J. Am. Chem. Soc. http://doi.org/qrw (2013)

The discovery of iron-based superconductors has inspired chemists and physicists to join forces to optimize their superconducting properties. One promising strategy is to change the chemical composition of the spacer layer between the iron-containing 'superconducting' layers. For example, iron selenide (FeSe) has a bulk superconducting temperature (Tc) of 8.5 K, but when placed in solutions of alkali metals in liquid ammonia it reacts to produce intercalates with Tc of up to 45 K. Now, Stefan Sadlmaier and colleagues present a method to monitor this reaction in situ, using time-resolved X-ray diffraction. They identify the metal–ammonia intercalates with increased Tc, but their technique also allows them to show that these are decomposition products from a fleeting phase that is much richer in ammonia. Moreover, they find that the absorption and desorption of ammonia in these intercalates occurs reversibly at room temperature. More generally, these results demonstrate the power of modern diffractometry in characterizing fleeting phases of matter, which should bode well for future studies of superconducting materials.