Venturing\ninto Unexplored Phase Space: Synthesis,\nStructure, and Properties of MgCo<sub>3</sub>B<sub>2</sub> Featuring\na Rumpled Kagome Network

MgCo<sub>3</sub>B<sub>2</sub>, a novel ternary boride\nin a previously\nunexplored phase space, was synthesized using the hydride route. In\nsitu powder X-ray diffraction and DFT calculations aided in the discovery\nof this compound, whose structure was then determined by single-crystal\nX-ray diffraction. Like the closely related CeCo<sub>3</sub>B<sub>2</sub>, MgCo<sub>3</sub>B<sub>2</sub> crystallizes in centrosymmetric\nspace group <i>P</i>6/<i>mmm</i> (<i>a</i> = 4.883(2) Å, <i>c</i> = 2.926(2) Å at 210 K, <i>Z</i> = 1). Unlike CeCo<sub>3</sub>B<sub>2</sub>, however, it\nadopts a disordered structure that features a rumpled Kagomé\nnetwork of Co atoms, and Mg atoms fill the channels of a Co–B\nframework. Although the structural disorder leads to motifs that are\nsimilar to those observed in MgNi<sub>3</sub>B<sub>2</sub> and other\nrelated ternary borides, no evidence of an ordered superstructure\nwas found by single-crystal X-ray diffraction or high-resolution powder\nX-ray diffraction. In the case of CeCo<sub>3</sub>B<sub>2</sub>, boron\natoms occupy the center of regular Co<sub>6</sub> trigonal prisms;\nin MgCo<sub>3</sub>B<sub>2</sub>, boron atoms are shifted from the\ncenter of the prism to form B–B dimers with roughly the same\nlength as those found in MgNi<sub>3</sub>B<sub>2</sub>. Magnetic susceptibility\ndata exhibit an unusual temperature dependence that cannot be convincingly\nmodeled by the modified Curie–Weiss equation, consistent with\nDFT calculations predicting a nonmagnetic ground state. Intrinsic\nsusceptibility at 300 K is 1.42 × 10<sup>–3</sup> emu/mol\nOe, which is comparable to that of paramagnetic YCo<sub>3</sub>B<sub>2</sub> and CeCo<sub>3</sub>B<sub>2</sub> with a similar structure\nand composition. This study showcases the efficacy of combining several\nmethodologies to discover new solids in unexplored phase spaces. This\napproach includes in situ PXRD data to monitor reactions of precursors\nupon heating, a diffusion-enhanced synthesis method, and DFT assessment\nof compound stability.