Crystal Structure Characterization by Powder Diffraction

The important role played by powder diffraction in the last twenty-five years both in its earlier fields of application (e.g., qualitative analysis) and in more challenging objectives (e.g., structure solution), explains today its relevant application in the investigation of microcrystalline materials of remarkable scientific and technological interest. The need to determine their atomic structure could not be satisfied in the past because they were not available in size and quality adequate for the more approachable single crystal diffraction technique.

The progress in the structure solution by powder diffraction data has also been accelerated by the power of modern computer programs and by the now accessible high quality of collecting data.

This Special Issue on “Crystal Structure Characterization by Powder Diffraction” covers a range of papers, discussing effective and innovative computational and methodological strategies implemented in computer programs for a correct solution process, as well as describing the structural determination processes of challenging polycrystalline materials, pointing out the essential and advanced contribution of powder diffraction in identifying unknown crystal structures.

The structural changes in chloride [Ni(Et₂en)₂(H₂O)₂]Cl₂ and bromide [Ni(Et₂en)₂]Br₂ complexes were investigated and elucidated by Konaka and Sasaki [1] under the temperature and humidity controls. The authors studied the transition phenomena accompanied by dehydration and hydration and their reverse processes by simultaneous measurements of powder X-ray diffraction (XRD) and differential scanning calorimetry data. The crystal structures of the complexes were determined from laboratory powder XRD data and a new form was also found.

The powerful use of powder diffraction in characterizing crystal compounds was stressed by Al Rahal et al. [2] for co-crystals, presenting and discussing two new structures of pharmaceutical interest: (1) a 1:1 co-crystal of furosemide with urea formed by liquid-assisted grinding; (2) a second polymorphic form of a 1:1 co-crystal of carbamazepine with indomethacin, formed by solvent evaporation. The study was enriched using the dispersion-corrected density functional theory as an increasingly valuable adjunct, not only for the crystal structure validation but also as part of the crystal structure refinement.

Of great interest and utility are the crystallographic computer programs used to characterize a large variety of polycrystalline materials (organic, inorganic and metal-organic) via the most modern and innovative algorithms implemented in them. An example is reported by Altomare et al. [3], describing the EXPO software that, designed for solving structures by powder diffraction data, can employ the two most widely used kinds of solution methods—Direct Methods working in the reciprocal space and Simulated Annealing proceeding in the real space. EXPO also allows one to suitably combine these two approaches for validating the structure solution. The authors also report some useful examples of structure characterization by EXPO to suggest a solution strategy leading towards the application of reciprocal-space methods or real-space methods, or both.

Another interesting subject is represented by the use of X-ray powder diffraction to analyse Sb–Te and Bi–Te binary system specimens, forming commensurately/incommensurately modulated long-period layer. These compounds stack structures known as homologous phases that comprise discrete intermetallic compounds, and X phases. In the latter, the homologous structures are not discrete but appear continuously, with varying stacking periods. The challenging aspect of these materials is that the regions over which these X phases exist have not yet been clarified. On this subject worked Kifune et al. [4] demonstrating that the X phase regions are located between Sb₂₀Te₃ and Sb₅Te₆ in the Sb–Te system and between Bi₈Te₃ and Bi₄Te₅ in the Bi–Te system. They also illustrate that the stacking periods of these X phases continuously changed along with the specimen composition.

The crystal structure of fosfomycin tromethamine was solved and refined using synchrotron X-ray powder diffraction data and optimized using density functional techniques by Butler et al. [5]. They reported that a network of discrete hydrogen bonds links the cations and anions into layers parallel to the ab-plane. The outer surfaces of the layers are composed of the methyloxirane rings of the anions and the methylene groups of the cations, showing that 93% of the atoms are consistent with an additional (pseudo) centre of symmetry.

Finally, an overview of Popović [6] on the use of X-ray powder diffraction as an ideal technique for the quantitative analysis of a multiphase sample is presented. The author concisely describes several simple well-known methods for quantitative X-ray diffraction phase analysis, such as (1) the matrix-flushing method, with the application of reference intensities; (2) the doping methods—(i) simultaneous determination of the fractions of several phases using a single doping and (ii) determination of the fraction of the dominant phase. The conditions to minimize systematic errors and precautions on how to obtain accurate and reliable results were also discussed. Several examples of the application of the doping methods were given.

The present Special Issue: “Crystal Structure Characterization by Powder Diffraction” reviews the progress in the application of powder diffraction for the characterization of innovative microcrystalline materials.