Intrinsic volumetric negative thermal expansion in the "rigid" calcium squarate.

The calcium squarate with a rigid framework is found to exhibit volumetric negative thermal expansion (NTE) with the coefficient -9.51(5) × 10-6 K-1 and uniaxial zero thermal expansion (ZTE, -0.14(4) × 10-6 K-1) over a wide temperature. Detailed comparison of the long-range and local structure sheds light on the fact that the anomalous thermal expansion originates from the transverse vibration of the bridging squarate ligand, although it has been tightly bonded by five calcium ions. We believe that this study can provide a deep insight into the origin of NTE and the structural flexibility of metal organic frameworks (MOFs).