On the question of metallic and superconducting poly(carbon diselenide)

Polymerization of carbon diselenide, CSe₂, at 5 kbar and ∼ 100°C has been reported to give a metallic (CSe₂)_n ladder polymer that superconducts near 6 K at 220 kbar pressure. Using a variety of techniques we find that the material synthesized (showing essentially the same x-ray diffraction pattern as previously published) is not (CSe ₂)_n but instead consists of a mixture of free, trigonal Se and an amorphous C-Se polymeric composition consistent with the formula (CSe_0.5)_n. Interestingly, possibly as a consequence of staged reactions during synthesis, the Se phase consists of a mixture of large, aggregated crystallites (∼600 Å in diameter) melting at ∼220°C and nonaggregated small crystallites (∼150-250 Å in diameter) showing a broad melting transition with an endothermic maximum at ∼180°C. Percolation of the C-Se polymeric composition probably provides the high observed electrical conductivity [σ(300 K)≈10-20 S cm^-1 and σ(300 K)/σ(8 K)≈2.5] in the presently available samples. The previously reported high pressure superconductivity is probably associated with the free Se phase in the samples, since selenium is a known superconductor in the 6 K and 220 kbar range.