Ordered low-temperature structure in K 4 C 60 detected by infrared spectroscopy

Infrared spectra of a ${\mathrm{K}}_{4}{\mathrm{C}}_{60}$ single-phase thin film have been measured between room temperature and 20 K. At low temperatures, the two high-frequency ${T}_{1u}$ modes appear as triplets, indicating a static ${D}_{2h}$ crystal-field stabilized Jahn-Teller distortion of the ${\mathrm{C}}_{60}{}^{4\ensuremath{-}}$ anions. The ${T}_{1u}(4)$ mode changes into a known doublet above 250 K, a pattern which could have three origins: a dynamic Jahn-Teller effect, static disorder between ``staggered'' anions, or a phase transition from an orientationally ordered phase to one where molecular motion is significant.