PXRD

Question 1

applications of X-ray diffraction

Answer 1

1. fingerprinting of solid-state materials
   -> different materials have different x-ray diffraction patterns
   -> unknown samples can be checked against database
2. forensic analysis
   -> small sample size which can be recovered (non-destructive technique)
3. geology
   -> evaluating purity of limestone
   -> determining extractability of metals from ore-bearing deposits
4. biology
   -> analysis of oligopeptides, co-factor, steroid hormones

Question 2

requirements for PXRD experiment

Answer 2

1. monochromatic (single wavelength) x-ray beam
2. crystalline material

Question 3

why is Be used?

Answer 3

z = 4 (light metal)

not a lot of e- (transparent to x-rays)

BUT v. toxic -> careful when handling

Question 4

effect of using metals further down group

Answer 4

absorb more of x-rays

x-rays interact with e- cloud

heavier metals have denser e- cloud

Question 5

crystals

Answer 5

= periodic arrangement of matter (repeats at regular intervals)

classified according to unit cells

Question 6

unit cells

Answer 6

smallest repeating volume unit which shows full symmetry of structure

Question 7

how many types of geometric shapes of unit cells are there?

Answer 7

7

known as CRYSTAL SYSTEMS

Question 8

how many Bravais lattices are there?

Answer 8

14

Question 9

how many LP in a P lattice type per unit cell?

Answer 9

1

Question 10

how many LP in a I lattice type per unit cell?

Answer 10

2

Question 11

how many LP in a F lattice type per unit cell?

Answer 11

4

Question 12

how many LP in a C lattice type per unit cell?

Answer 12

2

Question 13

crystal structure vs Bravais lattice

Answer 13

CRYSTAL STRUCTURE - gives information of the exact location of atoms or ions within the unit cell

BRAVAIS LATTICE - only gives information on where environments are identical (i.e. lattice points)
-> many compounds have same Bravais lattice type

Question 14

what are the 2 naming systems for planes?

Answer 14

1. WEISS INDICES - intercepts axes (a,b,c)
2. MILLER INDICES - reciprocals of the Weiss indices with the fractions cleared

Question 15

Weiss index = ∞ meaning?

Answer 15

parallel with axis

Question 16

what happens to the d-spacing as the Miller indices increase?

Answer 16

increases

Question 17

x-ray diffraction

Answer 17

x-rays “see” e- cloud

each LP acts as a secondary source of radiation

x-rays send each e- on an atom into vibration

each vibrating charge emits x-rays with the same frequency as x-ray beam = diffraction pattern

Question 18

what does Bragg’s law indicate?

Answer 18

determines the set of conditions needed to observe reflection of x-rays from a given (h,k,l)

no indication of amplitude

Question 19

2 methods of recording diffraction patterns

Answer 19

1. Debye-Scherrer camera (detector = film)
2. Automatic powder diffractometer (detector = digital)

Question 20

diffraction pattern composition

Answer 20

millions of microcrystallites

randomly oriented -> plane of reflected beam also random

gives rise to cone of reflection for each Miller index (h,k,l)

Question 21

what do the positions of the lines relate to?

Answer 21

hkl (d) of unit cell/lattice parameters

Question 22

selection rule for I-type lattice

Answer 22

Miller indices = even (2n) number

Question 23

selection rule for F-type lattice

Answer 23

either (even, even, even) or (odd, odd, odd)

Question 24

when is a reflection not observed for given h,k,l values?

Answer 24

when set of planes doesn’t contain all LPs in structure

Question 25

metal filter

Answer 25

1 or 2 atomic number before metal target in periodic table => ensures absorption edge falls between Kα and Kβ line