Elastic modules of solid C60: measurement and relationship with nanostructure

Abstract

Elastic moduli of solid C60 have been measured in single crystal of primary fullerite and diverse metastable phases of polymerized and amorphous C60 synthesized under high-pressure high-temperature (HPHT) conditions within the wide range of pressure p=5–13GPa and temperature T=500–1870K. Small sizes and heterogeneity of HPHT samples admit application of the specific microacoustic technique. A focused ultrasonic beam and ultra-short probing pulses have been employed to measure sonic velocities and study elastic module within an area of 100–300μm in diameter. Ultrasonic measurements have revealed permanent increase of elastic module of HPHT phases when there is rise in pressure and temperature. The bulk elastic module K and shear module G are compared with those in diamond for three-dimensional (3D) polymerized C60. Amorphous C60 produced at the highest pressure p=13GPa and temperature T=1600–1900K possesses unique mechanical properties which includes the highest value of the longitudinal sound velocity (cL=26km/s) among substances now known and values of the bulk module (K≈700–800GPa) substantially greater than the bulk elastic module in diamond (K≈440∼490GPa). Essential elastic heterogeneity of the most HPHT states has been shown including smooth distribution of elastic properties over a specimen for crystalline polymerized phases.Experimental results as well as model calculations developed on the base of the finding evidence that elastic properties correlate with state of fullerene molecules, character and number of covalent bonds and nanostructure of the matter. Measuring elastic properties is one of the effective ways to get information on nanostructure of solid C60, to classify and identify its diverse phases.