The van der Waals Ferromagnets Fe 5– δ GeTe 2 and Fe 5– δ –x Ni x GeTe 2 – Crystal Structure, Stacking Faults, and Magnetic Properties

Fe 5– δ GeTe 2 was synthesized by heating the elements at 1050 K and characterized by single crystal and powder X‐ray analysis. The structure [ R 3 m, a = 4.0376(4) Å, c = 29.194(6) Å] consists of Fe 5– δ Ge layers separated by tellurium double layers forming a van der Waals gap. The pronounced two‐dimensional character of Fe 5– δ GeTe 2 causes stacking faults along the c direction. Simulations of different stacking variants using the DIFFaX software reveal disorder occurring in domains. Magnetic measurements of Fe 5– δ GeTe 2 show ferromagnetism below 279 K with a saturation moment of 1.80 μ B at 1.8 K. Nickel substitution of the iron sites has little influence on the structure but changes the saturation moment, which passes through a maximum of 2.11 μ B in Fe 4.11 Ni 0.50 GeTe 2 . This indicates that structural influences as well as the dilution of the magnetic iron atoms play a decisive role.