Electrical transport properties of Na2C60 under high pressure

• We present direct resistance measurements on Na2C60 under high pressure up to 2 GPa in the temperature range ~90–500 K. We show that fresh, nominally pure Na2C60 exhibits a semiconductor-type conduction behavior at all experimental conditions, with a 1.2 eV band gap and a conduction strongly influenced by defect gap states. Contrary to Rb4C60 and Na4C60, both band gap and gap states have large pressure dependences. In contrast, a Na2C60 sample preserved in an oxygen-free environment for several months shows conduction by another mechanism, variable range 3D hopping. This is consistent with recent findings of nanoscale phase segregation in Na2C60 and can be understood in terms of conduction by intergrain hopping. Heating the sample at 2 GPa leads to a significant change in the resistivity, suggesting a structural transition. By characterizing the sample after the phase transformation with Raman spectroscopy and x-ray diffraction we identify a phase separation of the material into two main structures, linearly polymerized orthorhombic C60 and two-dimensionally polymerized Na4C60.

Ämnesord

NATURVETENSKAP  -- Fysik -- Den kondenserade materiens fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Condensed Matter Physics (hsv//eng)

Nyckelord

electrical resistivity

high pressure

solid-state phase transformations

hopping conduction

narrow band gap semiconductors

phase separation

Raman spectra

segregation

sodium compounds

X-ray diffraction

fulleride polymers

gap states

Condensed matter physics

Kondenserade materiens fysik

fysik

Physics

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