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.

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• Jahn-Teller effect
• Lattice dynamics
• Phase transitions
• Thermal expansion
• Specific heat measurements
• X-ray diffraction