Shuailing Ma

Manganese mono-boride, an inexpensive room temperature ferromagnetic hard material

We synthesized orthorhombic FeB-type MnB (space group: Pnma) with high pressure and high temperature method. MnB is a promising soft magnetic material, which is ferromagnetic with Curie temperature as high as 546.3 K, and high magnetization value up to 155.5 emu/g, and comparatively low coercive field. The strong room temperature ferromagnetic properties stem from the positive exchange-correlation between manganese atoms and the large number of unpaired Mn 3d electrons. The asymptotic Vickers hardness (AVH) is 15.7 GPa which is far higher than that of traditional ferromagnetic materials. The high hardness is ascribed to the zigzag boron chains running through manganese lattice, as unraveled by X-ray photoelectron spectroscopy result and first principle calculations. This exploration opens a new class of materials with the integration of superior mechanical properties, lower cost, electrical conductivity, and fantastic soft magnetic properties which will be significant for scientific research and industrial application as advanced structural and functional materials.

Discovery of superconductivity in hard hexagonal ε-NbN

Since the discovery of superconductivity in boron-doped diamond with a critical temperature (TC) near 4 K, great interest has been attracted in hard superconductors such as transition-metal nitrides and carbides. Here we report the new discovery of superconductivity in polycrystalline hexagonal ε-NbN synthesized at high pressure and high temperature. Direct magnetization and electrical resistivity measurements demonstrate that the superconductivity in bulk polycrystalline hexagonal ε-NbN is below ∼11.6 K, which is significantly higher than that for boron-doped diamond. The nature of superconductivity in hexagonal ε-NbN and the physical mechanism for the relatively lower TC have been addressed by the weaker bonding in the Nb-N network, the co-planarity of Nb-N layer as well as its relatively weaker electron-phonon coupling, as compared with the cubic δ-NbN counterpart. Moreover, the newly discovered ε-NbN superconductor remains stable at pressures up to ∼20 GPa and is significantly harder than cubic δ-NbN; it is as hard as sapphire, ultra-incompressible and has a high shear rigidity of 201 GPa to rival hard/superhard material γ-B (∼227 GPa). This exploration opens a new class of highly desirable materials combining the outstanding mechanical/elastic properties with superconductivity, which may be particularly attractive for its technological and engineering applications in extreme environments.

Pressure induced structural transition of small carbon nano-onions

Small carbon nano-onions (S-CNOs) were prepared by annealing nanodiamonds (ND) in an argon atmosphere. The structure and morphology of S-CNOs were determined by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) and the average grain size of the S-CNOs was about 8 nm. In situ high pressure Raman spectra of S-CNOs were investigated by diamond anvil cell experiments at pressures up to 22.5 GPa. A reversible structural transition occurred at about 7.4 GPa, resulting from the polygonization of S-CNOs. The structural transition pressure of S-CNOs is lower than that of large CNOs (L-CNOs) and onion-like carbon (OLC) nanospheres. S-CNOs derived from the annealing of ND have a high defect density, a large number of sp3 bonds and high free energy. In addition, high pressure can be generated in the interior of S-CNOs at high temperatures. The results indicated that nanotwinned diamond (nt-diamond) may be prepared using S-CNOs derived from the annealing of NDs as a raw material below 10 GPa, which is much lower than the pressure needed for synthesizing nano-polycrystalline diamond (NPD) and nt-diamond with other carbon resources (usually more than 15 GPa).

An ultra-incompressible ternary transition metal carbide

The ternary transition metal carbide Mo0.5W0.5C was synthesized under high pressure and high temperature and the crystalline structure was confirmed by Rietveld refinements as being hexagonal (Pm2). The mechanical properties, bulk modulus and Vickers hardness were also investigated by in situ high-pressure X-ray diffraction and Vickers microhardness testing, respectively. The fitting bulk modulus of the ternary transition metal carbide is 399.9 ± 9.3 GPa, which is as compressible as diamond, and its asymptotic Vickers hardness is 15.3 GPa, nearly 60% harder than molybdenum carbide. The high bulk modulus is attributed to the high valence electron density and the greater hardness compared with γ-MoC is due to the strong bond between tungsten and carbon atoms.

A first-principles investigation of a new hard multi-layered MnB2 structure

ReB2-type MnB2 has always been considered to be the ground-state structure of MnB2. However, subsequent theoretical study has revealed that this structure is easy to decompose into elemental Mn and B under ambient conditions, which motivated us to look for a stable MnB2 structure at high pressures. Using structure prediction algorithm USPEX and density functional theory calculations, we found a stable multi-layered MnB2 structure with space group Immm at high pressure. The calculated hardness of Immm-MnB2 is 22.5 GPa, which makes it a potential hard multifunctional material along with its conductive and magnetic properties. The hexagonal graphene-like boron networks of Immm-MnB2 contribute to its hardness and stability.

Revealing unusual rigid diamond net analogues in superhard titanium carbides

Transition metal carbides (TMCs) are considered to be potential superhard materials and have attracted much attention. With respect to titanium and carbon atoms, we confirm the pressure-composition phase diagram of the Ti–C system using structure searches and first-principles calculations. We firstly discovered stable TiC4 which was expected to be synthesized at high pressure, as well as metastable TiC2 and TiC3. These layered titanium carbides are diamond net analogues due to the unusual C-layers in the form of puckered graphene-like, diamond-like and double diamond-like C-layers. The existence of diamond-like C-layers might help to understand the formation of diamond. All the studied titanium carbides could be recoverable at ambient pressure and exhibited great mechanical properties (strong ability to resist volume and shear deformations, small anisotropy, and high hardness). Moreover, we crystallized the structure of TiC4 in other transition metal carbides and obtained five superhard TMC4s (TM = V, Zr, Nb, Hf and Ta). Interactions between layers were revealed to be the source of the great mechanical properties and high hardness through combining detailed analyses of electronic structure and chemical bonding, namely, weak ionic interactions of neighboring Ti- and C-layers and the strong covalent interactions of C- and C-layers.