Metastable β-NdCo₂B₂: A Triclinic Polymorph with Magnetic Ordering

Metastable phases increase the diversity of structure-property relationships, facilitating the study and comparison of materials. In this article, we present the synthesis and characterization of a metastable polymorph of NdCo2B2, synthesized by arc melting, which we designate as β-NdCo2B2. Single-crystal X-ray diffraction reveals that it crystallizes in a new triclinic structure type with space group P1¯ (#2). The structure is composed of 3D Co-B channels filled by Nd zig zags, a large departure from the thermodynamically stable tetragonal ThCr2Si2-type polymorph. β-NdCo2B2 is closely related to the BaRh2Si2 structure type, providing a connection to the "122-type"channel-containing RM2B2 compounds (R = rare-earth element, M = Ni, Ru, Os, and Ir) that was previously absent for rare-earth cobalt borides of this stoichiometry. Differential scanning calorimetry shows that this new polymorph transforms irreversibly into the tetragonal form upon heating above 530 °C. β-NdCo2B2 displays metallic conductivity and magnetically orders into a complex ferromagnetic or ferrimagnetic state at 7.7 K. We present density functional theory calculations throughout to further explore the stability, magnetism, and electronic structure. β-NdCo2B2 demonstrates that new compounds with interesting structural and magnetic properties can be found even in well-studied chemical systems, such as the ternary transition metal borides.