Theoretical Simulation and Physical Properties of MgN8 Crystal Structure under High Pressure
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摘要: 基于密度泛函理论第一性原理的方法，使用CALYPSO结构搜索技术结合VASP软件，在0～100 GPa压强范围内对MgN8的晶体结构进行预测，并对预测的结构进行系统研究。结果表明：在常压下，空间群为P4/mbm的α-MgN8晶体结构的焓值最低；当压强达到24.3 GPa和68.3 GPa时发生相变，分别相变成空间群为P4/mnc的β-MgN8相和空间群为Cmcm的γ-MgN8相，两次相变均为对应体积坍塌的一级相变。电子性质计算结果表明，α-MgN8相的导带与价带之间具有3.09 eV的带隙，表明该结构具有非金属性；β相和γ相具有明显的金属特征。Bader电荷转移计算表明，随着压力的增加，Mg原子向N原子转移的电荷逐渐增多。Abstract: Based on the first principle of density functional theory, the crystal structure of MgN8 was predicted in the pressure range of 0–100 GPa by using CALYPSO structure search technique and VASP software. After systematically studying the predicted structure, it was found that the enthalpy of α-MgN8 crystal with space group P4/mbm was the lowest at ambient pressure. The phase was changed to β-MgN8 phase of P4/mnc and γ-MgN8 phase of Cmcm when the pressure reached 24.3 GPa and 68.3 GPa, respectively. And both of the phase transitions were the first order phase transition of corresponding volume collapse. The calculated results of electronic properties suggested that the existence of a band gap of 3.09 eV between the conduction band and valence band of α-MgN8 phase revealed the non-gold properties of the structure, whereas the obvious metal characteristics appeared in the β-MgN8 phase and γ-MgN8 phase. Bader charge transfer calculation showed that the charge which transferred from Mg atom to N atom, increased gradually with the increase of pressure.
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Key words:
- high pressure /
- first-principles /
- crystal structure prediction /
- MgN8 /
- charge transfer /
- phase transition
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图 1 MgN8的焓差曲线以及α-MgN8、β-MgN8、γ-MgN8相体积随压强的变化关系
Figure 1. Calculated enthalpies per formula unit of pressure of MgN8 and the calculated volume versus pressure of α-MgN8, β-MgN8 and γ-MgN8
图 2 α-MgN8、β-MgN8和γ-MgN8相的晶体结构
Figure 2. Crystal structures of α-MgN8, β-MgN8 and γ-MgN8
图 3 α-MgN8、β-MgN8、γ-MgN8相声子色散关系和声子态密度
Figure 3. Phonon-dispersion curves and the phonon density of states of α-MgN8, β-MgN8 and γ-MgN8
图 4 α-MgN8、β-MgN8、γ-MgN8相的能带结构和电子态密度
Figure 4. Band structure and partial density of states of α-MgN8, β-MgN8 and γ-MgN8
图 5 α-MgN8、β-MgN8、γ-MgN8相的电子局域函数
Figure 5. Electron localization function of α-MgN8, β-MgN8 and γ-MgN8
表 1 α-MgN8、β-MgN8和γ-MgN8相的平衡态晶格常数
Table 1. Lattice parameters of α-MgN8, β-MgN8 and γ-MgN8 in the equilibrium state
Phase Pressure/GPa a/Å b/Å c/Å α/(°) β/(°) γ/(°) α-MgN8 (P4/mbm) 0 5.913 5.913 6.572 90.0 90.0 90.0 β-MgN8 (P4/mnc) 24.3 6.219 6.219 3.724 90.0 90.0 90.0 γ-MgN8 (Cmcm) 68.3 4.167 4.167 8.680 90.0 90.0 51.1 
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表 2 α-MgN8、β-MgN8和γ-MgN8相的平衡态晶格常数和原子位置
Table 2. Lattice parameters and atomic coordinate of α-MgN8, β-MgN8 and γ-MgN8 in the equilibrium state
Phase Pressure/GPa Atoms Wyckoff position x y z α-MgN8 (P4/mbm) 0 Mg 2b 0.000 0.000 0.500 N1 2a 0.000 0.000 0.000 N2 2c 0.000 0.500 0.500 N3 4e 0.000 0.000 0.821 N4 4f 0.000 0.500 0.915 N5 4h 0.859 0.359 0.500 β-MgN8 (P4/mnc) 24.3 Mg 2a 0.000 0.000 1.000 N 16i 0.416 0.219 0.823 γ-MgN8 (Cmcm) 68.3 Mg 8g 0.624 –0.376 1.250 N1 16h 0.226 –0.135 0.617 N2 16h 1.115 0.471 1.386
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表 3 α-MgN8、β-MgN8、γ-MgN8相电荷转移
Table 3. Calculated Bader charges of α-MgN8, β-MgN8 and γ-MgN8
Phase Pressure/GPa Atoms Number Charge value/e Charge transfer/e α-MgN8 (P4/mbm) 0 Mg 1 6.26 1.74 N1 1 4.84 0.16 N2 1 4.86 0.13 N3 2 5.04 –0.04 N4 2 5.46 –0.46 N5 2 5.52 –0.52 β-MgN8 (P4/mnc) 24.3 Mg 1 6.06 1.94 N 8 5.24 –0.24 γ-MgN8 (Cmcm) 68.3 Mg 1 5.80 2.20 N1 4 5.28 –0.28 N2 4 5.27 –0.27
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