Negative thermal expansion in cubic FeFe(CN)6 Prussian blue analogues

Negative thermal expansion (NTE) behavior is an interesting physical phenomenon, but the number of NTE materials is limited. In this study, a new NTE compound has been found, FeFe(CN)6 Prussian blue analogue, where the average linear coefficient of thermal expansion (αl) is −4.260 × 10−6 K−1 between 100 and 450 K. The NTE properties and local vibration dynamics have been investigated by joint experiments of synchrotron X-ray diffraction, X-ray pair distribution function, and extended X-ray absorption fine structure spectroscopy. It has been observed that the Fe–C/Fe–N bonds expand with increasing temperature, while the unit cell shrinks in FeFe(CN)6. The vibration directions of both Fe–C and Fe–N prefer to be perpendicular to the linkage of Fe–CN–Fe rather than being parallel. More pieces of evidence indicate that the transverse vibrations of N atoms dominate the NTE behavior of FeFe(CN)6. The present results prove directly that the transverse thermal vibrations of C and N atoms are crucial for the occurrence of the NTE of Prussian blue analogues.