Crystallographic, magnetic and electronic structures of a new layered ferromagnetic compound Cr2Ge2Te6

Cr2Ge2Te6 is a new layered material belonging to the lamellar ternary M2X2Te6 chalcogenides family (where M is a 3+ oxidation state metal and X2 a silicon or a germanium pair). It was synthesized from a stoichiometric mixture of the elements and heated in a sealed evacuated quartz tube for 20 d at 700 degrees C. The crystal symmetry is rhombohedral, of space group R3, with the following hexagonal cell parameters: a=b=0.68275(4) nm, c=2.05619(9) nm, V=0.830 1(1) nm3 and Z=3. The crystal structure of Cr2Ge2Te6 was solved using both X-ray single-crystal diffraction and neutron powder diffraction data. Growth defects were detected and investigated using high-resolution electron microscopy. The magnetic structure and properties of Cr2Ge2Te6 have been determined by neutron powder diffraction and susceptibility and magnetization measurements. Below 61 K, Cr2Ge2Te6 is ferromagnetic with spins aligned along the c axis of the cell ( mu (Cr3+)=2.80(4)mu B at 5 K). The thermal evolution of the magnetic moments below Tc is given. At room temperature, Cr2Ge2Te6 presents a rho =0.02 Omega cm resistivity. Above Tc, the thermal evolution of the resistivity can be fitted as rho =exp(A/kT), where A=0.2 eV. Band-structure and crystal orbitals of Cr2Ge2Te6 have been calculated using the extended Huckel method and indicate no gap but highly localized Cr 3d states, giving evidence of a hopping mechanism for Cr2Ge2Te6, electrical conductivity.