Understanding the role of guest ions in the control of thermal expansion of FeFe(CN)6

Understanding the role of guest species in framework materials is important to successfully achieve the control of thermal expansion. In this work, the thermal expansion of the Prussian blue analogue FeFe(CN)6 has been effectively tailored from negative, to zero to positive through the insertion of Na+ ions. To shed light on the role of Na+ ions on thermal expansion behavior, a joint study was conducted by means of high-resolution synchrotron X-ray diffraction, EXAFS spectroscopy and lattice dynamics calculations. It has been found out that the insertion of Na+ ions reduces the structural flexibility of the CN linkages and affects the low-energy phonons, thus allowing the tuning of the overall thermal expansion. This work demonstrates the possibility of precisely controlling the thermal expansion of open-framework materials by inserting guest ions and clarifies the physical mechanism underlying this control.