Structure and physical properties of Na₄C₆₀ under ambient and high pressures

The structure and physical properties of two-dimensional polymeric Na₄C₆₀ (body-centered monoclinic, space group 12/m) are studied in a wide temperature region from 12 to 300 K at 1 bar, and in a pressure region up to 53 kbar at 300 K. The temperature dependence of lattice constants suggests a structural anomaly below 100 K where the variation of spin susceptibility is observed from electron spin resonance. The thermal expansion of the unit-cell volume V is smaller than that of monomeric Rb₃C₆₀ and K₃C₆₀. The compressibility of c is larger than that of a and b, which can be well explained by the repulsion between Na ions. The compressibility of the center-to-center distance in the (101) plane is ∼ 1/3 times smaller than that in the (101) plane, which can be well explained by the formation of the polymer chains. Further, a possibility of a three-dimensional polymerization is discussed on the basis of the pressure dependence of C₆₀···C₆₀ distances.