Peierls-distorted monoclinic MnB(4) with a Mn-Mn bond.

Tetraborides of chromium and manganese exhibit an unusual boron-atom framework that resembles the hypothetical tetragonal diamond. They are believed to be very hard. Single crystals of MnB4 have now been grown. The compound crystallizes in the monoclinic crystal system (space group P21 /c) with a structure that has four crystallographically independent boron-atom positions, as confirmed by (11) B MAS-NMR spectroscopy. An unexpected short distance between the Mn atoms suggests a double Mn-Mn bond and is caused by Peierls distortion. The structure was solved using group-subgroup-relationships. DFT calculations indicate Mn(I) centers and paramagnetism, as confirmed by magnetic measurements. The density of states shows a pseudo-band gap at the Fermi energy and semiconducting behavior was observed for MnB4 .