The enhanced negative thermal expansion in less-oxygen-vacancies copper pyrophosphate

For oxides, controlling the concentration of oxygen vacancy is a useful way to optimize their functional properties. However, when it comes to the field of negative thermal expansion (NTE), much less attention has been paid to the effect of oxygen vacancy on NTE, though oxide-typed NTE materials account for more than 40% of the total NTE materials. Here, we report that the linear coefficient of thermal expansion at 250–350 K of copper pyrophosphate (i.e., Cu 2 P 2 O 7 ) can be significantly improved from –13.88 to –36.60 ppm/K when the synthesis temperature is raised from 1073 to 1373 K. The combined study including X-ray absorption near edge structure, neutron powder diffraction, and X-ray diffraction has confirmed the selective vacancies in the O1 site for low-temperature synthesized samples, which suppress both the phase-transition and framework-structure driven NTE simultaneously. Our result proposes a new approach for optimizing the NTE effect of oxides.