Is orthorhombic iron tetraboride superhard?

Abstract

Millimeter-sized FeB4 bulks were synthesized under high pressure of 15 GPa and 1600 °C. Multiple analysis methodologies including XRD, SEM, TEM, and Raman spectroscopy verified this new phase with an oP10 crystal structure. A relatively low hardness of 15.4 GPa, which is consistent with hardness estimated from both macroscopic and microscopic hardness models, excludes FeB4 as a superhard material. Magnetization and resistance measurements do not indicate superconductivity in FeB4 for temperature as low as 2.5 K. Previously reported superconductivity in FeB4 needs a further investigation, especially for structural imperfections, unidentified phases, and/or contaminations.