Quaternary Solid Solutions in the (M_[1-x]M'_x)₂AlB₂ and (Mo_[1-x]M'_x)AlB Systems

The MAB phases are layered ternary transition metal borides (TMB), with atomically laminated crystal structures comprised of transition metal, M, boride layers (M-B) interleaved by single or double layers of A, typically Al. Quenching studies were performed on Cr₂AlB₂, Cr₃AlB₄, and Fe₂AlB₂ to investigate the exact reaction mechanism for MAB formation. It was determined that during formation, all MABs undergo a topotactic transformation, retaining the structure of the MB building blocks in the final MAB form. Using the reaction mechanism determined from these quenching studies, (M_[1-x]M'_x)₂AlB₂ solid solutions were synthesized via pressureless sintering along a two-step reaction process, first by synthesizing the M_[1-x]M'_xB binary boride solid solutions, then reacting them with Al. For the (Mo_[1-x]M'_x)AlB solid solutions, two-part single-step synthesis was employed. A range of solid solutions between existing MABs and 3d and group 5-6 transition metals were attempted. Powder X-ray diffraction of select systems show lattice parameter shifts and unit cell expansions indicative of successful solid solution formations. In this work, a full range of (Fe_[1-x]Mn_x)₂AlB₂ solid solutions were synthesized, and their magnetic properties were characterized. It was determined that over the entire range of Mn loading, solid solutions exist as ferromagnetic, canted antiferromagnetic and antiferromagnetic. Increased Mn loading leads to a reduction in T_c, however during evaluation of the magnetocaloric effect, it was found that a Mn loading of x = 0.2 has a slightly greater relative cooling power than Fe₂AlB₂ between 0 - 20 °C. Polycrystalline (Mn_[1-x]Cr_x)₂AlB₂ solid solutions were synthesized for the first time and are found to be continuous over the entire range of substitution. Bulk synthesis of these solid solutions was also attempted via vacuum hot pressing in single and two step reactions. For (Mn_[1-x]Cr_x)₂AlB₂, scanning electron microscopy and energy dispersive spectroscopy was used to confirm single phase bulk solid solutions. Overall, the only successful quaternary MAB solid solutions produced via pressureless sintering were between atomic nearest neighbors of existing MAB phases. A full range of solubility (x = 0 - 1) exists between MABs of the same structure (MoAlB and WAlB in M₂A₂B₂; Fe₂AlB₂, Mn₂AlB₂ and Cr₂AlB₂ in M₂AlB₂). Keywords: Fe₂AlB₂, MAB Phases, Magnetic materials, Phase diagrams, Ternary, Solutions, Solid, Magnetocaloric, Transition metal borides