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[en] Nanosized powder of manganese diboride (MnB2) was synthesized by the microwave heating method for the first time. A stoichiometric mixture of manganese dioxide (MnO2) and magnesium dodecaboride (MgB12) was used as raw material, where magnesium dodecaboride served as the reducing agent and a source of boron. A domestic microwave oven was used to generate microwave radiation. Phase composition of the initial mixture and the obtained product were examined by XRD studies, of the initial mixture and formation of a single-phase final product, MnB2. Average particle size of synthesized manganese diboride (40 nm) was calculated on the basis of Scherer’s formula
[en] The monoborides of Mn, Fe and Co are ferromagnetic and the saturation magnetisation indicates that the numbers of 3d electrons are 8.1, 8.9 and 9.7 respectively. A three band model described by MB 2spα* 3dβ 4sγ where 2sp implies hybridised 2s, 2p states, has been proposed for these borides. Covalent 2-sp bonding is discussed and the requirement of unsaturated covalency implies empty 2sp levels at the Fermi surface. These levels can receive scattered conduction electrons and may cause the electrical properties to differ from those of the pure transition metals. Expressions are derived relating the resistivity in the neighbourhood of the Curie temperature to the density of electronic states and the results are applied to Ni and MnB. Unfortunately the matrix elements for the scattering transitions and the 3d band form are unknown factors. It seems, however, necessary to postulate a high density of 2sp states to explain experimental data
[en] MnB4 was newly synthesized to crystallize in a monoclinic P21/c structure, different from previous experimental and theoretical reports. Here, based on first-principles calculations, we perform a comparative study of geometric and energetic features, mechanical behaviors, electronic property and chemical bonding of the experimentally identified monoclinic MnB4, as well as orthorhombic CrB4 and FeB4. The results demonstrate that the presence of distorted rhomboidal-B4 units and one-dimensional Mn chains in the monoclinic MnB4 breaks the structural symmetry and lowers the total energy in comparison to the orthorhombic phase. The opening of band gap in MnB4 is induced by Peierls-paired Mn atoms, differing from the metallic behaviors of recently studied tetraborides. Specifically, the preservations of covalent bonding in distorted boron-rhomboids in MnB4 explain the relatively higher incompressibility and hardness. - Graphical abstract: P21/c-type structure for MnB4 characterizes rhomboid-B4 units and alternately short and long Mn–Mn chains. - Highlights: • The novel P21/c-type MnB4 compound is studied by first-principles calculations. • P21/c-type MnB4 has lower total energy and relatively higher stability. • An energy gap opening is found for P21/c-type MnB4 induced by Mn atom pairs. • P21/c-type MnB4 exhibits excellent mechanical properties. • The mechanical properties of TcB4 and ReB4 in P21/c structure are also studied
[en] In an effort to develop super hard materials, MnB_4 was recently synthesized for the first time in single crystalline form, with micro scale crystals of about 200 μm length. Subsequently, the magnetic properties have been studied experimentally, and band structure calculations have been carried out. Based on these calculations and preliminary resistivity measurements, it was argued that the material is semiconducting, although a definite conclusion could not be reached. In order to determine whether MnB_4 is a semiconductor or a metal we have carried single crystal resistivity measurements at temperatures 2 to 300 K. For this purpose a setup for measuring micro-scale samples was developed and characterized. The setup is based on a modified two point configuration and the resistivity of MnB_4 was measured as function of temperature. With these measurements MnB_4 was identified to be a semiconductor.
[en] Recently, single crystalline MnB_4 was synthesized for the first time, yielding microscale crystals with dimensions of the order of 200 μm. Based on band structure calculations, it was argued that the material is semiconducting as result of a Peierls distortion. Conversely, in a study of polycrystalline material it was concluded that the material is a weakly ferromagnetic metal. To establish if MnB_4 is a semiconductor we have carried out single crystal four point resistivity measurements. For this purpose a setup for measuring microscale samples was developed and characterized. Qualitatively, we find semiconducting behavior (increasing resistivity for decreasing temperature), although a band gap could not be derived because of a non-linear Arrhenius plot. Our data are consistent with MnB_4 being a pseudogap/small gap material as proposed. A pronounced sample dependence of the transport properties points to the presence of impurity states. For the single crystals no ferromagnetic signatures could be obtained, suggesting an extrinsic cause of it in polycrystalline material.
[en] Magnetization anisotropy at a given intensity of a magnetic field and magnetization curves in antiferromagnetic (T>Tsub(c)) and ferromagnetic (T< Tsub(c)) states have been studied on MnB2 monocrystals possessing a hexagonal symmetry and a magnetic phase transition from antiferromagnetism to weak ferromagnetism. It is established that the antiferromagnetism axis is parallel to a certain separated axis of the  type. The axes of difficult and easy magnetization in a ferromagnetic state lie in a basal plane and are mutually perpendicular, the first of them being directed along the above  axis. The results obtained testify in favour of s-d exchange nature of a magnetic phase transition in this compound
[en] It has been shown that MnB is the only ferromagnetic phase occurring in the Mn-B system. The magnetisation per unit mass at 0 K and in infinite field strength has been found to be 163 corresponding to a Bohr magneton value 1.92 per Mn atom. The Curie temperature in zero field is 300 C. The significance of this magnetic data is discussed
[en] The magnetic structures of a series of Y2(Fe1-xMnx)14B samples, with x equal to 0.03, 0.10, 0.25, and 0.37, have been studied by powder neutron diffraction and Moessbauer spectroscopy. Y2(Fe0.97Mn0.03)14B and Y2(Fe0.9Mn0.1)14B are ferromagnetic at both 295 and 85 K, Y2(Fe0.63Mn0.37)14B is paramagnetic at both 295 and 85 K, whereas Y2(Fe0.75Mn0.25)14B is paramagnetic at 295 K and is partially ordered at 78 K. The magnetic structure of Y2(Fe0.75Mn0.25)14B is explained in terms of the preferential Mn occupancy of the transition metal 8j2 site in the Y2Fe14B structure. Small amounts of Mn located in this site are very effective in disrupting the long-range ferromagnetic coupling