Strong Negative Thermal Expansion in a Low-Cost and Facile Oxide of Cu2P2O7.

Negative thermal expansion (NTE) behaviors have been observed in various types of compounds. The achievement in the merits of promising low-cost and facile NTE oxides remains challenging. In the present work, a simple and low-cost Cu2P2O7 has been found to exhibit the strongest NTE among the oxides (αV ∼ -27.69 × 10-6 K-1, 5-375 K). The complex NTE mechanism has been investigated by the combined methods of high-resolution synchrotron X-ray diffraction, neutron powder diffraction, X-ray pair distribution function, extended X-ray absorption fine structure spectroscopy, and density functional theory calculations. Interesting, the direct experimental evidence reveals that the coupling twist and rotation of PO4 and CuO5 polyhedra are the inherent factors for the NTE nature of Cu2P2O7, which is triggered by the transverse vibrations of oxygen atoms. The present new NTE material of Cu2P2O7 also has been verified for the practical application.