Negative Thermal Expansion of Undulating Coordination Layers through Interlayer Interaction.

The negative thermal expansion (NTE) of solid-state materials is of significance in various fields, but a very rare phenomenon. In this study, we carried out a meta-analysis for the anisotropic thermal expansion behavior of fifteen two-dimensional coordination polymers [M(salen)]2[M'(CN)4(solvent)] (M = Mn, Fe; M' = MnN, ReN, Pt, Pt(I2)x; x = 0.18, 0.45, 0.85, 1.0; solvent = H2O, MeOH, MeCN) with a newly synthesized [Fe(salen)]2[MnN(CN)4(MeCN)]. Consequently, we successfully demonstrate the unusual NTE of the undulating coordination layers by an expansion deformation of the layers via strong interlayer interaction within the layer stacking. Notably, the layer volume of [Mn(salen)]2[ReN(CN)4] with its powder form decreases with a large NTE coefficient, αlayer-volume = -27 × 10-6 K-1 (100-500 K). This is a significantly large value despite the increase in layer thickness along the layer contraction based on the anisotropic transformation of undulating layers. Conversely, the analysis demonstrates that the chemical modification of the layers to enhance intralayer interaction rather than interlayer interaction switches a direction of the layer anisotropy, yielding positive thermal expansion materials with the coefficient of the layer volume reaching +92 × 10-6 K-1.