THE COMPRESSIBILITY OF RB3C60 DERIVED BY X-RAY EXPERIMENTS UNDER HIGH-PRESSURE

The pressure dependence of the lattice parameter a(p) of Rb3C60 has be en investigated at 300 K and pressures up to 6 GPa using a diamond anv il cell and angular dispersive X-ray scattering. Rb3C60 shows an anoma lous increase of the bulk modulus under pressure, similar to the C-60 fullerite [1]. This behaviour is caused by the extremely small compres sibility of the C-60 molecules, which can be considered by a modificat ion of a standard EOS. The experimental a(p) data of Rb3C60 are in an excellent agreement with a modified Birch equation, resulting in a bul k modulus of 20.5 GPa. Starting with the obtained modified EOS of Rb3C 60 we can calculate the T-c(a(p)) behaviour of this material by using the experimental T-c(p) data of Sparn et al. [2]. In contrast to Zhou et al. [3] the obtained T-c(a(p)) dependence of Rb3C60 is linear in th e entire investigated pressure range and agrees with the T-c(a(M)) val ues from several superconducting M(3)C(60) compounds.