Surface phase diagrams of pristine and hydroxylated barium hexaferrite surfaces from first-principles atomistic thermodynamics

Barium hexaferrite (BHF) is a ferrimagnet, whose hexagonal unit cell presents five iron crystallographic sites along the c-axis. At the nanoscale, BHF nanoparticles grow in the form of platelets, characterized by a low thickness along its principal magnetization axis (c-axis), displaying uniaxial magnetic anisotropy with the easy axis pointing perpendicular to the platelet. This unique property of BHF nanoplatelets has lead to a variety of novel applications, which often require surface functionalization. However, it has been observed that the nanoplatelets display two different surface morphologies depending on the stage/conditions of preparation. To ground these experimental observations, we employ the ab initio thermodynamics framework to perform systematic investigation of the thermodynamic stability of BHF bulk terminations under a wide range of chemical conditions. We calculate the surface phase diagrams of pristine and hydroxylated bulk terminations along the c-axis. For pristine terminations, two different iron terminated surfaces are preferred: the barium containing 2b termination (Ba-rich conditions) and the 4f2 iron terminated surface (Ba-poor conditions). In the presence of water, the hydroxylated oxygen-terminated surfaces (12k-O) are identified as the most stable ones at Ba-poor conditions and low pH values, whereas the hydroxylated 2b surface is preferred at high pH and Ba-rich conditions.

Automated summary

Barium hexaferrite (BHF) is a ferrimagnet that grows as platelets at the nanoscale, showing uniaxial magnetic anisotropy. The surface morphology of BHF nanoplatelets varies depending on the preparation conditions. Using ab initio thermodynamics, we found that the most stable surface terminations of BHF are the hydroxylated oxygen-terminated surfaces (12k-O) under Ba-poor and low pH conditions, and the hydroxylated 2b surface under high pH and Ba-rich conditions.