Uniaxial negative thermal expansion behavior of <b> <i>β</i> </b>-CuSCN
Jiaqi Wang, Qilong Gao, Yaxing Gao, Yan Luo, Juan Guo, Qiang Sun, Erjun Liang
Abstract
Negative thermal expansion (NTE) as an interesting physical behavior is promising for thermal expansion control engineering. β-CuSCN consists of linear chain units with NTE along the c-axis. The NTE mechanism of β-CuSCN is investigated by variable temperature x-ray diffraction, temperature- and pressure-dependent Raman spectra, and first-principles calculations. It is found that the quasi rigid unit modes associated with the rotations of S–C≡N–Cu chains driven by Cu and S antiphase transverse vibrations and longitudinal acoustic and transverse acoustic modes involving the collective motions of atoms have large negative Grüneisen parameters, contributing significantly to the NTE of c-axis. Translational and librational motions of C≡N units, in which C and N atoms vibrate in the same and opposite directions have much smaller negative Grüneisen parameters, contribute only a minor part to the NTE, which is different from the known NTE mechanism of cyanides and Prussian blue analogous.