Significant Enhancement of Negative Thermal Expansion Under Low Pressure in Cu₂P₂O₇

Much effort is made to achieve the negative thermal expansion (NTE) control, but rare methods reached the improvement of intrinsic NTE. In the present work, a significantly enhanced NTE is realized in Cu₂P₂O₇ by applying low pressure. Especially, the volumetric coefficient of thermal expansion (CTE) of Cu₂P₂O₇ reached to -50.0 × 10^-6 K^-1 (150-325K) under 0.25 GPa, which is increased by 47.5% compared to its NTE in a similar temperature range under atmosphere pressure. This character enables a more effective manifestation of the thermal compensation role of Cu₂P₂O₇ in composites. The enhanced NTE mechanisms are analyzed by high pressure synchrotron X-ray diffraction, neutron diffraction at variable temperature and pressure, as well as density functional theory (DFT) calculations. The results show that applied pressure accelerates the contraction of the distance between adjacent CuO layers and CuO columns. Meanwhile, the low-frequency phonon contribution to NTE in α-Cu₂P₂O₇ is improved. This work is meaningful for the exploration of methods to enhance NTE and the practical application of NTE materials.