CRYSTALLOGRAPHIC, MAGNETIC AND ELECTRONIC-STRUCTURES OF A NEW LAYEREDFERROMAGNETIC COMPOUND CR2GE2TE6

Cr2Ge2Te6 is a new layered material belonging to the lamellar ternary M(2)X(2)Te(6) chalcogenides family (where M is a 3(+) oxidation state metal and X(2) a silicon or a germanium pair). It was synthesized from a stoichiometric mixture of the elements and heated in a sealed evacu ated quartz tube for 20 d at 700 degrees C. The crystal symmetry is rh ombohedral, of space group R ($) over bar 3, with the following hexago nal cell parameters: a = b = 0.68275(4) nm, c = 2.05619(9) nm, V = 0.8 301(1) nm(3) and Z = 3. The crystal structure of Cr2Ge2Te6 was solved using both x-ray single-crystal diffraction and neutron powder diffrac tion data. Growth defects were detected and investigated using high-re solution electron microscopy. The magnetic structure and properties of Cr2Ge2Te6 have been determined by neutron powder diffraction and susc eptibility and magnetization measurements. Below 61 K, Cr2Ge2Te6 is fe rromagnetic 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 resis tivity can be fitted as rho = exp(A/kT), where A = 0.2 eV. Band-struct ure and crystal orbitals of Cr2Ge2Te6 have been calculated using the e xtended Huckel method and indicate no gap but highly localized Cr 3d s tates, giving evidence of a hopping mechanism for Cr2Ge2Te6 electrical conductivity.