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 (α_V ∼ -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.