Mechanochemical Synthesis and Characterization of Nanostructured ErB₄ and NdB₄ Rare-Earth Tetraborides

Rare-earth borides have become very popular in recent decades with high mechanical strength, melting point, good corrosion, wear, and magnetic behavior. However, the production of these borides is very challenging and unique. The production of ErB₄ and NdB₄ nanopowders via mechanochemical synthesis (MCS) is reported in this study first time in the literature. Er₂O₃ or Nd₂O₃, B₂O₃, and Mg initial powders are mechanically alloyed for different milling times to optimize the process. Rare-earth borides with MgO phases are synthesized, then MgO is removed with HCl acid. The nanostructured rare-earth tetraboride powders are analyzed using X-ray diffraction (XRD). Based on the XRD, ErB₄ powders are produced successfully at the end of the 5 h milling. However, the NdB₄ phase does not occur as the stoichiometric ratio, so the B₂O₃ amount is decreased to nearly 35 wt%. When the amount of B₂O₃ is decreased to 20 wt%, NdB₄ and NdB₆ phases are 50:50 according to the Rietveld analysis. However, a homogenous NdB₄ phase is obtained with 30 wt% loss of B₂O₃. The average particle sizes of ErB₄ and NdB₄ powders are nearly 100.4 and 85.6 nm, respectively. The rare-earth tetraborides exhibit antiferromagnetic-to-paramagnetic-like phase transitions at 18 and 8.53 K, respectively.