W0029: Structure Studies on and Superconductivity in the Polymeric Phase of Na2CsC60

Structure Studies on and Superconductivity in the Polymeric Phase of Na₂CsC₆₀, B. Morosin and J. E. Schirber, Sandia Nat’l Lab, Albuquerque, NM; G. H. Kwei, Lawrence Livermore Nat’l Lab, Livermore, CA; J. D. Jorgensen, Argonne Nat’l Lab, Argonne, IL.

Neutron diffraction and complementary superconducting measurements provide a clear and unambiguous demonstration that the single C-C bonded polymeric phase in Na₂CsC₆₀, accessible by pressure, is a superconductor with the transition temperature about 3 K lower than that of its Pa3 phase.

Margadonna et al. [J. Sol. State Chem. 145 471,1999] reported in some detail the evolution of the structure of Na₂CsC₆₀ with temperature and pressure. These results are in general in good agreement with our earlier study [Morosin, et al., Recent Adv. in Chem. and Phys.of Fullerenes and Related Matl 3 (1998) 446], including the onset of the polymeric phase near 0.6 Gpa. However, the two studies differ primarily in the phase purity of the sample above the pressure induced transition, with our data not showing any evidence of the Pa3 phase. Rietveld structure refinements on various points in our p-T phase used a crystal structure model consisting of single C-C bonded polymeric chains of -C₆₀^3—ions, paralleling the c-axis and separated by Cs⁺ (1/2, 0, 0) and Na⁺ (x= 0, y~1/4, z=1/2) ions, as derived from the octahedral and tetrahedral positions of the Pa3 structure. Our P2₁/a lattice parameters (17 K, 500psi Ne) are a_o = 13.776(3)Å, b_o = 14.343(3)Å, c_o = 9.375(2)Å, β = 133.084(7)^o, and cell volume = 1353.9(5)Å³.

The polymeric phase of Na₂CsC₆₀ may be accessed by pressurizing a sample at room temperature to 5 kbar, using a non penetrating pressure medium, followed by cooling to below 200 K [Schirber et al., Physica C260 173, 1998]. This structural transition is reversible, and the cubic phase is recovered when the sample is returned to ambient conditions. This polymeric phase is superconducting and has both a lower T_c and smaller pressure dependence of T_c than the corresponding values for the cubic phase. By analogy with the observed behavior in Na₂CsC₆₀, the polymeric phase of isomorphic Na₂RbC₆₀ may be either non-superconducting or have T_c below the range of experiments to date.