Venturing into Unexplored Phase Space: Synthesis, Structure, and Properties of MgCo3B2 Featuring a Rumpled Kagomé Network

Abstract

MgCo₃B₂, a novel ternary boride in a previously unexplored phase space, was synthesized using the hydride route. In situ powder X-ray diffraction and DFT calculations aided in the discovery of this compound, whose structure was then determined by single-crystal X-ray diffraction. Like the closely related CeCo₃B₂, MgCo₃B₂ crystallizes in centrosymmetric space group P6/mmm (a = 4.883(2) Å, c = 2.926(2) Å at 210 K, Z = 1). Unlike CeCo₃B₂, however, it adopts a disordered structure that features a rumpled Kagomé network of Co atoms, and Mg atoms fill the channels of a Co–B framework. Although the structural disorder leads to motifs that are similar to those observed in MgNi₃B₂ and other related ternary borides, no evidence of an ordered superstructure was found by single-crystal X-ray diffraction or high-resolution powder X-ray diffraction. In the case of CeCo₃B₂, boron atoms occupy the center of regular Co₆ trigonal prisms; in MgCo₃B₂, boron atoms are shifted from the center of the prism to form B–B dimers with roughly the same length as those found in MgNi₃B₂. Magnetic susceptibility data exhibit an unusual temperature dependence that cannot be convincingly modeled by the modified Curie–Weiss equation, consistent with DFT calculations predicting a nonmagnetic ground state. Intrinsic susceptibility at 300 K is 1.42 × 10^–3 emu/mol Oe, which is comparable to that of paramagnetic YCo₃B₂ and CeCo₃B₂ with a similar structure and composition. This study showcases the efficacy of combining several methodologies to discover new solids in unexplored phase spaces. This approach includes in situ PXRD data to monitor reactions of precursors upon heating, a diffusion-enhanced synthesis method, and DFT assessment of compound stability.