State Key Lab of Superhard Materials, Jilin University, Changchun 130012, China.
J Chem Phys. 2012 Aug 21;137(7):074501. doi: 10.1063/1.4745186.
By employing first-principles metadynamics simulations, we explore the 300 K structures of solid hydrogen over the pressure range 150-300 GPa. At 200 GPa, we find the ambient-pressure disordered hexagonal close-packed (hcp) phase transited into an insulating partially ordered hcp phase (po-hcp), a mixture of ordered graphene-like H(2) layers and the other layers of weakly coupled, disordered H(2) molecules. Within this phase, hydrogen remains in paired states with creation of shorter intra-molecular bonds, which are responsible for the very high experimental Raman peak above 4000 cm(-1). At 275 GPa, our simulations predicted a transformation from po-hcp into the ordered molecular metallic Cmca phase (4 molecules/cell) that was previously proposed to be stable only above 400 GPa. Gibbs free energy calculations at 300 K confirmed the energetic stabilities of the po-hcp and metallic Cmca phases over all known structures at 220-242 GPa and >242 GPa, respectively. Our simulations highlighted the major role played by temperature in tuning the phase stabilities and provided theoretical support for claimed metallization of solid hydrogen below 300 GPa at 300 K.
采用第一性原理的元动力学模拟，我们在 150-300 GPa 的压力范围内探索了 300 K 下固态氢的结构。在 200 GPa 时，我们发现常压无序六方密堆积（hcp）相转变为绝缘部分有序 hcp 相（po-hcp），这是有序类石墨烯 H(2)层和其他层弱耦合、无序 H(2)分子的混合物。在这个相中，氢仍处于配对状态，形成较短的分子内键，这就是实验中 Raman 峰超过 4000 cm(-1)的原因。在 275 GPa 时，我们的模拟预测了从 po-hcp 到有序分子金属 Cmca 相（4 个分子/晶胞）的转变，先前的研究认为这种相仅在 400 GPa 以上才稳定。300 K 下的吉布斯自由能计算证实了 po-hcp 和金属 Cmca 相在 220-242 GPa 和 >242 GPa 范围内在所有已知结构上的能量稳定性。我们的模拟强调了温度在调节相稳定性方面的重要作用，并为声称在 300 K 下 300 GPa 以下固态氢的金属化提供了理论支持。
