Benefits of hard X-ray radiation
X-ray powder diffractometers are generally equipped with X-ray sources using Cu anodes. For specific applications, however, switching to shorter wavelength 'harder' radiation, like that obtained from X-ray tubes with Mo or Ag anodes, gives significant improvements over Cu, or even allows experiments that are impossible to do with Cu radiation. Often, people make use of synchrotron radiation, but new developments on source, optics and detector technologies allow a variety of hard radiation experiments in the home lab.
Below we present some of the key advantages of hard radiation.
Larger penetration depth
For inorganic materials, the penetration depth of Cu Kα is often only a few micrometres (see Table 1). In order to extract information from a larger sample volume, hard radiation is an advantage. Thanks to the larger penetration depth, it is possible to observe crystallographic changes in working devices, such as Li-ion batteries, and perform high-pressure experiments with diamond anvil cells. Hard radiation allows the use of transmission geometry for inorganic samples.
Optimizing the instrument
Effective excitation of hard radiation requires a higher acceleration voltage than used with Cu tubes. The Empyrean system is designed for continuous operation at 60 kV. X-ray tubes with Mo or Ag radiation, optimized to work with mirror optics, are developed in our in-house tube factory. The optical path is carefully optimized to prevent stray radiation, essential for proper PDF data free of instrument artifacts. The proprietary GaliPIX3D detector based on Pixirad technology features a high-quality CdTe sensor allowing 100% detection efficiency up to 50 keV photon energy.
Better crystallographic information
Preparing samples in glass capillaries is the best way to minimize preferred orientation effects of powdered samples. Working with Mo radiation in combination with optics focusing on the detector allows the usage of large-diameter capillaries which increases the irradiated volume of the sample without a loss in angular resolution or excessive absorption problems. Moreover, the use of Mo radiation suppresses the fluorescence from transition metal ions. In this example, we present a detailed investigation of the crystal structure of Fe(IO3)3 from powder data. Table 2 reports the atomic coordinates derived from the Rietveld refinement. As expected, we obtained an excellent precision of the position of the iodine atom but also of the lighter atoms such as the oxygen atoms. In particular, the error bars on these lighter atoms are 10 times better than previously reported [2,3].
In operando studies of batteries in the lab
Batteries are complex systems and in order to understand their functioning in detail it is important to be able to measure their phase composition and structure in real time and under non-equilibrium conditions . This is possible with X-rays by using radiation that is sufficiently penetrating to go through pouch cells or thin prismatic batteries. Moreover, the possibility of performing such measurements in the home lab is crucial for aging studies, which are typically time consuming since the battery needs to be charged and discharged a large number of times.
In Fig. 2 is possible to see the modification in intensity and peak position for a LiCoO2 cathode during a charge and discharge cycle. The so-called 'unit-cell breathing' is clearly visible, due to insertion of lithium ions in between the cobalt oxide layers which results in peak shift; but also the fact that part of the cathode material is not participating in the charge-discharge cycle and therefore reduces the charge capacity of this specific cathode. This information is of great value to battery manufacturers since understanding it can result in better batteries to fuel our future devices and cars.
 T. Degen et al. (2014), Powder Diffraction, 29 (S2), S13-S18.
 M. Jansen (1976), Journal of Solid State Chemistry, 17, 1-6.
 C. Galez et al. (2006), Journal of Alloys and Compounds, 416, 261-264.
 V. K. Peterson et al. (2017), IUCrJ, 4, Part 5, 540-554.