Magnetic properties and phase transitions of the metallic CuX2dichalcogenides (X=S, Se, Te) with pyrite structure

Susceptibility, NMR, electrical resistivity and lattice parameter measurements have been performed between 1.4 and 300K on the CuX2 dichalcogenides with pyrite structure. For CuS2 it appears that two phase transitions occur: at TQ=160K observe a second order transition (decrease of the magnetic susceptibility, increase of the NMR linewidth, change in delta a/ delta T and also in delta rho / delta T); at TR=47K a first-order transition takes place (jump in the lattice parameter, sharp anomaly in the magnetic susceptibility). In the case of CuSe2 and CuTe2, a magnetic transition is observed for T=Tc(Tc=31K and 26K respectively) when special conditions are used for the sample preparation (synthesis temperature or composition). The authors discuss only the case of CuSe2 and show that the magnetic transition which occurs at 31K (paramagnetic to weak ferromagnetic) is related to the presence of vacancies. Below Tsc=2.40K all the CuSe2 compounds present superconducting properties, even those which exhibit the magnetic transition at 31K. The electronic structure of the CuX2 compounds is discussed from the NMR and susceptibility measurements.