  Infrared spectra of a K4C60 single-phase thin film have been measured between
room temperature and 20 K. At low temperatures, the two high-frequency T1u
modes appear as triplets, indicating a static D2h crystal-field stabilized
Jahn-Teller distortion of the (C60)4- anions. The T1u(4) mode changes into the
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.
