Studies On Synthesis Of Manganese Borides At High Pressure-High Temperature And Their Pressure Induced Phase Transitions

Transition metal borides have excellent physical and chemicalproperties, such as high melting point, high hardness and high chemicalstability, etc. And manganese borides also bear these excellent properties.Recently, Sezgin Aydin et al. studied the ReB₂-type structure MnB₂, TcB₂and ReB₂with the first-principle calculations. Their results showed thatthe ReB₂-type structure of those compounds were more stable than theAlB₂-type structure. Although ReB₂-type MnB₂has relatively low bulkand shear modulus, it has a hardness as high as43.9GPa, surpassing thestandard value40GPa of superhard materials, which indicates that theReB₂-type MnB₂may be a potential superhard material. Wang et al.studied the structures and properties of Mn-B compounds with the firstprinciple calculations, their results showed that both the ReB₂-type MnB₂and the monoclinic structure MnB₄might be potential superhard materials.At present, the experimental and theoretical studies on the manganese borides concentrate on ambient pressure, however, the high pressureexperimental synthesis and high pressure structural stability studies ofMn-B systems are rare.We synthesized a series of manganese borides at high temperatureand high pressure. In this thesis, we will discuss the synthesis temperature,pressure effects on the preparation of various manganese borides, and wewill analyze their growth mechanism. The in-situ high pressuresynchrotron radiation x-ray diffraction （XRD） and high pressure electricaltests were employed to study the Mn-B systems, combined with the firstprinciple calculations. The main original results are listed as following:1. We prepared several kinds of manganese borides at hightemperature and high pressure. And the synthesis temperature, holdingtime, the Mole ratio of the raw materials were discussed, and wesynthesized the single phases of Mn₂B, Mn₃B₄, MnB, MnB₂, and MnB₄containing with the impurities of MnBx. We synthesized the AlB₂-typeMnB₂at different conditions, indicating that AlB₂-type MnB₂is morestable within our synthesizing condition. For successive synthesis ofmanganese borides, excessive boron is necessary. And the formationmechanism of different products will also be discussed. The results showthat the Gibbs free energy of these compounds plays a key role in finalproduct. When we tested on the bulk Mn₃B₄sample, the productexhibited a preferred orientation on （002） crystal plane, which may be ascribed to the lower Gibbs free energy on the （002） crystal plane growthat high temperature and high pressure.2. We carried out the in situ high-pressure synchrotron X-raydiffraction （XRD） experiments and high-pressure electrical measurementson MnB₂. We observed lattice parameters changed at6.2GPa,18.44GPaand31GPa, and the lattice volume collapsed at31GPa. However, therewere no diffraction peaks found either dispress or occurance, whichsuggests that no crystal structural phase transition occurs. Combined withthe first-principles calculations, we found that it could be explained withthe electronic phase transition at19.36GPa and the magnetic phasetransition at31GPa. By fitting the equation of state （EOS）, we got bulkmodulus of175.11±3.49GPa and209.76±13.47GPa of phase I andphase II, and the phase II is more difficult to compress than phase I.We use the silicone oil as a pressure transmission medium in anotherexperiment, and we also observed the lattice parameters changed at11.21GPa and19.84GPa, which were consistent with the first experimentusing argon as a pressure transmission medium. However, no variation ofthe lattice parameters and lattice volume was found at31GPa. The reasonof the difference may be related to the choice of the pressure transmissionmedium. These results show that the choice of different pressuretransmission medium may play an important role on the experimentalresults in the high pressure experiments. 3. We also carried out the in situ high pressure synchrotron XRDtests on Mn₂B, MnB₄, MnB and Mn₃B₄. The results showed that thesematerials are stable in the pressure range we studied, and there was nostructural phase transition. We obtained the EOS and zero-pressure bulkmodulus of Mn₂B, MnB₄, MnB and Mn₃B₄by fitting the experimentalP-V data. We calculated the unit cell parameters of Mn₂B, MnB₄, MnBand Mn₃B₄, and found that the crystal axes of them exhibit an anisotropiccharacteristic. We explain the characteristics of this anisotropy with theircrystal structures and electronic structures.