Orientational ordering in C₇₀. Evidence for three distinct phase transitions

Abstract

Variable-temperature X-ray diffraction studies of C₇₀ suggest the occurrence of two phase transitions around 350 and 280 K where the high-temperature phase is fcc and the low-temperature phase is monoclinic, best described as a distorted hcp structure with a doubled unit cell; two like-phases (possibly hcp) seem to coexist in the 280-350 K range. Application of pressure gives rise to three distinct transitions associated with characteristics pressure coefficients, the extrapolated values of the transition temperatures at ambient pressure being around 340, 325 and 270 K. Pressure delineates closely related phases of C₇₀ just as in the case of C₆₀ which exhibits two orientational phase transitions at high pressures.