Lecture 7 Flashcards
Crystalline and Amorphous
Crystalline- possesses long range order
Amorphous- possesses short range order (tetrahedral coordination of Si by O
Crystalline to Amorphous
A crystalline solid is heated –> atoms begin to vibrate due to increase in energy –>bonds break and long range order is lost
Amorphous to Crystalline
Amorphous liquid is cooled –>atoms lose energy –> they assemble into crystalline clusters –> clusters grow by addition of atoms from the liquid
Glass formation
When a liquid is cooled it can be difficult for atoms to be arranged in a crystalline array (involves bond breaking/making and the molecules are large) –> as it is cooled the atoms lose mobility–> amorphous liquid structure is frozen solid –> amorphous or glassy solid
Silica glass
Silicon and oxygen are covalently bonded and to transform the amorphous liquid into a crystalline
Unit cells
Single crystals formed from crystals of high symmetry have properties which are largely isotropic
Single crystals formed from unit cells of low symmetry have properties which may be highly anisotropic
Metallic crystal structures
Non- directional
Desirable to have electrons orbit the maximum number of nuclei
Maximum packing density of atoms
Metallic crystal structures pt 2
Face centred cubic -Al, Cu, Ni, Ag, Au, Pt, Pb Body centred cubic -Cr, Mo, Ta, W Hexagonal close packed -Zn, Cd, Co
Face centred cubic
Atoms touch lies along the face diagonal
Each atom is coordinated by 12 nearest neighbours
Filling space for spherical atoms of 74%
Body centred cubic
Centre and corner atoms touch along the body diagonal
Coordination number is 8
Space filling at 68%
Hexagonal close packed
Each atom is coordinated by 12 nearest neighbours
Space filling of 74%
Polymorphism
The ability to adopt different crystal structures depending on the temperature (and pressure). Structural changes are accompanied by a volume change
Polymorphism (Iron)
bcc fcc bcc
Polymorphism (Titanium)
hcp bcc
Metals have simple crystal structures because
- non-directional bonding
- all atoms have the same size
- close packing giving the lowest energy
