Powder X-ray diffraction analysis

Identification of different crystalline phases and the determination of their relative concentrations in mixtures based on the diffraction pattern analysis recorded from the powder samples: precise determination of the unit cell parameters of the known substance to detect isomorphic impurities, indexing of diffraction patterns, and determination of unit cell parameters and possible space group for new compounds.

Qualitative phase analysis is performed by comparing the experimental values ​​of the interplanar spaces and relative intensities with standard diffraction patterns, because each substance has its own "picture" of the lines location of the diffraction pattern. Qualitative phase analysis allows separating and identifying the individual phases of a heterogeneous system. The objects of study in the phase analysis are metals, alloys, chemicals, minerals and ores. Using X-ray diffraction analysis it is possible to determine the composition of non-metallic inclusions in metals (oxides, sulfides, nitrides, carbides), the distribution of alloying elements in multiphase alloys. Widespread use of X-ray diffraction well explained by a well-developed theory, simple sample preparation, the relative rapidity of obtaining quality results, the preservation of the samples without changes after the experiment, the use of polycrystalline material, possibility of mass measurements and the ability to distinguish between polymorphs, the possibility of obtaining experimental diffraction, and data on the structural characteristics of the individual phases and their amounts along with data on the phase composition.

Quantitative phase analysis is the second stage, when the qualitative phase composition is already known. Quantitative X-ray phase analysis based on the correlation between the diffraction intensities of the test phase and its content in a multiphase polycrystalline sample. However, even with the same content of the phase the intensities of the diffraction reflections could vary depending on the value of the average absorption coefficient of X-rays in the sample. So you need to either find this relationship and to determine the absorption coefficient of the sample or use methods to eliminate the influence of the absorption factor. Several methods of quantitative phase analysis are known:

  • mixing method based on a comparison of the diffraction lines intensities of the test phase, and the lines intensities of the standard material, the amount of which is set in advance in the mixture;
  • method of independent standard, in which the sample and the standard are measured consequentially;
  • method of homologous pairs – X-ray diffraction measurement of a series of alloys or cther crystalline mixtures and defining of the lines of different phases with the same intensity;
  • superposition method based on a comparison of the sample diffraction pattern with the individual components in the pure form, consistently;
  • no standard method of measurement is based on the fact that the line intensity is proportional to the phase volume content phase and measuring the absolute intensity of the lines of each phase or the ratio of intensities of the different phases, the concentration of each phase could be determine.

A full analysis of the powder X-ray diffraction pattern including precise determination of the unit cell parameters, the quantitative analysis of multiphase mixtures, refinement of the crystal structure (atomic coordinates, site occupation factors), the determination of particle size and micro strains, textural characteristics, etc.