Powder to become crystal clear

The development of X-ray free electron lasers (XFELs) has opened up new opportunities for experiments that seem impossible now to become a reality in the near future. One of the new capabilities of XFELs is to collect single-crystal diffraction data from randomly oriented sub-micron-sized crystals using serial femtosecond crystallography (SFX) (Chapman et al., 2011).

Many important materials, such as zeolites, are polycrystalline (powders) and cannot be grown as single crystals. Furthermore, different types of samples (multiphase) may be mixed during production of a material; for example, zeolite NU-87 may occur as an impurity in zeolite TNU-9 (Hong et al., 2007). X-ray diffraction from such samples will usually result in a one-dimensional powder pattern (Fig. 1, left). Because of the relatively large molecular size (76 non-hydrogen atoms in the case of TNU-9), the powder diffraction pattern from a zeolite can be difficult to interpret (Gramm et al., 2006). The powder diffraction pattern from a mixture of TNU-9 and NU-87 would be impossible to process.

Figure 1 A one-dimensional powder diffraction pattern seen using conventional methods (left) may potentially be analysed as three-dimensional single-crystal patterns using serial crystallography (right).

Powder samples are essentially a mixture of sub-micron-sized (typically 100 nm) single crystals. The latest sample handling techniques, such as liquid jet injectors, can deliver the crystals to the beam one at a time and the extremely intense XFEL beam can capture a diffraction image of each crystal in a sub-nanosecond time scale (Spence et al., 2012). In this issue, Zhang et al. (2015) have proposed the use of serial crystallography to turn powder diffraction into single-crystal diffraction (Fig. 1). A test has been performed using simulated diffraction patterns. The test sample is a mixture of zeolites TNU-9 and NU-87 with crystal grain sizes as small as 100 nm. X-ray diffraction snapshots by SFX were simulated and processed using the program suite CrystFEL (White et al., 2012). Identification according to the primitive unit-cell volume determined from individual snapshots was able to separate the whole set of snapshots into two subsets, which matched the two zeolites in the sample. Monte Carlo integration in CrystFEL was then applied to them separately. Two sets of three-dimensional single-crystal diffraction intensities could then be derived. The crystal structures of the two zeolites were solved using the direct methods program SHELXD (Sheldrick, 2008) with default parameters.

Turning one-dimensional diffraction from polycrystalline (powder) samples, particularly from multiphase samples, into three-dimensional single-crystal diffraction patterns has long been regarded as a difficult, if not impossible, task. Zhang et al.'s proof-of-principle study has demonstrated that with the latest XFEL and sample delivery technology, single-crystal diffraction patterns can be collected from multiphase polycrystalline samples, processed, and then the molecular structures can be solved ab initio. This technique promises to open up new avenues for the study of many important polycrystalline materials that cannot be analysed by conventional X-ray powder diffraction methods.