Structure and microhardness of low pressure polymerized fullerite C-60

We have carried out low-temperature x-ray diffraction studies on C-60 fulle rite polymerized by low quasi-hydrostatic pressure of 1.1 GPa at T=563 K. I t is established that at room temperature in freshly prepared samples three phases mainly coexist, viz. a compressed cubic phase with the lattice para meter a = 13.94 Angstrom, an orthorhombic O' phase with the lattice paramet ers a = 9.12 Angstrom, b = 9.82 Angstrom, c = 14.60 Angstrom, and a rhomboh edral phase of symmetry R3m with the parameters a = 9.20 Angstrom and c = 2 4.27 Angstrom. Mechanical grinding or annealing at 573 K entails depolymeri zation of sintered samples and restoration of the fee structure of pristine C-60. During annealing in air, intercalation of fullerite lattice by oxyge n molecules occurs as well as a substantial amount of some new phase is for med, most probably with tetragonal symmetry, the chemical composition and s tructure of which have not been determined. The microhardness of polymerize d C-60 is higher than that of single crystal samples roughly four-fold at r oom temperature and by a factor of 2.6 at liquid nitrogen temperature. Anal ysis shows that polymerization and grain boundaries give contributions to t he microhardness of sintered samples but we did not succeed in separating t hese two contributions. The temperature dependence of the microhardness of polymerized samples exhibits a jump in the vicinity of 260 K, where pristin e C-60 fullerite undergoes the fee-sc phase transition. We think that this jump is due to a partial destruction of the polymerized state under indento r as a result of shear straining in inhomogeneous stress fields. Annealing of polymerized C-60 at a temperature of 573 K, which restores the fee phase , leads to a considerable decrease in microhardness, the hardness "jump" ne ar the fee-sc transition extends in temperature and shifts to lower tempera tures. (C) 1998 American Institute of Physics. [S1063-777X(98)00912-8].