Manganese Tetraboride, MnB4: High-Temperature Crystal Structure, p-n Transition, 55Mn NMR Spectroscopy, Solid Solutions, and Mechanical Properties.

The structural and electronic properties of MnB₄ were studied by high-temperature powder X-ray diffraction and measurements of the conductivity and Seebeck coefficient on spark-plasma-sintered samples. A transition from the room-temperature monoclinic structure (space group P2₁/ c) to a high-temperature orthorhombic structure (space group Pnnm) was observed at about 650 K. The material remained semiconducting after the transition, but its behavior changed from p-type to n-type. ⁵⁵Mn NMR measurements revealed an isotropic chemical shift of −1315 ppm, confirming an oxidation state of Mn close to I. Solid solutions of Cr_{1− x}Mn _xB₄ (two phases in space groups Pnnm and P2₁/ c) were synthesized for the first time. In addition, nanoindentation studies yielded values of (496±26) and (25.3±1.7) GPa for the Young's modulus and hardness, respectively, compared to values of 530 and 37 GPa obtained by DFT calculations. [ABSTRACT FROM AUTHOR]