Elastic properties of carbon phases obtained from C₆₀ under pressure: the first example of anisotropic disordered carbon solid

We observe an anisotropy of the propagation velocities of longitudinal and transverse ultrasonic waves, as well as of the hardness, for disordered graphite-like samples obtained from the C₆₀ fullerite, which is heated to different temperatures under a pressure of 7.5 GPa. The anisotropy of the elastic properties and the hardness is connected to the additional pressure component that occurs in the quasi-hydrostatic experimental conditions. The elastic characteristics of the samples are determined. We propose a model description relating the observed properties of superhard sp² carbon to its possible structural features and to the mechanism of its formation.