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Chapter 7

Chapter 7. X-Ray diffraction. Contents. Basic concepts and definitions Waves and X-rays Crystal structure Bragg’s law X-ray techniques. Basic concepts. Crystal chemistry: study of relationship of internal crystal structure to the physical and chemical properties of minerals Crystals:

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Chapter 7

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  1. Chapter 7 X-Ray diffraction

  2. Contents • Basic concepts and definitions • Waves and X-rays • Crystal structure • Bragg’s law • X-ray techniques

  3. Basic concepts • Crystal chemistry: study of relationship of internal crystal structure to the physical and chemical properties of minerals • Crystals: • Lattice structure, with unit cell as basic building block • Atoms and inter-atomic distances: 0.1 – 0.5 nm • For investigation of crystals - waves with wavelengths in this order • Visual light: wavelength 400 – 700 nm • X-Rays and neutron waves: 0.01 – 0.5 nm

  4. Basic concepts • Quantum shells orbits wherein electrons move: • K, L, M, N, O, P, Q • No of electrons: 2N2 (N = 1,2,3,4,5,6 or 7) • M: 2 x 32 = 18 • Sub-shells – dividing quantum shells • s, p, d, f • No of electrons: 4K – 2 (K = 1, 2, 3 or 4) • d: (4 x 3) – 2 = 10

  5. Basic concepts • Generation of X-rays • Electrons emitted by source (heated W filament) • Accelerate electrons by using an electric field (control strength of electric field to control speed of electrons) • Electrons collide with metal anode (Cu or Mo) • Very high-energy radiation emitted: X-rays* • How: • Accelerated electrons displace electrons of inner shell of atom • Electron from higher shell fills electron ‘gap’ and release excess energy as X-ray photons • Energy and wavelength of these photons correspond to particular electronic transition of given atom of the metal anode

  6. Generation of X-rays

  7. Waves • Amplitude • Wavelength • Path/phase difference • Fig 7.4 • Interference of waves • Constructive • Destructive

  8. Interference of waves

  9. Bragg’s Law • Based on the diffraction of X-rays or neutrons from crystal surfaces at certain angles • Basis of developing powerful tool for studying crystals in the form of X-ray and neutron diffraction • How: • When X-rays (or neutrons) hit an atom the movement of electrons (or spinning of nuclei) cause re-radiation of waves with the same frequency • Rayleigh scattering • Scattered waves interfere with each other constructively or destructively to produce peaks at certain angles • Produces a diffraction pattern from which the analyses of the crystal structure is done

  10. Bragg’s law • When X-rays are scattered from a crystal lattice, peaks of scattered intensity are observed which correspond to the following conditions: • The angle of incidence = angle of scattering • The path length difference is equal to an integer number of wavelengths. • The condition for maximum intensity contained in Bragg's law above allow us to calculate details about the crystal structure, or if the crystal structure is known, to determine the wavelength of the x-rays incident upon the crystal. • Bragg’s equation: nλ = 2dsinθ • n: integer determined by the order given • λ: the wavelength of x-rays • d: spacing between atomic lattice planes • θ: angle between the incident ray and the scattering planes

  11. Bragg’s law

  12. Techniques: X-ray diffractometer • The x-rays are collimated into a strong X-ray absorber (usually lead) • Narrow resulting x-ray beam strike the crystal • Rotate crystal and detector to satisfy Braggs law for diffraction

  13. Techniques: Diffractometer

  14. Techniques: Diffractometry of powders • Randomly oriented small crystals or ‘crystallites’ • Reflections scanned and record intensity as function of diffraction angle • The list of θ angles with different intensities are converted to d-spacings • Identify crystals by comparing with diffractions patterns of known minerals

  15. Techniques: Diffraction pattern

  16. Chapter 8 Physical properties of crystals Definitions (Further self-study of this chapter is optional)

  17. Definitions • Thermal conductivity: • Transfer of heat through a mineral through thermal vibrations • High in metals and minerals with significant metallic bonding • Thermal expansion • Expansion of a crystal (increase in volume) with an increase in temperature

  18. Definitions • Piezoelectricity: • The ability of a crystal to change its shape slightly (undergoes strain) when an electrical field is applied to it (also vice versa: applying stress can induce an electric field on these crystals) • Only possibly with some crystals with no centre of symmetry • Pyroelectricity • The ability of a prismatic crystal to develop opposite electric charges on opposite ends when heated • Common in trigonal tourmaline crystals • Magnetisism • The ability of a crystal to produce a magnetic moment when a magnetic field is applied to it • Only possible when crystal contains atoms or ions with unpaired electrons • Strongest: Fe3+ and Mn2+ - has five unpaired 3d-electrons • Fe2+ - has four unpaired 3d-electrons

  19. Chapter 3Q2

  20. Chapter 3Q5

  21. Chapter 4 Q8

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