A new explanation for negative thermal expansion

Most materials expand when heated because increased atomic vibrations push atoms slightly farther apart. However, some unusual materials, such as α‑Cu₂V₂O₇, instead shrink when heated, a phenomenon known as negative thermal expansion. Although this behaviour had been observed before, its underlying mechanism was not well understood. In this study, the researchers examined α‑Cu₂V₂O₇ from 5 K to 800 K using neutron diffraction, synchrotron X‑ray diffraction, Raman spectroscopy, and first‑principles calculations. They found that the material exhibits three distinct thermal‑expansion regimes: almost no expansion below 35 K, strong negative thermal expansion between 35 K and 550 K, and normal positive expansion above 550 K.

The origin of this behaviour lies in how copper atoms move within distorted CuO₆ like octahedra. At the lowest temperatures, a quantum effect called the second‑order Jahn-Teller effect pushes the copper atoms off‑centre, but this motion is partly suppressed by the onset of antiferromagnetic ordering, which stabilises the structure and produces near‑zero thermal expansion. As the temperature increases, the second‑order Jahn-Teller effect weakens, allowing the copper atoms to shift back toward the centre of their octahedra, but in opposite directions along different structural chains. This anti‑off‑centering motion compresses the Cu-Cu zigzag chains and also reduces the spacing between neighbouring chains, pulling the structure inward and producing the observed negative thermal expansion.

The researchers also found that the copper atoms have unusually large vibrational freedom along one axis, which helps enable this motion. Raman spectroscopy revealed an anomalous broadening of a low‑frequency vibrational mode, providing evidence for electron-phonon coupling that further supports the proposed mechanism. Together, these effects explain the unusual thermal behaviour of α‑Cu₂V₂O₇ and offer valuable insight for designing materials with controlled thermal expansion, which is important for precision engineering, electronics, and composite materials that must remain dimensionally stable across temperature changes. Meanwhile, this mechanism, centered on the Jahn–Teller effect, can be extended to a wide range of transition metal oxide systems, providing a universal theoretical foundation for systematically explaining the anomalous thermal expansion behavior of such materials.

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Negative thermal expansion and associated anomalous physical properties: review of the lattice dynamics theoretical foundation by Martin T Dove and Hong Fang (2016)

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