We report a robust honeycomb boron layers sandwiching manganese layers compound, MnB, synthesized by high pressure and high temperature. First-principle calculation combined with X-ray photoelectron spectrum unravel that the honeycomb boron structure was stabilized by filling the empty π-band via grabbing electrons from manganese layers. Honeycomb boron layers sandwiching manganese layers is an extraordinary prototype of this type of sandwiched structure exhibiting electronic conductivity and ferromagnetism. Hydrostatic compression of the crystal structure, thermal expansion, and the hardness testing reveal that the crystal structure is of strong anisotropy. The strong anisotropy and first-principle calculation suggests that B-B bonds in the honeycomb boron structure are a strong directional covalent feature, while the Mn-B bonds are soft ionic nature. Sandwiching honeycomb boron layers with manganese layers that combine p-block elements with magnetic transition metal elements could endow its novel physical and chemical properties.

Hardness is an essential but complex property of materials. Although it is generally accepted that a high hardness is related to a high covalent bond density, the determinant of hardness is always unclear. To overcome the restriction of the high density of covalent bonds, the low-boron content transition metal borides (TMBs) are chosen to explore a new way to enhance hardness. We fix the density of covalent bonds and modulate the hardness by designing perpendicular boron zigzag chain (Bzc) skeletons (pe-Bzcs) and parallel Bzc skeletons (pa-Bzcs) in α-MoB (I41/amd) and β-MoB (Cmcm). We utilize pe-Bzcs in α-MoB to enhance the shear modulus via less slippage than in pa-Bzcs. Pe-Bzcs generate a higher grain boundary density in α-MoB than in β-MoB to create a nano-polycrystal morphology under high pressure and high temperature. Hence, the hardness of α-MoB (18.4 GPa) is greater than that of β-MoB (12.2 GPa), which can be attributed to the higher shear modulus and higher density of the grain boundary caused by pe-Bzcs. This work suggests a new idea that modulating boron covalent bond substructures is an effective way to enhance hardness even in low-boron content TMBs. This finding is significant for the design new hard or superhard functional materials.

This work synthesized a high hardness and superconductive polycrystalline Mo3C2 material by the HPHT method. Mo3C2 exhibits superconductivity below 8.2 K and its hardness is far higher than that of the traditionally used superconductive materials.

The D7b-type structure Mn3B4 was fabricated by high-temperature and high-pressure (HPHT) methods.