Metastable β-NdCo₂B₂: A Triclinic Polymorph with Magnetic Ordering

Abstract

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 NdCo 2 B 2 , synthesized by arc melting, which we designate as β-NdCo 2 B 2 . Single-crystal X-ray diffraction reveals that it crystallizes in a new triclinic structure type with space group P1̅ (#2). The structure is composed of 3D Co−B channels filled by Nd zig zags, a large departure from the thermodynamically stable tetragonal ThCr 2 Si 2 -type polymorph. β-NdCo 2 B 2 is closely related to the BaRh 2 Si 2 structure type, providing a connection to the "122-type" channel-containing RM 2 B 2 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-NdCo 2 B 2 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-NdCo 2 B 2 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.

Path Less Traveled: A Contemporary Twist on Synthesis and Traditional Structure Solution of Metastable LiNi12B8

“…When it comes to borides, the challenge of synthesizing metastable borides is even greater due to the high melting point of elemental boron (∼2350 K), thus requiring unconventional synthetic methods. Rare examples of metastable borides include Na 2 BP 2 obtained via an oxidative elimination pathway, 36 β-NdCo 2 B 2 synthesized via rapid cooling of an arcmelted ingot, 37 and Ni 7 B 3 from low-temperature solution synthesis, 22 making metastable borides an underexplored class of materials. Motivated by these studies, we pursued the preparation of metastable borides in the Li−Ni−B system through soft topochemical deintercalation route.…”