  We report crystal structure, electronic structure, and magnetism of manganese
tetraboride, MnB4, synthesized under high-pressure high-temperature conditions.
In contrast to superconducting FeB4 and metallic CrB4, which are both
orthorhombic, MnB4 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 MnB4 to the insulating state, we find indications for a sizable
paramagnetic effective moment of about 1.7 muB/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 MnB4 a
structurally simple but microscopically enigmatic material; we argue that its
properties may be influenced by electronic correlations.
