Structure and Properties of the Non‐Centrosymmetric Manganese(II) Borate Mn 5 (BO 3 ) 3 OH

The rose‐colored compound Mn 5 (BO 3 ) 3 OH was synthesized under high‐pressure/high‐temperature conditions of 2.5 GPa and 973 K in a Walker‐type multianvil press. Single‐crystal X‐ray structure determination yielded the non‐centrosymmetric monoclinic space group Pm (no. 6) with the lattice parameters a = 329.71(1), b = 1384.92(5), c = 902.37(3) pm, and β = 107.92(1)°. The presence of the hydroxide group is ascertained by vibrational spectroscopy. Magnetization experiments in the range from 300 to 14 K revealed paramagnetic behavior with antiferromagnetic ordering below 14 K and an effective magnetic moment of 12.2 µ B per formula unit. The acentric nature of the material was verified by second harmonic generation measurements, yielding a considerably strong intensity (I sample /I quartz = 11.14). Additionally, thermal investigations showed a several‐step transformation to Mn 2 OBO 3 upon heating under air.