Question

Do you expect to get a different x-ray diffraction pattern for two samples of pure Aluminum: Sample #1 – a cube 1cm x 1cm x 1cm in size, or Sample #2 - a powder of aluminum with a particle size diameter of 1 mm?

Answer 1

The first sample(#1) refers to single crystal diffraction. Whereas ths Sample #2 refers to powder diffraction.

Single-crystal X-ray Diffraction is a non-destructive analytical technique which provides detailed information about the internal lattice of crystalline substances, including unit cell dimensions, bond-lengths, bond-angles, and details of site-ordering. This technique provides the structure within a single crystal, which does not necessarily represent the bulk of the material. Therefore, additional bulk characterization methods must be utilized to prove the identity and purity of a compound.

Unlike single-crystal XRD, powder XRD looks at a large sample of polycrystalline material and therefore is considered a bulk characterization technique. The powder pattern is considered a "fingerprint" for a given material; it provides information about the phase (polymorph) and crystallinity of the material. Typically, powder XRD is used to study minerals, zeolites, metal-organic frameworks (MOFs), and other extended solids. Powder XRD can also be used to establish bulk purity of molecular species.

The diffraction pattern of single crystal is comprised of spots instead of lines like in the double slit experiment. In fact, these “spots” are 2D slices of 3-dimensional spheres. Crystallographers use a computer program to integrate the resulting spots in order to determine the shape and intensity of the diffracted X-rays.  In a powder sample, the X-rays interact with many tiny crystals in random orientations. Therefore, instead of seeing spots, a circular diffraction pattern is observed. The intensities of the diffracted circles are then plotted against the angles between the ring the beam axis (denoted 2θ) to give a 2 dimensional plot known as a powder pattern.