Metal-insulator and structural phase transition observed by ESR spectroscopy and x-ray diffraction in KC60

We have performed electron spin resonance (ESR) spectroscopy and x-ray diff raction experiments at low temperature on KC60 single crystals. ESR data re veal the occurrence of a metal-insulator phase transition at about 50 K. In the same temperature range, we observe the stabilization of a superstructu re which doubles the volume of the unit cell. We suggest that displacements of the K atoms and a modulation of the C-60 charge may be involved in the mechanism of this phase transition. These results shed new light on the sub tle interplay of structure, dimensionality, and electronic properties in th e AC(60) fullerides.