Uniaxial negative thermal expansion behavior of β -CuSCN

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.