We report crystal structure, electronic structure, and magnetism of manganese tetraboride, ${\text{MnB}}_4$, synthesized under high-pressure, high-temperature conditions. In contrast to superconducting ${\text{FeB}}_4$ and metallic ${\text{CrB}}_4$, which are both orthorhombic, ${\text{MnB}}_4$ features a monoclinic crystal structure. Its lower symmetry originates from a Peierls distortion of the Mn chains. This distortion nearly opens the gap at the Fermi level, but despite the strong dimerization and the proximity of ${\text{MnB}}_4$ to the insulating state, we find indications for a sizable paramagnetic effective moment of about 1.7 μ${}_B$/f.u., ferromagnetic spin correlations, and, even more surprisingly, a prominent electronic contribution to the specific heat. However, no magnetic order has been observed in standard thermodynamic measurements down to 2 K. Altogether, this renders ${\text{MnB}}_4$ a structurally simple but microscopically enigmatic material; we argue that its properties may be influenced by electronic correlations.

• Received 25 December 2013

DOI:https://doi.org/10.1103/PhysRevB.89.064108