................................................................................................... 10 Crystal density ........................................................................................................................... 11

2 Linear density ............................................................................................................................. 11 Planar density ............................................................................................................................. 11 FINDING OUT THE TYPE OF CRYSTAL STRUCTURE USING X-RAY DIFFRATION ... 11

3 What is the difference between short range and long range order? Short range order: bonded to each other with no repeating pattern, amorphous aka non crystalline (ex: water, glass) Long range order: Bonded together in a regular, repeating 3D pattern forming a crystal (think brick wall) What are the 2 sorts of packing, and how do their energy differ? Random packing, non dense. (blue) Ordered packing, dense. (red) You can see that for random packing, bond length is further between atoms and bond energy is higher whereas ordered packing creates shorter bond lengths with lower bond energy What are the differences between the 2 types of materials? Crystalline materials: atoms are packed in periodic, 3D arrays (e.g. metals, ceramics, polymers) Non crystalline materials aka amorphous: atoms have no periodic packing (e.g. complex structures, rapid cooling) What is the crystal structure? It is the arrangement of atoms, it determines the properties of materials. Important distinction: Atomic structure is nb of protons, neutrons, electrons in an atom Crystal structure is the arrangement of atoms in the material. Crystalline or amorphous? Metals, ceramics, glasses, polymers Metals: majoritairement crystalline Ceramics: crystalline Glasses: amorphous Polymers: amorphous or semy-crystalline. What are space lattices? It is the pattern created by the atom, specific to its type, temperature, and pressure based on the most stable configuration possible. What are unit cells? It is the smallest unit of a space lattice that still exhibits the lattice pattern of the crystal and that still has the chemical and physical properties of the material.