Sample Preparation Matters

http://www.datech-scientific.co.uk/analysis/xrd/

What is X-ray Powder Diffraction (XRD)

X-ray powder diffraction (XRD) is a rapid analytical technique primarily used for phase identification of a crystalline material and can provide information on unit cell dimensions. The analysed material is finely ground, homogenised, and average bulk composition is determined. Fundamental Principles of X-ray Powder Diffraction (XRD) Max von Laue, in 1912, discovered that crystalline substances act as three-dimensional diffraction gratings for X-ray wavelengths similar to the spacing of planes in a crystal lattice. X-ray diffraction is now a common technique for the study of crystal structures and atomic spacing. X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample. These X-rays are generated by a cathode ray tube, filtered to produce monochromatic radiation, collimated to concentrate, and directed toward the sample. The interaction of the incident rays with the sample produces constructive interference (and a diffracted ray) when conditions satisfy Bragg’s Law (nλ=2d sin θ). This law relates the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. These diffracted X-rays are then detected, processed and counted. By scanning the sample through a range of 2θangles, all possible diffraction directions of the lattice should be attained due to the random orientation of the powdered material. Conversion of the diffraction peaks to d-spacings allows identification of the mineral because each mineral has a set of unique d-spacings. Typically, this is achieved by comparison of d-spacings with standard reference patterns. All diffraction methods are based on generation of X-rays in an X-ray tube. These X-rays are directed at the sample, and the diffracted rays are collected. A key component of all diffraction is the angle between the incident and diffracted rays. Powder and single crystal diffraction vary in instrumentation beyond this. Applications X-ray powder diffraction is most widely used for the identification of unknown crystalline materials (e.g. minerals, inorganic compounds). Determination of unknown solids is critical to studies in geology, environmental science, material science, engineering and biology. Other applications include:

• Characterisation of crystalline materials

• Identification of fine-grained minerals such as clays and mixed layer clays that are difficult to determine optically

• Determination of unit cell dimensions

• Measurement of sample purityWith specialised techniques, XRD can be used to:

• Determine crystal structures using Rietveld refinement

• Determine of modal amounts of minerals (quantitative analysis)

• Characterise thin films samples by:

1. Determining lattice mismatch between film and substrate and to inferring stress and strain

2. Determining dislocation density and quality of the film by rocking curve measurements

3. Measuring superlattices in multilayered epitaxial structures

4. Determining the thickness, roughness and density of the film using glancing incidence X-ray reflectivity measurements

5. Make textural measurements, such as the orientation of grains, in a polycrystalline sample

Strengths and Limitations of X-ray Powder Diffraction (XRD)

• Powerful and rapid (< 20 min) technique for identification of an unknown mineral

• In most cases, it provides an unambiguous mineral determination

• Minimal sample preparation is required

• XRD units are widely available

• Data interpretation is relatively straight forwardLimitations

• Homogeneous and single phase material is best for identification of an unknown

• Must have access to a standard reference file of inorganic compounds (d-spacings, hkls)

• Requires tenths of a gram of material which must be ground into a powder

• For mixed materials, detection limit is ~ 2% of sample

• For unit cell determinations, indexing of patterns for non-isometric crystal systems is complicated

• Peak overlay may occur and worsens for high angle ‘reflections’