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

The motion of the C-2 dumbbells in K2C2 in the low-temperature (LT-K2C2, I4(1)/acd, Z=8) and high-temperature (HT-K2C2, Fm (3) over barm, Z=4) modifications was examined by a combination of synchrotron powder diffraction and C-13 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 C-2 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 mus. 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.