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 CuX 2 dichalcogenides with pyrite structure. For CuS 2 it appears that two phase transitions occur: at T Q =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 T R =47K a first-order transition takes place (jump in the lattice parameter, sharp anomaly in the magnetic susceptibility). In the case of CuSe 2 and CuTe 2 , a magnetic transition is observed for T=T c (T c =31K and 26K respectively) when special conditions are used for the sample preparation (synthesis temperature or composition). The authors discuss only the case of CuSe 2 and show that the magnetic transition which occurs at 31K (paramagnetic to weak ferromagnetic) is related to the presence of vacancies. Below T sc =2.40K all the CuSe 2 compounds present superconducting properties, even those which exhibit the magnetic transition at 31K. The electronic structure of the CuX 2 compounds is discussed from the NMR and susceptibility measurements.