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.…