The negative thermal expansion behavior in Prussian blue analogue Zn3[Fe(CN)6]2: A first-principles study

We have studied the electronic and thermal expansion properties in Prussian blue analogue (PBA) Zn3[Fe(CN)6]2 by first-principles calculation. The results show that Zn3[Fe(CN)6]2 is an insulator with an indirect band gap. The composition of valence band maximum (VBM) and conduction band minimum (CBM) can be obtained. It is demonstrated that Fe-C and Zn-N are strong ionic bonds and C-N is a strong covalent bond combining qualitative and quantitative analysis. The negative thermal expansion (NTE) behavior has been confirmed. It is found that low-frequency phonons play a significant role. Especially, acoustic phonons have the most negative mode/total Grüneisen parameter, suggesting their greatest contribution to the NTE behavior. From the mode analysis of low-frequency optical phonons, we can know that the NTE behavior is closely associated with transverse vibration in the same direction of bridge atoms (C and N atoms) as well as polyhedral movements and distortions (FeC6 octahedron and ZnN4 tetrahedron).