Fe₃GeTe₂ and Ni₃GeTe₂ – Two New Layered Transition‐Metal Compounds: Crystal Structures, HRTEM Investigations, and Magnetic and Electrical Properties

Abstract

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 [P63/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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Magnetic structure and phase stability of the van der Waals bonded ferromagnetFe3−xGeTe2

“…Consistent with this, we refine a larger concentration of vacancies on Fe(2) compared to Ref. 18, and our refined U 11 for Ge is also larger than previously reported. However, the previous study did report a strong anisotropy for the displacement parameters of Ge, and a similar U 33 to that shown in Table II was reported.…”

“…Fe 3 GeTe 2 is an itinerant ferromagnet, with Curie temperatures of 220 and 230 K reported, and an estimated spontaneous magnetization of 1.6µ B /Fe at 0 K. 1,18 Previous anisotropic magnetization measurements on crystals grown using chemical vapor transport suggested that the c-axis is the easy axis, and an anisotropy field of at least 5 T was demonstrated at 10 K. 1 The Fe(2) position was reported to have a small concentration of vacancies (17%), but chemical characterization suggested the composition is Fe 3 GeTe 2 .…”

“…This compares to a=3.9110(10)Å and c=16.022(3)Å previously reported for crystals with a composition of Ni 2.95 GeTe 2 obtained from refinement of single crystal x-ray diffraction data. 18 Thus, for both To investigate the phase width of Fe 3−x GeTe 2 , polycrystalline samples were produced via solid-state reactions with nominal compositions Fe 3−x GeTe 2 with −0.1 ≤ x ≤ 0.4. These materials were characterized using powder x-ray diffraction, EDS, and magnetization measurements.…”

“…However, the previous study did report a strong anisotropy for the displacement parameters of Ge, and a similar U 33 to that shown in Table II was reported. 18 In the binary compound Fe 1.60 Ge 2 , it has been suggested that the Ge atoms actually move off of their site and the symmetry is potentially broken.…”

“…In order to avoid beaminduced hopping and preserve observation of the intrinsic structure, images were collected rapidly on unirradiated regions by summing 20 frames, each one acquired within 1 s. EEL spectrum images were acquired using a spacing of 0.87Å and a dwell time of 0.2 s. Beam-induced structural effects were also noted for Ni 3 GeTe 2 in Ref. 18.…”