C₆₀ crystals containing C₇₀ at various dopant levels, from 0 to 10%, have been grown via the novel method of 'solvent dilution'. Their crystallographic structures were examined with X-ray diffractometry, and their order-disorder phase transitions with differential scanning calorimetry. An extra HCP lattice structure was clearly seen in more highly doped C₆₀. On the other hand, both the phase transition temperature and the heat of transition decreased with rising C₇₀ concentration. This suppression of the order-disorder transition may be explained by a weakening of the correlation among C₆₀ molecules due to their dilution by the dopant.