Talk:X-Ray Diffraction Analysis (XRD)
Author: Hans-Jürgen Schwarz, NN
back to Analysis of Salts
Abridged version[edit]
Basic principle: in a crystal, the atoms are arranged at well-defined intervals and form a crystal lattice. The basic model of the defined crystal lattice is repeated periodically. The distances of these periods (lattice spacing) are in the range of 0.02 to 2.5 A (Å- angström? Ampere, absorbance etc) . The lattice spacings define the crystal lattice planes, and their position in the crystal is determined by the integer multiple of the crystal lattice spacing. The radiation is now diffracted on the crystal lattice planes at the corresponding wavelength (x- rays), comparable to the diffraction of light on a crystal.
Incident x-rays on a crystals cause a number of reflections, which relate to the incident angle of the rays and the specific crystal lattice planes. Here the Bragg equation applies:
n * λ = 2 sin Θ
n = integer,
λ = wavelength of the incident x-rays,
d = interplanar spacing or spacing between lattice planes
Θ = glancing or grazing angle- the angle between the incident ray and the lattice planes.
Glanzwinkel - Winkel zwischen einfallendem Röntgenstrahl und Kristallgitterebene.
If the wavelength of the x-rays is known, the crystalline phase can be determined from the position of the diffraction reflections. X-ray diffraction cannot provide much information on amorphous, i.e. non-crystalline substances such as glass.
In practice it is most common to measure powder samples, because then the micro-crystallites are oriented into all direction (statistically). Thus, it is possible to detect nearly all crystal lattice planes without moving the sample (sometimes the sample holder is rotated). With certain minerals such as the phyllosilicates, a preferred orientation is obtained on "normal" sample preparation but special measures may prevent this.
There are several possibilities for the detection of diffracted radiation. A photographic film with a suitable camera can be used for recording the diffraction, reflections according to the diffraction angle and the intensity of the diffracted radiation, (Debeye-Scherrer-, Guinier- and Gandolfi-camera). However, it is equally possible to detect the reflections with a counter tube or a solid state detector (Goniometer method). The detector follows the desired area of the angle and determines the intensity of the reflection for each angle. The intensity of the x-rays as a function of the angle are recorded in the diffractogram. Today, tables with XRD data are kept in databases available for computer-assisted analysis. These provide a tool for the reliable determination of all crystalline organic and inorganic substances (approx. 50,000 sample plots, 35,000 of these for inorganic substances).
Advantage:
The x-ray diffraction provides a qualitative, a semi-quantitative and in some cases also a quantitative determination of crystalline substances. In conservation and restoration, it delivers particularly good results for the study of pigments, salts, rock samples, corrosion products and ceramic materials. The material quantities needed for analysis, can vary between some milligrams, when using a powder diffractometer, down to a few microgram. Single crystal sample holders using the diffractometer method and Debye-Scherrer or Guinier and Gandolfi method, can work with even less. The investigation method is non-destructive, i.e. the sample can be re-used for subsequent investigations.
Disadvantage: For phase mixtures- only phases from a proportion of 1-5% can be detected. SLeithaeuser 10:05, 8 September 2012 (CEST) please extra double-check terms in bold