The High-pressure Structure Studies On Transition Metal Based Intermetallic Silicides

The intermetallic silicides have the excellent characteristics of high melting point, high oxidation resistance, high electrical and thermal conductivity, etc. This is mainly due to its structure and the electronic structure of the micro aspects.The structure of metal silicides was often tunned by introducing internal stress or element doping in the lattice in industry. The high pressure techniques as a clean and effective research method may also play a similar effect on tunning crystal lattice structures or electronic structures. Therefore, the research on the structure evolution of intermetallic silicides under high pressure can build the internal contact between structures and their macroscopic physical properties, and could play an important role in guiding producing excellent-performance electronic device materials or coating materials.In this paper, using the advanced synchrotron X-ray diffraction techniques with a symmetric diamond anvil cell, the high-pressure structure evolution of three binary transition metal based intermetallics TaSi2, TiSi2and Cu5Si were studied, and the structural evolution of TaSi2under high-temperature high-pressure was also studied. In addition, the enthalpy-pressure and energy-volume relations, the evolutions of the structures and the electronic structures，and the elastic properties of these three transition-metal silicides under high pressure were studied using the first-principle calculations and CASTEP package, and found that the theoretical and experimental values agree well with each other.First, the crystal structure evolution of C40type TaSi2was studied. And the structure diffraction data were collected and refined by Rietveld method. It was found that the diffraction peaks of the hexagonal structure with space group P6422is very similar to that of the orthogonal structure with space group C222for TaSi2above the pressure10GPa. The enthalpy-pressure relation、energy-volume relation and the structural elastic stability of C40type TaSi2were calculated and analyzed by the first-principle calculation under high pressure, it was found that no elastic softening occured within our experimental pressure range, and meets the elastic criterion. The calculations for the high pressure band structure and density of state show that TaSi2has metallicity. The crystal structure evolution of C40type TaSi2was investigated under high temperature high pressure by experimental and theoretical methods. It was found TaSi2goes through a structural phase transition during a temperature decreasing. It indicates that the thermal effect of temperature plays a positive role to the structural phase transition. TiSi2and TaSi2both belong to TMSi2type transition metal silicide, but the structure of TiSi2is distinct with TaSi2. TiSi2mainly has C54-and C49-type structures. By the in situ high pressure X ray diffraction data collected and refined by Rietveld method. The axis compressibility ratio-pressure relation was gained. By fitting the unit cell volume-pressure data to Birch-Murnaghan equation of state, it was found the bulk modulus and the axial modulii of TiSi2are both anisotropic. Through the calculations of enthalpy-pressure and energy-volume relations for the C54-and C49-type structures, the elastic anisotropy and stability for C54-type structure, the band structure and the density of state, it is found that C49structure has lower energy than C54-type structure. The elastic anisotropy increases with the pressure increasing and the elastic softening occured at60GPa for C54-type TiSi2. The calculation of the electronic structure shows that the C54-type TiSi2is metallic.The high-pressure structure stability of Cu5Si was studied using in situ synchrotron X-ray diffraction technique. The scope of pressure was expanded to50GPa, it was found that the lattice parameter and the unit cell volume of Cu5Si show a singular platforms in the range of11.7to15.3GPa. This paper argues that an isostructural phase transition occurs under high pressure with no space group changing. By the "supercell law" of first-principle calculations, the supercell model of Cu5Si crystal structure was built. The structure evolution and the origin of this isostructural phase transition were calculated and analyzed."Supercell Law" is an approximate calculation method, which could introduce some calculation errors, but the calculation results are consistent with the experimentally observed phenomena on Cu5Si. The calculations of the band structure and the density of state under high pressure show that Cu5Si has the metallicity.