Understanding Large Negative Thermal Expansion of NdFe(CN)₆ through the Electronic Structure and Lattice Dynamics

Abstract

A large negative thermal expansion (NTE) (αv = −4.1 × 10–5 K–1, 100–525 K) has been discovered in NdFe­(CN)6. Here, the synchrotron X-ray diffraction and lattice dynamics calculations using the density functional theory were conducted to understand the NTE in NdFe­(CN)6. The information obtained on the bond nature of the Nd–NC–Fe linkage and on the atomic thermal vibrations suggests that the transverse vibrations of the −NC– group, in particular from N atoms, produced the NTE in NdFe­(CN)6. This is corroborated by the calculated Grüneisen parameters, which confirm the relationship between NTE and CN atomic vibrations. The results provide a helpful contribution toward the realization of new materials with negative or controllable thermal expansion.

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

“…Moreover, the perpendicular MSD of O atoms decreases sequentially, i.e., Th 2 O­(PO 4 ) 2 > U 2 O­(PO 4 ) 2 > Zr 2 O­(PO 4 ) 2 > Hf 2 O­(PO 4 ) 2 , implying a progressive decrease in the transverse vibrations of O atoms, like the order of the AAV. The fundamental role of transverse vibrations in NTE is well-recognized in framework materials, such as ZrW 2 O 8 , ScF 3 , NdFe­(CN) 6 , etc. Therefore, Zr 2 O­(PO 4 ) 2 and Hf 2 O­(PO 4 ) 2 , due to their reduced structural flexibility and smaller transverse vibrations of O atoms, exhibit low PTE in contrast to the NTE shown by Th 2 O­(PO 4 ) 2 and U 2 O­(PO 4 ) 2 , which have more structural flexibility and larger transverse vibrations.…”