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 (2D) 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 (α_a, α_c∼22 MK^-1) in the ac plane and negative thermal expansion (NTE) in the b direction (α_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 modeling 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 substitution 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.