Phase transition in K2C2 as studied by synchrotron X-ray powder diffraction and solid-state 13C NMR spectroscopy

Abstract

The motion of the C₂ dumbbells in K₂C₂ in the low-temperature (LT-K₂C₂, I4₁/acd, Z = 8) and high-temperature (HT-K₂C₂, Fmm, Z = 4) modifications was examined by a combination of synchrotron powder diffraction and ¹³C solid-state NMR spectroscopy. For LT-K₂C₂, the experimental data are consistent with a motion restricted to a double cone with the cone angle becoming wider with increasing temperature. In HT-K₂C₂, the C₂ dumbbells are less restricted and undergo a fast reorientation that averages out the large chemical shift anisotropy (292 ppm) found in LT-K₂C₂. Hence, the disorder observed in XRPD experiments is caused by this dynamic process with a characteristic time constant much shorter than 4 μs. However, a distinction between an isotropic free rotation (Pauling model) and a random exchange process between distinct directions consistent with cubic symmetry (Frenkel model) is not possible on the basis of the experimental data obtained.