X-RAY POWDER DIFFRACTION is a process of identifying the crystal structure (i.e. atomic spacing) of a powder sample using x-rays. It can tell you both what elements are present as well as differentiating between polymorphs.

PRODUCING X-RAYS

1. Electrons are freed from a tungsten filament (aka cathode) by heating, where the valence electrons are free to move with very little energy input. Normally the electrons would stay within the metal because it becomes (+) charged when the electrons leave, drawing them back. However, here the cathode is given a (-) charge when the cathode is heated so the electrons can escape.
2. The freed electrons are accelerated by an electric field (established between the cathode and anode) and collide with a piece of copper (aka the anode).
3. The accelerated electrons will knock out electrons in the inner shell of the copper. This is an unstable configuration for the copper.
4. Electrons in the outer shell will fall down to fill the vacancies and in doing so, emit X-rays at a very specific wavelength (k\alpha_{1}, k\alpha_{2}, and k\beta)

XRD PRIMARY OPTICS: filters that clean up the emitted x-rays from the source before they hit the mounted, powdered sample

1. Suller Slits are pieces of evenly-spaced metal foils that correct the direction of the x-ray beam
2. Divergence Slits are slits in lead slabs that limit the divergence (aka “spread”) of the x-ray beam
3. Monochromators are filters that only allow specific wavelengths through (e.g. filters out radiation from all electrons transitions in copper except k\beta)

The result of the XRD Primary Optics is a less intense but more precise beam of electrons from the source hitting the sample.

XRD SECONDARY OPTICS: filters that clean up the x-rays after they have been diffracted by the powdered sample but before they hit the detector

1. Selection Filters reduce the diffusion of diffraction x-rays due to amorphous crystals or scattering in air
2. Suller Slits to correct the spread of the beam
3. Monochromators to only measure certain wavelengths in the detector

XRD DETECTOR recieves x-rays from samples after being processed by secondary optics

1. X-rays collide with a phosphor screen (aka a scintillator) coating a thallium-doped sodium crystal. Phosphor is a transition metal/rare earth compound that emits light via luminescence when struck with photons (this process = photoluminescence).
2. The thallium-doped sodium iodide crystal is a photocathode so it then produces blue (visible) light, where the number of electrons that are produced by the photocathode is proportional to the number of x-ray photons which hit it.  The mounted sample and detector both rotate, and different materials (and their polymorphs) will have characteristic regions with higher density of x-rays.
3. The blue light is converted to a voltage via a photomultiplier tube.

Source: Klein & Philpotts, Earth Materials, 2nd Editions.