Fe3GeTe2 and Ni3GeTe2 -: Two new layered transition-metal compounds:: Crystal structures, HRTEM investigations, and magnetic and electrical properties

Fe3GeTe2 and Ni3GeTe2 are two new air-stable, black-metallic solids. They were characterized by single-crystal X-ray crystallography, high resolution transmission electron microscopy (HRTEM), and preliminary magnetic measurements. Both compounds crystallize in the hexagonal system [P6(3)/ mmc, Z = 2; Fe3GeTe2: a = 399.1(1) pm, c = 1633(3) pm; Ni3GeTe2: a = 391.1(1) pm, c = 1602.0(3) pm], and represent a new structure type with a pronounced macroscopic and microscopic layer character. They show close structural relationships to iron/nickel germanium alloys. Each layer in the title compounds represents a sandwich structure with two layers of tellurium atoms covering a triple-layer Fe3Ge (Ni3Ge) substructure on both sides. Assuming full occupancies for the Fe and Ni sites, a mixed-valence formulation for the transition-metal atoms according to (M2+)(M3+)(2)(Ge4-)-(Te2-)(2) (M = Fe, Ni) may be concluded. A slightly reduced occupancy for one Fe/Ni position, however, indicates a more complicated local structural situation. This is confirmed by weak residual electron density in the van der Waals gap and by the results of detailed HRTEM and electron-diffraction experiments for Ni3GeTe2. The latter results show variations in the arrangement of Ni atoms, as well as vacancies and a misfit of in-plane disordered hexagonal layers. Fe3GeTe2 shows Curie-Weiss behavior above and ferromagnetism below 230 K, while Ni3GeTe2 exhibits temperature-independent paramagnetism in the measured temperature range and a metallic behavior of the electrical resistance. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006.