Semimetallic and semiconductor properties of some superhard and ultrahard fullerites in the range 300-2 K

Electrical resistivity and magnetoresistance were measured on samples with disordered structures synthesized from pure C-60 and C-70 at pressures in t he range 8-12.5 GPa and temperatures of 900-1500 K. Different types of beha viour were observed: semimetallic, VRH and semiconducting, depending on the degree of disorder and the particular short-range order of the samples. A negative magnetoresistance was observed at T < 10 K on samples with a semim etallic type of conductivity synthesized at 8 cpa pressure. The temperature dependence of resistivity in the sample with a disordered crystalline stru cture based on 3D-polymerized C-60 molecules fits Mott's law for hopping co nductivity. T-3/2, T-2 and T-4 dependencies of conductivity are observed fo r samples with densities of 2.8 and 3.05 g/cm(3) synthesized at a pressure of 12.5 GPa, The effect of hydrostatic pressure on the resistivity of cross -linked layered carbon structures obtained from C-60 at P = 8 GPa, T = 1600 K was investigated up to 0.6 GPa at room temperature. An approximately lin ear decrease of resistivity was observed with a very small value of the der ivative d ln rho/d rho = 0.06 /GPa, which correlates with a very low compre ssibility of the material. (C) 2000 Elsevier Science Ltd. All rights reserv ed.