Collecting Diffraction Data

1
Collecting Diffraction Data

• Theory and Practice

2
X-Ray Diffraction

• Treat diffraction as if it were a reflection from
  a set of equivalent, parallel planes of atoms in
  crystal.

3
Define the Unit Cell

• Six numbers
• Three unique edges
• Three unique angles
• Types
• Triclinic
• Monoclinic
• Hexagonal
• Cubic
• Orthorhombic

4
Crystals and Reflections

• Crystals lattices represent regularly spaced
  planes
• Lattice indices defined by 3 numbers hkl
• H of planes in direction a
• K/l of planes in b and c

5
Braggs Law

• True-rays reflected from planes are in phase with
  each other strong beam
• Large dmany reflections

6
Reciprocal Lattice

• -The spacing of reflections in the lattice on the
  film
• Sphere of reflection-when a reciprocal lattice
  point comes in contact with all circles of r
  wave-1

Fig 4.11 Pg 58 Fig 4.10 Pg 56
7
Limiting Sphere

• Distance of 2/wave of the origin
• Any reciprocal lattice point within can be
  rotated into sphere of reflection measurement
• Fig4.12 pg 59

8
Maximizing by Minimizing

• Limiting sphere- of reciprocal lattice pts
  reflections that can be produced
• Shorter wavelengths larger sphere of reflection
  (more measurements)

9
Unit-cell Symmetry

• Redundancy improves accuracy
• Space groups
• Primitive
• Internal
• Face-centered
• Symmetry operations
• Translation, rotation, reflection

10
X-ray Sources

• Monochromatic source
• Gives only one sphere of reflection
• X-ray tubes
• Low angles
• Rotating anode tubes
• 10x stronger than x-ray tubes
• Particle storage rings
• Powerful selectable monochromatic

11
Particle Storage Rings

• Advantages
• Data obtained in seconds-minutes
• Selectable wavelength helpful in solving phase
  problem

12
Detectors

• Image plate detectors
• Record reflections of varying intensities
• Area detectors
• Wide range and direct collection by computer
• Gives a better determination of position

13
Diffractometers

• Fixed X-ray source
• Goniostat
• Moves crystal in any orientation wrt beam and
  detector
• Movable scintillation-counter

14
Cameras

• Precession camera
• Crystal quality, unit-cell dimension, symmetry
• Simplest diffraction pattern
• Rotation/oscillation camera
• Measure large umbers of reflections simultaneously

15
Determining Unit Cell Dimensions

• Inversely proportional to the real lattice
  spacings

Fig 4.26 Pg 81
16
Data Processing What is indexing? Scaling?
Averaging?What is the end product? Data can be
collected with 2D or 3D processing in mind. Any
ideas what this means? (Hint think about the
precision of l and what that means for the
circumference of the Ewald sphere, and the
irregularities in the real space lattice that
necessarily generates irregularities in the
reciprocal lattice.)