Pressure and temperature evolution of the structure of the superconductingNa2CsC60 fulleride

The structural properties of the Na2CsC60 fulleride have been studied by sy nchrotron X-ray powder diffraction at both ambient and elevated pressures. Complementary neutron diffraction measurements at high pressure were also p erformed. 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 M. The pressure dependence of the structure of solid Na2CsC60 at amb ient 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 stru cture is primitive cubic with a = 14.1329(3) Angstrom (space group Pa (3) o ver bar). When a pressure of 0.76 GPa is reached, an incomplete phase trans ition 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) Angstrom, b =14.224(6) Angstrom, c = 9.408(3) Angstrom, and be ta = 133.71(1)degrees (space group P2(1)/a), isostructural with the low-tem perature polymer phase of Na2RbC60. The cubic and polymeric phases of Na2Cs C60 coexist up to 0.90 GPa. The pressure evolution of the monoclinic lattic e constants a, b, and c to 8.63 GPa reveals the presence of substantial ani sotropy in the compressibility along the three axes. The structure is least compressible along the c axis, which defines the polymeric C-C bridged ful leride chains and is most compressible along the interchain b direction. (C ) 1999 Academic Press.