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 T_c is given. At room temperature, Cr₂Ge₂Te₆ presents a rho =0.02 Omega cm resistivity. Above T_c, 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.