Mechanism of Pressure and Carbon Content Regulating Physical Properties of BCxO Compounds
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摘要: 结合粒子群优化算法生成的候选结构和第一性原理的稳定性分析,预测出新型B-C-O化合物B4C6O4。B4C6O4具有带隙宽度约2.25 eV的直接带隙半导体属性。研究同属BCxO系列,且结构具有相似性的B4C6O4、B2CO2和B4CO4,发现C含量的降低会导致体系带隙增大,三者的分子式体积随C含量的降低而降低,且100 GPa的高压对三者体积均形成高达20%的压缩。高压导致B2CO2和B4C6O4的带隙持续降低,而B4CO4的带隙先升后降。应力-应变模拟结果表明,3种BCxO(x = 3/2, 1/2, 1/4)化合物均具有较高的极限拉伸应力,同时应力引起的应变会影响3种BCxO化合物的带隙。力学性能研究表明,3种BCxO化合物均具有高弹性模量和高硬度等特点。常压下BCxO的最高声子振动频率均高于30 THz,且由高到低分别为B4CO4、B2CO2、B4C6O4,压力作用使该体系结构的键能持续增强。Abstract: A novel B-C-O compound, B4C6O4, was predicted by combining the candidate structure generated by the particle swarm optimization algorithm and first-principles stability analysis. B4C6O4 has a direct bandgap semiconductivity characteristic with a bandgap width of about 2.25 eV. B4C6O4, B2CO2 and B4CO4 have similar structures and belong to the BCxO series. It was found that the decrease of carbon content led to the increase of the band gap of the system, and the molecular formula volume decreased synchronically with the decrease of carbon content, and the high pressure of 100 GPa compressed the volume of the three as high as 20%. The band gaps of B2CO2 and B4C6O4 continue to decrease due to the effect of high pressure, while the band gap of B4CO4 rise first and then fall. The stress-strain simulation results showed that the three BCxO compounds (x = 3/2, 1/2, 1/4) all have high ultimate tensile stress, and the strain would affect the band gaps of the three BCxO compounds. The mechanical properties of three BCxO compounds showed that they all had high modulus of elasticity and hardness. The highest phonon vibration frequencies of BCxO under chamber pressure are higher than 30 THz, and the relationship is B4CO4 > B2CO2 > B4C6O4. The effect of high pressure will cause the continuous enhancement of the bond energy of the system.
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Key words:
- high pressure /
- carbon content /
- stability /
- mechanical properties /
- electrical properties
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图 4 基于HSE06计算得到的常压下B4C6O4的电子能带结构和态密度:(a)原胞,(b)单胞(水平红线、暗青色曲线和金色曲线分别代表费米能级、VBM和CBM)
Figure 4. Calculated electronic band structures and density of states of B4C6O4 phases via HSE06 with primitive cell (a) and unit cell (b) at ambient pressure (The horizontal red line, dark cyan curve and gold curve represent the Fermi energy level, VBM and CBM, respectively.)
图 5 基于PBE计算常压下BCxO原胞结构的电子能带结构和态密度(水平红线、暗青色曲线和金色曲线分别代表费米能级EF、VBM和CBM)
Figure 5. Calculated electronic band structures and density of states of three BCxO phases with primitive cell via PBE at ambient pressure (The horizontal red line, dark cyan curve and gold curve represent the Fermi energy level EF, VBM and CBM, respectively.)
表 1 常压下B4C6O4的原子坐标
Table 1. Atomic Wyckoff positions of B4C6O4 at ambient pressure
Atom Wyckoff site x y z B 4d 0.299 0.500 0.267 C1 2b 0 0.500 0.991 C2 4c 0.250 0.250 0.491 O 4e 0 0.279 0.746 表 2 常压和100 GPa下B4C6O4的弹性常数
Table 2. Elastic parameters of B4C6O4 at ambient pressure and 100 GPa
GPa Pressure C11 C12 C13 C22 C23 Ambient 693.28 15.78 77.13 542.70 167.35 100 1394.79 146.17 335.20 974.72 497.69 Pressure C33 C44 C55 C66 Ambient 516.07 255.45 197.31 222.21 100 1000.58 517.68 363.80 292.39 表 3 恒温常压条件下B4C6O4在分子动力学过程中的动态结构信息
Table 3. Structural information of B4C6O4 during molecular dynamics under constant temperature and ambient pressure
Time/ps $\,\rho $/(g·cm−3) B―B bond length/Å Atom distance/Å C2―C2 O―O B―B 0 3.130 1.842 2.720 3.038 2.757 0.6 3.175 1.776 2.677 3.013 2.775 1.2 3.137 1.869 2.697 3.026 2.722 1.8 3.066 1.827 2.741 3.010 2.810 2.4 3.123 1.846 2.711 3.063 2.806 3.0 3.128 1.884 2.738 3.033 2.798 -
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