3.4 applications of crystal structure

x-ray 10^-10

angstroms, same spacing as in crystals

constructive interference

• crest lines up with crest

• trough with trough

• amplified signal

  • delay by ½ lambda lose signal

  • delay by a whole number lamda keep signal

destructive interference

everything cancels and you get no signal

atoms are the

scattering center

path length distance comes from

• x-rays scatter off deeper planes travel farther

• scattered x-rays that will have constructive or distructive interference

peaks in x-ray diffraction occur

at specific angles

path length difference for x-rays scattereed off of two different planes

is a whole number of the wavelength

constructive interference

change theta

path length difference changes

peaks are

constructive

the noise that looks to be at one level

distructive interference

more symmetry

fewer/sharper peaks

less symmetry

less symmetry more peaks

more impurities

more impurities more peaks

every XRD pattern is

unique

• a fingerprint for crystalline materials

components of XRD

• number of peaks

• spacing of peaks

• relative heights of peaks

d decreases

2 theta increases

a decreases

2 theta increases

a decreases

d decreases

heat a material and it expands

a and d increase

2 theta decrease

number of peaks

• based on symmetry and purity

• two crystals of same lattice types should have the same number of peaks

location of peaks

• interplanar spacing

• even with same lattice type of different materials have different bond lengths and thus different peak locations

lattice parameter expands

a increases; d increases ; 2 theta

same number of peaks; all shift to smaller theta