Anomalous thermal expansion of strontium squarate trihydrate induced by hydrogen-bond weakening

Blending of various chemical bonds with different strengths within a framework is expected to induce some anomalous physical properties. Herein, a novel metal squarate framework containing covalent bonds, coordination bonds and hydrogen bonds was synthesized and it showed a uniaxial negative thermal expansion (NTE). A combination of in situ powder/single-crystal X-ray diffraction, Raman spectroscopy and density-functional theory (DFT) calculations revealed that this anomalous thermal expansion originates from a hydrogen bond-triggered wine-rack-like distortion. This study can provide a new strategy for the rational design of hybrid NTE compounds.