First-principle Calculations Of Physical Properties Of Transitional Metal Borides

The transition metal borides(TMBs) have attracted attention as the potential hard material candidates due to the outstanding properties such as high hardness, high strength, ultra–incompressibility and good thermal stability. In this dissertation, the crystal structure, mechanical, thermodynamical, dynamical and electronic properties of transitional metal borides are investigated by using first-principles calculations. The main conclusions are as follows:(1) The structural, thermodynamical, mechanical, dynamical and electronic properties of 5d transitional metal triborides TMB3(TM=Hf-Au) are investigated by density functional theory(CASTEP code). For each TMB3(TM = Hf-Au), five structures are considered, i.e., m-Al B2(modified Al B2), Os B3, Fe B3, WB3 and Tc P3 structures. Thermodynamic stability of compounds is predicted and the formation enthalpy increases from Ta B3 to Au B3. The calculated phonon dispersion curves demonstrate that each TMB3 in the most stable structure is dynamically stable. The calculated densities of states show that they are all metallic. Among the considered compounds, Os B3-Re B3(Os B3-Re B3 represents Re B3 in Os B3 structure, the same hereinafter) has the largest shear modulus(261 GPa), bulk modulus(355 GPa) and Young modulus(630 GPa). WB3-WB3 has the second largest shear modulus(257 GPa). This suggests that Os B3-Re B3 and WB3-WB3 might be potential ultra-incompressible and superhard materials.(2) The phase stability and mechanical properties of ruthenium borides Ru7B3, Ru B, Ru2B3, Ru B2, Ru B3 and Ru B4 have been investigated systemically by first-principles calculations within density functional theory. The results show that Re B2-Ru B2,Tc P3-Ru B3 and Mo B4-Ru B4 are more thermodynamically and mechanically stable than other structures at ambient conditions. Further analysis on density of states unravel that the high bulk modulus and high shear modulus as well as small Poisson’s ratio of ruthenium borides are derived from the force of covalent bonding covalent bonding(B-B and Ru-B). Combing enthalpy-pressure relationship with convex hull, it is interesting to note that the synthesized Ru7B3-Ru7B3, Ru2B3-Ru2B3 and hypothetical WB-Ru B, Re B2-Ru B2 should be ground state phases at zero pressure; while at 60 GPa, the predicted Mo B4-Ru B4 becomes a stable phase; Ru7B3-Ru7B3, WB-Ru B, Re B2-Ru B2 and Mo B4-Ru B4 are the most stable phases at about 100 GPa. High pressure is advantageous to synthesis of B-rich ruthenium borides.(3) For tantalum borides, include t I12-Ta2 B, o C8-Ta B, o C22-Ta5B6, o I14-Ta3B4 and h P3-Ta B2 have been synthesized experimentally. Thus, the crystal structure of Ta B3 is predicted by using the evolutionary algorithm(USPEX code) first, it is indicated that the most stable phase of Ta B3 is o C16-Ta B3 in the orthorhombic Imma sructure. And then, various properties of o C16-Ta B3 have been investigated by first-principles calculations(VASP software). The calculated convex hull indicates that at ambient conditions, the ground state phases are t I12-Ta2 B, o C8-Ta B, o C22-Ta5B6, o I14-Ta3B4, and h P3-Ta B2; while at 75 GPa, they are t I12-Ta2 B, o C8-Ta B, o C22-Ta5B6, o I14-Ta3B4, h P3-Ta B2 and o C16-Ta B3; o C8-Ta B, o C22-Ta5B6, o I14-Ta3B4, o C16-Ta B3 are the most stable phases at 120 GPa. Relative stabilities of the experimentally known phases and the predicted one(Ta B3) are also studied. The mechanical, dynamical and electronic properties are discussed. The calculated results indicate that the predicted o C16-Ta B3 lies on the line of the convex hull above 75 GPa. Combining the estimated hardness(41.2 GPa) and indentation strength(22.8 GPa), it is suggested that o C16-Ta B3 is hard or potential superhard material. For predicted o C16-Ta B3, further experimental synthesis could be rewarding.