x ray crystal structures (copy) 3

amorphous material properties (3)

• broad melting point

• globular

• isotropic - no order

• no diffraction

• yield n flow under pressure

crystalline material properties (4)

• narrow melting point

• regular faces and edges

• anisotropic - very ordered

• diffracts light

• fracture/cleave under pressure

what are the three crystallisation steps

• supersaturation

• formation of nuclei

• crystal growth

why do crystals diffract

because molecules are arranged in an ordered fashion

what happens when an x ray beam strikes a crystal surface at theta θ?

part of the beam is scattered by the layer of atoms at the surface

why do x rays scatter

the electron clouds of the atoms in the crystal

what does amount of scatter depend on?

size of atom

why are x rays scattered in an identical way

as a crystal has atoms in identical position

what is visible light between

400 - 700 nm

what do lattice points define?

unit cells

what is a unit cell

• smallest repeating internal unit that has the symmetry characteristic of the solid- atom, ion or molecule

what are the seven basic crystal systems

• cubic

• tetragonal

• orthorhombic

• rhombohedral

• hexagonal

• monoclinic

• triclinic

cubic unit cells properties

• all sides equal

• all sides equal 90

whats a primitive cubic cell (PC)

atom in each corner of a cube

what is the net number of atoms of pc?

1

whats a body centered cube (BCC)

atom in each corner and the centre

what is the net number of atoms of bcc?

2

whats a face centred cube (FCC)

atom in every corner and on each face

what is the net number of atoms of fcc?

4

what causes diffraction spots

x rays striking a single crystal

what does each diffraction spot correspond to?

single point in lattice

Formula you should know

2d sinθ=nλ