Effect of bond on negative thermal expansion of Prussian blue analogues MCo(CN)6 (M = Fe, Ti and Sc): a first-principles study

Negative thermal expansion (NTE) is an abnormal physical behavior that has promising applications for high precision thermal control. Since Prussian blue analogues have the two central linking atoms of -C≡N-, they have large structure flexibility and are suitable to explore new NTE materials. However, understanding the nature of structure flexibility from the point of view of chemical bonding is important and urgent. Here, we adopt for the first time first-principles calculations to predict that the cubic TiCo(CN)₆ and ScCo(CN)₆ have NTE behavior. The calculated results for M in MCo(CN)₆ (M = Fe, Ti and Co) indicated that the Sc-N bond is the strongest, but with the weakest direction dependence among the M-N bonds in the three systems. The lattice dynamics calculations results revealed that the low-frequency phonon vibration modes for NTE in MCo(CN)₆ have much stronger relationship with the M-N bond feature. The present work reveals the important role of the related bond in the NTE open-framework materials.