Insight into the Relationship between Negative Thermal Expansion and Structure Flexibility: The Case of Zn(CN)₂-Type Compounds

Abstract

High structure flexibility can lead to large negative thermal expansion (NTE), but the reason is not clear. In this work, first-principles calculations have been carried out to investigate the relationship between NTE and structure flexibility in Zn(CN) 2type compounds. Smaller bulk modulus corresponds to larger compressibility, thus making the crystal structure more flexible and more suitable for NTE. It indicated that the ionic nature of the bond and the bond length jointly affect the structural flexibility and then act on the transverse vibration of C and N atoms. The results of lattice dynamic suggested that higher structural flexibility promotes a greater number of low-frequency optical modes with negative Gruneisen parameters, resulting in a larger NTE. This work also gives us new insight into the design of NTE materials.

Origin of Near-Zero Thermal Expansion in A2O(PO4)2 Oxides over an Ultrawide Temperature Range

“…From the calculated charge densities in the [1 1 0] and [−1 1 1.7] planes with a contour interval of 0.1 e/Å 3 (Figure ), it is clear that the A–O1, A–O2, and A–O4 bonds are all ionic bonds due to the low charge density between the atom pairs. The nature of ionic bonds is jointly influenced by the bond length and the electronegativity difference between atoms . Since the electronegativity of these four A elements is very close (Th (1.3), U (1.38), Zr (1.33), and Hf (1.3)), this means that the bond length mainly influences the strength of the A–O ionic bonds.…”