Jahn-Teller distortions induced strong negative thermal expansion inα-Cu₂V₂O₇

Negative thermal expansion (NTE) refers to volume contraction upon heating, but the intrinsic complexity of its physical mechanisms presents a fundamental challenge.α-Cu₂V₂O₇exhibits significant anisotropic NTE over a wider temperature range; however, its NTE mechanism has not been clearly elucidated. Herein, we systematically investigate the NTE mechanism ofα-Cu₂V₂O₇using neutron powder diffraction, synchrotron radiation x-ray diffraction, and temperature- and pressure-dependent Raman spectra, and density functional theory calculations across 5-800 K. The structure exhibits a second-order Jahn-Teller (SOJT) effect, which is the primary cause of off-centering within the quasi-CuO₆octahedra. As temperature increases, the SOJT effect weakens, reducing the distortion of the driving force for off-centering; this causes the Cu atoms to shift in opposite directions, increasing symmetry. The anti-off-centering displacement of the Cu atoms toward the O_4(long)atoms in the quasi-CuO₆octahedra compresses the Cu···Cu zigzag chains and reduces the spacing between orthogonal chains, resulting in the NTE behavior ofα-Cu₂V₂O₇. This study reveals a novel mechanism whereby the SOJT effect governs the displacement and symmetry of Cu atoms, providing crucial insight into the origin of NTE behavior inα-Cu₂V₂O₇. These findings could help the community advance the understanding of NTE in anisotropic materials.