NdB₆ ceramic nanoparticles: First principles calculations, mechanochemical synthesis and strain engineering

Borides are usually hard and brittle materials; however, we report the synthesis of superplastic nanostructured NdB₆ ceramic powders, counter to the conventional wisdom that borides are always brittle. We investigate that through strain engineering, NdB₆ can be made extremely ductile if the lattice is compressively strained and highly defected, based on transmission electron microscopy (TEM) and density functional theory (DFT) calculations. In this study, the synthesis conditions were designed based on CALPHAD modelling, and the superplastic NdB₆ powders were successfully obtained through mechanochemical synthesis (MCS) of Nd₂O₃, B₂O₃ and Mg initial materials in a high-energy ball mill. Following MCS, the powders were purified in a hydrochloric acid (HCl) containing aqueous solution in order to leach out MgO by-product. The purified powders were characterized using X-ray diffractometry (XRD), Helium (He) gas pycnometry, scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), particle size analysis (PSA) and magnetometry techniques, which demonstrated NdB₆ nanoparticles with an average particle size of 118 nm belonging paramagnetic behavior at cryogenic temperatures. DFT calculations have been carried out through to investigate the structural, mechanical, electronic, optical, thermodynamic and magnetic properties of NdB₆. The impact of various defects was examined, which revealed the significance of boron vacancies and compressive strains in the superplastic form of NdB₆.