Jahn–Teller distortions induced strong negative thermal expansion in <i>α</i> -Cu <sub>2</sub> V <sub>2</sub> O <sub>7</sub>
Xiangkai Hao, Shibo Zhao, Qilong Gao, Yongqiang Qiao, Andrea Sanson, K Matan, G Gitgeatpong, Qiang Sun, Juan Guo, Feng Jin, Lijun HE, Shogo Kawaguchi, Erjun Liang, Jun Chen
Abstract
Abstract Negative thermal expansion (NTE) refers to volume contraction upon heating, but the intrinsic complexity of its physical mechanisms presents a fundamental challenge. α -Cu 2 V 2 O 7 exhibits significant anisotropic NTE over a wider temperature range; however, its NTE mechanism has not been clearly elucidated. Herein, we systematically investigate the NTE mechanism of α -Cu 2 V 2 O 7 using 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-CuO 6 octahedra. 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-CuO 6 octahedra compresses the Cu···Cu zigzag chains and reduces the spacing between orthogonal chains, resulting in the NTE behavior of α -Cu 2 V 2 O 7 . 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 in α -Cu 2 V 2 O 7 . These findings could help the community advance the understanding of NTE in anisotropic materials.