Polymorphism in Cu-substituted GaV4S8: structural and electronic properties

The hexagonal and cubic phases of Ga1−xCuxV4S8 are obtained by different methods of preparation. The reaction of elements above 900°C gives hexagonal phases for large range of x=0.02–0.5. These are metallic and show enhanced paramagnetism. The reduction of oxides by H2S at a lower temperature of 700°C gives non-stoichiomertic compositions of cubic-V4 cluster compound GaV4S8. The solubility of Cu-atoms in cubic phase is less than 10% and above x=0.2 the samples contain a mixture of phases, CuxVS2, GaxVS2 and CuGaS2. The cubic phases are insulating and show Mott's Variable Range Hopping conduction. The non-stoichiomerty and the Cu-substitution reduce the resistivity and thermopower. For x=0.15 and 0.20, the additional peaks are observed in X-ray patterns. These compositions showed a sharp metal to insulator transition on cooling below 180K. The transitional behaviour is very similar to that previously reported intercalated VS2 compound AlxVS2. The transport and magnetic properties of these phases are discussed in terms of the clustering interactions among V-atoms and the localisation of carriers on the metallic clusters frequently found in V-chalcogenides.