University10 min

X-ray diffraction: Bragg's law in practice

Bragg's law, the 2θ angle trap, and the method for going from a diffractogram peak to an interplanar spacing, then to the unit-cell parameter.

The essentials

1

Why X-rays

Their wavelength, λ≈0.5\lambda \approx 0.5–2.5 A˚2.5\ \text{Å}, is of the same order as interatomic distances (C–C ≈1.54 A˚\approx 1.54\ \text{Å}): the crystal behaves as a diffraction grating. Laboratory line to know: Cu KαK_\alpha, λ=1.5406 A˚\lambda = 1.5406\ \text{Å}. Reminder: 1 A˚=10−10 m=100 pm1\ \text{Å} = 10^{-10}\ \text{m} = 100\ \text{pm}.
2

Bragg's law

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3

θ and 2θ

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4

The cubic cell

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5

Single crystal or powder

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