Near-zero Thermal Expansion in Coordination Polymer Cd(1,2,4-triazole)2(H2PO4)2

Zero thermal expansion is a rare but desirable physical property for materials. Here, we report an unprecedented near-zero thermal expansion (nZTE) behavior in a two-dimensional coordination polymer (CP) Cd(1,2,4-triazole)2 (H2PO4)2 (Cd-Tz) across a broad temperature range of 25 K - 400 K. Using multi-temperature high-resolution single-crystal X-ray diffraction, we investigated the structural dynamics of the wine-rack-like framework of Cd-Tz and compared it with that of positive thermal expansion (PTE) CPs Zn-Tz and Mn-Tz with similar framework topology. We show that nZTE in Cd-Tz is a consequence of two monotonic and opposing trends with PTE (a, c ~ 22 MK-1) in the ac plane and negative thermal expansion (NTE) in the b direction (b ~ -47 MK-1). A new mechanism for the uniaxial NTE is established based on concerted out-of-plane single atom libration of similarly oriented adjacent triazole rings in the wine-rack motifs. X-ray electron density analysis and modelling of low-temperature vibrational anharmonicity reveals the role of Cd-triazole bonds and hydrogen bonds in promoting the single atom libration. Tuning the metal−ligand bonding strength through minor doping of Zn/Mn (~8/12 %) in the Cd-Tz lattice results in a prominent shift from nZTE to NTE behavior in the solid solution phases.