Pressure and Temperature Evolution of the Structure of the Superconducting Na2CsC60 Fulleride

The structural properties of the Na2CsC60 fulleride have been studied by synchrotron X-ray powder diffraction at both ambient and elevated pressures. Complementary neutron diffraction measurements at high pressure were also performed. We find no evidence for a monomer polymer phase transition on cooling at ambient pressure, despite the adopted slow cooling procedures, with the structure remaining strictly cubic, even after prolonged standing at 200 K. The pressure dependence of the structure of solid Na2CsC60 at ambient temperature was followed up to 0.56 GPa by neutron diffraction and up to 8.63 GPa by synchrotron X-ray diffraction. At ambient pressure, the structure is primitive cubic with a=14.1329(3) ? (space group Pa ). When a pressure of 0.76GPa is reached, an incomplete phase transition to a low-symmetry structure, accompanied by a large volume decrease (2.7(1)%), is encountered. This phase was characterized as monoclinic with a= 13.745(5) ?, b=14.224(6) ?, c=9.408(3) ?, and =133.71(1)° (space group P21/a), isostructural with the low-temperature polymer phase of Na2RbC60. The cubic and polymeric phases of Na2CsC60 coexist up to 0.90 GPa. The pressure evolution of the monoclinic lattice constants a, b, and c to 8.63 GPa reveals the presence of substantial anisotropy in the compressibility along the three axes. The structure is least compressible along the c axis, which defines the polymeric C–C bridged fulleride chains and is most compressible along the interchain b direction.