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

Abstract

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 exibility and are suitable to explore new NTE materials. However, understanding the nature of structure exibility from the point of view of chemical bonding is important and urgent. Here, we adopt for the rst time rst-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.

Effect of H2O Molecules on Thermal Expansion of TiCo(CN)6

“…The thermal expansion difference of the Ti–N bond is much larger than that of the Co–C bond, indicating that the NTE driving force of TiCo­(CN) 6 is mainly the transverse thermal vibrations of the Ti–N bonds. These results were also consistent with the previous report; i.e., the total charge density in the (001) plane of TiCo­(CN) 6 shows that the Co–C bonds are weak covalent, while the Ti–N bonds display a strong electrovalent bonding nature. With H 2 O molecule insertion, the “apparent” thermal expansion of the Ti–N bond is switched to the PTE behavior in TiCo­(CN) 6 ·2H 2 O, while thermal expansion of the Co–C bonds displays the same behavior as that of TiCo­(CN) 6 , suggesting that the H 2 O molecules have an effect mainly on the chemical bond of Ti–N.…”

“…30 i.e., the total charge density in the (001) plane of TiCo(CN) 6 shows that the Co−C bonds are weak covalent, while the Ti−N bonds display a strong electrovalent bonding nature. With H 2 O molecule insertion, the "apparent" thermal expansion of the Ti−N bond is switched to the PTE behavior in TiCo(CN) 6 • 2H 2 O, while thermal expansion of the Co−C bonds displays the same behavior as that of TiCo(CN) 6 , suggesting that the…”

Octahedral tilting in Prussian blue analogues

“…This is interesting in its own right, as transverse low-energy phonons—such as tilts—typically underlie negative thermal expansion. 72–74…”

“…84,85 Yet, there are no confirmed cases of such distortions and a computational study found these modes, particularly the unconventional tilt, to be more energetically unfavourable compared to conventional tilts. 72 This is presumably due to the strong preference for a linear M′–C–N arrangement, as discussed above, whereas the unconventional distortions require nonlinear binding at both ends of the linker. 86 While this simplifies the study of distortions in PBAs, as only conventional modes need to be considered, it also limits the diversity of possible distortions.…”

Theoretical Study of Abnormal Thermal Expansion of CuSCN and Effect on Electronic Structure

“…Detailed discussion of the QHA can be found in the references (Kuzkin and Krivtsov, 2015). In our previous work, the QHA provided a reasonable description of the dynamic properties of the compound (Liu et al, 2015;Chang et al, 2018;Li et al, 2020).…”