Transition metal pyrophosphates $A_2{M}_2{\mathrm{O}}_7$ ($A=\mathrm{Zn}$, Cu, Mg, Fe, Co, Ni, Cr) have recently gained significant attention owing to their giant negative thermal expansion (NTE) properties and the relevant applications. Of them, $\mathrm{Z}{\mathrm{n}}_2{\mathrm{P}}_2{\mathrm{O}}_7$ serves as a model one with ${\mathrm{A}}^{2+}$ ions being Jahn-Teller inactive, exhibits a very low thermal expansion in the α phase at low temperature and giant NTE during $\alpha \to \beta$ phase transition. Yet, the underlying mechanism remains unclear. With temperature-dependent x-ray diffractions (XRD) and Rietveld analysis, a new concept of cooperative pseudo Jahn-Teller effect (PJTE) was proposed to account for such unique thermal expansion properties. Detailed analysis shows the strong interactions between neighboring $\mathrm{Z}{\mathrm{n}}^{2+}$ ions, which result in the abrupt elongation of Zn1-O10 in Zn3-O4 bond lengths in neighboring $\mathrm{Zn}1{\mathrm{O}}_6$ and $\mathrm{Zn}3{\mathrm{O}}_6$ octahedra, and hence the splitting of the hexagonal rings along shared O4-O10 edges and a distinct volume increase upon lowering the temperature during a $\beta \to \alpha$ phase transition. Further density functional theory calculations confirm that the $d$-electron energy levels of Zn split and tend toward lower energy states under the phase transition, which facilitates deformation and leads to the NTE. Moreover, owing to the strong cooperative PJTE, $\mathrm{Z}{\mathrm{n}}_2{\mathrm{P}}_2{\mathrm{O}}_7$ is confined in the adiabatic potential energy surface (APES) well, giving rise to the very low thermal expansion behavior in the low temperature α phase.

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• Electronic structure
• Jahn-Teller effect
• Lattice dynamics
• Phase transitions
• Pseudo Jahn-Teller effect
• Thermal expansion
• Ordered compounds
• Transition metals
• Density functional calculations
• X-ray diffraction