Acta Physica Sinica, Volume. 68, Issue 9, 096201-1(2019)
To search new hard or superhard materials in transition-metal light-element compounds is a current research focus. Most of the past researches focused on binary phases such as transition metal borides, carbides and nitrides, while the researches on ternary phases were relatively rare. The single crystals Nb3B3C and Nb4B3C2 were synthesized recently by using Al-Cu alloys as auxiliary metals and their structures were determined by Hillebrechtand Gebhardt. In the present paper, 29 TM3B3C and 29 TM4B3C2 configurations are constructed by TM atoms (TM = Sc to Zn, Y to Cd, Hf to Hg) replacing Nb atoms in the known Nb3B3C and Nb4B3C2 configuration. By calculating the formation energy from first-principles density functional theories, only 13 TM3B3C and 11 TM4B3C2 phases are stable compared with the three elemental substances of TM, boron and carbon. However compared with the most competing phases, only Ta3B3C, Nb3B3C and Nb4B3C2 phases are stable thermodynamically. The metastable Ta4B3C2 phase at 0 K becomes stable when temperature is higher than 250 K. Thus two new phases of Ta3B3C and Ta4B3C2 are uncovered to be stable thermodynamically. Global structure searches conducted by the popular USPEX and CALYPSO softwares prove the Ta3B3C and Ta4B3C2 phases to be the most favorable energetically. By calculating the phonon dispersion curves of the Ta3B3C and Ta4B3C2 phase, no imaginary phonon frequencies are observed in the whole Brillouin zone, which demonstrates the dynamical stability of the Ta3B3C and Ta4B3C2 phase. The calculated elastic constant of the Ta3B3C and Ta4B3C2 phases satisfy the criteria of mechanical stability, showing that the Ta3B3C and Ta4B3C2 phase are stable mechanically. The calculations of band structure and density of state show that the Ta3B3C and Ta4B3C2 phases are both conducting, which mainly arises from the d electrons of Ta atoms. The calculated bulk modulus and shear modulus of the Ta3B3C and Ta4B3C2 phases show their brittle nature. The hardness of Ta3B3C and Ta4B3C2 phase are nearly the same and calculated to be about 26 GPa by Chen’s and Tian’s models, which illuminates that the two phases are hard but not superhard.
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Qian-Ku Hu, Yi-Ming Hou, Qing-Hua Wu, Shuang-Hong Qin, Li-Bo Wang, Ai-Guo Zhou.
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Received: Jan. 27, 2019
Accepted: --
Published Online: Oct. 29, 2019
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