Synthesis, characterization, properties, first principles calculations, and X-ray photoelectron spectroscopy of bulk Mn5SiB2 and Fe5SiB2 ternary borides

Abstract

Herein, we synthesize fully dense, bulk, predominantly single-phase, polycrystalline samples of the layered ternary transition metal borides Mn5SiB2 and Fe5SiB2 by reactively hot-pressing Mn, Fe, FeB, Si, and B powders. The atomic structures were imaged using high-resolution scanning transmission electron microscopy and revealed high-crystal quality. Elongated striped defects, confined below the nanometer in width, were observed. Selected area electron diffraction further accentuates the high-crystal quality by discrete spots of pattern, that is expected from a tetragonal crystal structure along the [001] zone axis. With Vickers hardness values of 12.1 ± 0.4 GPa, and 12.7 ± 0.1 GPa, for Mn5SiB2 and Fe5SiB2 respectively, these borides are relatively soft. The room temperature electrical resistivities were 1.5 ± 0.1 and 1.2 ± 0.1 μΩ m, for Mn5SiB2 and Fe5SiB2, respectively. The binding energies of the Mn, Fe and Si measured by X-ray photoelectron spectroscopy bolster the idea that the bonds are quite metallic in character. Density functional theory (DFT) calculations confirm that the ground states of both compounds are ferromagnetic as observed experimentally. We also use DFT to predict the elastic and electronic properties. In both compounds, the density of states at the Fermi level are dominated by the d-orbitals of the transition metals. Neither material was readily machinable with conventional tooling, but is so with sharp cobalt steel bits or electro-discharge machining (EDM).