Novel metal borides: structure, high-pressure behavior and properties.

Metal borides are an important class of compounds having a number of remarkable properties like superconductivity, low compressibility, and high hardness. Therefore synthesis of novel metal borides and their investigations have a great interest for materials science and technology. Novel metal borides, FeB$_{4}$, Fe$_{2}$B$_{7}$, FeB$_{50}$, and MnB$_{4}$, have been obtained in a series of high pressure and high temperature experiments in multi-anvil apparatus. The current work is focused on their crystal structures determined by single crystal X-ray diffraction and physical properties, such as compressibility and hardness. Rather low compressibility of all studied iron borides has been revealed in X-ray diffraction experiments in diamond anvil cells where single crystals were compressed up to 40-50 GPa. In particular, bulk modulus of FeB$_{4}$ was found to be K$=$252(5) GPa. This value correlates very well with a very high hardness (Hv$=$65(5) GPa) of this material. Additionally, FeB$_{4}$ shows phonon-mediated superconductivity below 2.9 K, thus opening a class of novel materials with a highly desirable combination of physical properties.