Pressure-Induced Polymerization of One-Dimensional Nitrogen Chains in K2N2
doi: 10.11858/gywlxb.20240719
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摘要: 采用先进的粒子群晶体结构搜索方法对K2N2在0～150 GPa压强范围内进行晶体结构预测，结果表明，K2N2的基态稳定相为单斜C2/m结构，且在1.7、3.6和122 GPa压强下的结构分别为Na2N2型、Cmmm、C2/c。体积随压强的变化关系显示C2/m→Na2N2型、Na2N2型→Cmmm和Cmmm→C2/c这3个相变均为一级相变，对应的体积坍塌分别为14.4%、22.5%和4.0%。在K2N2高压相变过程中，K原子的配位数从5增加到10，并伴随着N-N成键性质的变化，即从基态C2/m结构中的准分子N＝N双键聚合为高压C2/c相中的N―N单键链。C2/m、Na2N2型、Cmmm相表现出金属性，而高压C2/c相表现出半导体（带隙为2.0 eV）性质。电子结构计算和电子局域函数分析表明，K2N2的高压结构相变来源于高压下K-p孤对电子的激活及其与N原子的成键。Abstract: The crystal structure prediction of K2N2 in the pressure range of 0–150 GPa using an advanced particle swarm crystal structure search method was conducted. The results show that the stable ground state phase of K2N2 is a monoclinicC2/mstructure, and three high-pressure structures including Na2N2-type,Cmmm, andC2/care identified at pressures of 1.7, 3.6, 122 GPa, respectively. The volume dependence on pressure shows that the three phase transitions, i. e.,C2/m→Na2N2-type, Na2N2-type→Cmmm, andCmmm→C2/c, are all first order phase transitions, corresponding to volume collapses of 14.4%, 22.5%, and 4.0%, respectively. During the high pressure phase transitions of K2N2, the coordination number of K atom increases from 5 to 10, and a change in the nature of the N-N bonding from N＝N dimmer in the ground state ofC2/mstructure to N―N single bond chain in the high-pressureC2/cphase is accompanied. The high-pressureC2/cphase exhibits semiconducting properties with a band gap of 2.0 eV, whileC2/m, Na2N2-type, andCmmmphases have metallic behaviors. Electronic structure calculation and electron-localized function analysis indicate that the high-pressure structural phase transition of K2N2 is due to the K-plone-pair electrons activation and their participation in bonding with N atoms under high pressure.
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Key words:
- high pressure /
- K2N2 /
- structure predictions /
- phase transitions /
- electronic structures
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Figure 1. Crystal structure of C2/m, Na2N2-type, Cmmm, and C2/c phases (The large and small spheres represent K and N atoms, respectively.)
Figure 2. Calculated phonon curves of the C2/m (a), Na2N2-type (b), Cmmm (c), and C2/c (d) phases at selected pressure points
Figure 3. (a)–(b) Enthalpy differences of the different predicted structures relative to the Na2N2-type structure under pressure;(c)–(d) pressure dependence of the volume per f. u. of each structure for K2N2
Figure 4. Total and site projected DOSs of C2/m (a), Na2N2-type (b), Cmmm (c), and C2/c (d) phases
Figure 5. Projected weights of N-p orbitals in the band structures of C2/m (a), Na2N2-type (b), Cmmm (c), and C2/c phases (d) (The Fermi level is indicated by horizontal lines and the black solid lines denote the energy structures of each phase of K2N2)
Figure 6. Contours of ELF for the C2/m on the (010) plane (a), Na2N2-type on the (010) plane (b), Cmmm on the (100) plane (c), and C2/c on the (010) plane (d)
Table 1. Optimized structural parameters of the C2/m, Na2N2-type, Cmmm, and C2/c phases of K2N2
Phase Pressure/GPa Lattice parameters dN-N/Å Atomic fractional coordinates C2/m 0 a = 7.562 Å, b = 3.912 Å, c = 10.923 Å,
α = γ = 90°, β= 134.955°1.192 K 4i (0.613, 0, 0.260)
N 4i (0.519, 0, 0.456)Na2N2-type 2.5 a = 3.326 Å, b = 4.375 Å, c = 5.694 Å,
α = β = γ = 90°1.220 K1 1e (0, 0.500, 0)
K2 1c (0, 0, 0.500)
N 2t (0.500, 0.500, 0.393)Cmmm 20.0 a = 6.896 Å, b = 5.104 Å, c = 2.855 Å,
α = β = γ = 90°1.254 K 4g (0.694, 0, 0)
N 4j (0, 0.877, 0.500)C2/c 135.0 a = 6.971 Å, b = 4.099 Å, c = 4.510 Å,
α = γ = 90°, β= 81.342°1.474, 1.373 K 8f (0.669, 0.909, 0.198)
N 8f (0.041, 0.887, 0.102)
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Table 2. Calculated Bader charges of K and N atoms in C2/m, Na2N2-type, Cmmm, and C2/c phases
Phase Pressure/GPa Atom Charge value/e Charge transfer/e C2/m 0 K
N8.509 (×2)
5.491 (×2)+0.491
−0.491Na2N2-type 2.5 K1
K2
N8.407
8.303
5.645 (×2)+0.593
+0.697
−0.645Cmmm 20.0 K
N8.310 (×2)
5.690 (×2)+0.690
−0.690C2/c 135.0 K
N8.377 (×2)
5.623 (×2)+0.623
−0.623
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