Distinct Jahn-Teller–induced negative thermal expansion behaviors in Cr2P2O7

Negative thermal expansion (NTE) is an intriguing phenomenon that is not only scientifically interesting but also technologically significant. Herein, using temperature-dependent high-resolution synchrotron radiation x-ray diffraction, together with dilatometry and specific heat capacity analyses, we systematically study the NTE behaviors of $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{P}}_{2}{\mathrm{O}}_{7}$ and the new physical mechanisms beyond the low-frequency phonon vibration modes within a framework structure. We show that $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{P}}_{2}{\mathrm{O}}_{7}$ can exhibit successively near zero and giant NTE from 100 to 320 K and 320 to 355 K, respectively. In the temperature range of giant NTE, we further clear that giant NTE behavior originates from the intense repulsion between Cr-Cr atoms and result in abrupt contraction along both the a- and c-axes, respectively. However, contrary to the current understanding of NTE in oxides, we confidently confirm that the transverse vibrations of diphosphate bridging oxygen in $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{P}}_{2}{\mathrm{O}}_{7}$ have no contribution to such giant NTE through a detailed analysis of the correlation between temperature dependent P-O-P angles and Cr--Cr distance. Instead, the Jahn-Teller distortion in $\mathrm{Cr}{\mathrm{O}}_{6}$ octahedra is responsible for the unique thermal expansion behaviors totally. These findings not only extend the NTE family in framework structure materials and new NTE mechanisms, but also propose a new approach to realize NTE materials with the Jahn-Teller effect at the level of electron spin.