Near‐Zero Thermal Expansion in Coordination Polymer Cd(1,2,4‐Triazole) ₂ (H ₂ PO ₄ ) ₂

Abstract

Abstract 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 (H 2 PO 4 ) 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.