β-MnO2高压相的从头计算模拟

 基于密度泛函理论（DFT），用完全势能线性缀加平面波方法（FPLAPW）计算模拟了MnO₂的同质多相变体（金红石型结构、CaCl₂型结构、黄铁矿型结构），通过计算，预测了β-MnO₂在5 GPa时从金红石型结构转变为CaCl₂型结构，在20 GPa时进一步转变为黄铁矿型结构。另外，还总结和比较了几种金红石型结构二氧化物的压致相变特点，得出第Ⅳ主族元素的金红石型结构氧化物有很好的规律性，发生的相变序列基本一致；并随着金属阳离子半径的增大，发生相变的压力值也相应地递减，各氧化物的体积模量值相应地减小。     

用甲醇-氢混合气源在不锈钢上生长金刚石薄膜的研究

 利用甲醇-氢（CH₃OH-H₂）混合气体为气源，30 nm厚的无定形硅为过渡层，借助于微波等离子体化学气相沉积（MWCVD）成功地将金刚石薄膜生长在不锈钢上，其最低生长温度可至420 ℃，并且甲醇-氢混合气体比传统的甲烷-氢（Ch₄-H₂）更具优势，测试表明这种金刚石薄膜有希望作为耐磨层在工业上应用。     

Cu100-xZrx金属玻璃的电阻-压力特性

 本文在0~0.8 GPa的压力范围内对金属玻璃Cu_100-xZr_x（x=70，75）进行了室温下的电阻测量。利用实验得到的负的电阻-压力系数α_p及推广的Ziman理论着重计算并讨论了压力下d波相移η₂(E_F)项对α_p的贡献。     

高压下Al的零温电子结构和物态方程

 用LMTO法（线性Muffin-Tin轨道法），计算了金属铝的超高压电子结构及零温物态方程。Al的压缩比到10，压力达10 TPa。根据第一原理计算结果，在带结构方面，s带总是处于Fermi能以下，随着压力的增加，s、p和d的带宽增加，随后则杂化程度增加，所以s、d轨道的电子占据数连续变化。上述变化对压力的影响也是连续的，换言之，从s→d转变没有理由说明Al在0.5 TPa附近冲击Hugoniot的斜率的拐弯现象。而这一点则是与Altshuler的观点不同。物态方程的第一原理计算结果表明，bcc结构比fcc结构要软，但因差别不很大，即使发生fcc→bcc的转变，也不会引起Hugoniot的质的变化（拐弯）。Al的LMTO物态方程还表明，我们以前所采用的半经验的冷压误差可达30%。为此，根据LMTO结果，给出新的冷压表达式p=∑_i=0⁵a_iδ^i/3，δ=Ω₀/Ω，其中a₀=-7.327 79，a₁=18.754 3，a₂=10.209 7，a₃=-2.523 53，a₄=-4.787 2，a₅=6.065 94，拟合误差小于3%。     

面心立方的金属氢结构与能量的全量子力学计算

本文利用电子瞬时配对形成未饱和共价键的物理模型来划分通道，将改进的排列通道量子力学方法推广应用于面心立方的金属氢结构与能量的理论计算。结果发现，当中心原子与周围顶角间距Ｒ０＝１．７０ａ０时，总能量有一级小值－７．７５４（ｈ．ａ．ｕ．），表明金属氢的面心立方结构是稳定存在的，从而通过定量理论计算验证了金属氢高压合成机理的合理性。     

高压对以沸石分子筛基质的Eu（Ⅲ）稀土发光材料荧光光谱影响的研究

 本文研究了高压下无机微孔材料的相转变，并讨论了压力对离子交换的Eu（Ⅲ）NaA和Eu（Ⅲ）NaY两种以沸石分子筛为基质的稀土发光材料发光性质的影响。实验结果表明，对于不同基质材料，压力对Eu（Ⅲ）离子的光谱结构的影响，尤其是对⁵D₀→⁷F₁磁偶极跃迁与⁵D₀→⁷F₂电偶极跃迁强度比（I_m/I_e）的影响十分显著。对于Eu（Ⅲ）NaA样品来说，I_m/I_e值随压力的增加而增加，而对于Eu（Ⅲ）NaY样品，I_m/I_e值随压力的增加而减少。     

Hg1-xCdxTe在高压下的结构、状态方程与相变

 利用X射线粉末衍射方法,在室温高压下观察到了Hg_1-xCd_xTe（x=0.19）的相变。实验是在DAC高压装置上完成的,压力从0逐步加至10.1 GPa。在常温常压下Hg_1-xCd_xTe（x=0.19）具有闪锌矿结构。从实验结果看到,在压力为3 GPa和6.8～8.3 GPa之间有两个结构相变存在。初步认为,后一个相变与Hg_1-xCd_xTe（x=0.19）的金属化有密切关系。通过计算,得到了它在相变前的状态方程,并且与二元HgTe化合物在相变规律上进行了比较。     

固体氢在极端压强下的超导性能

氢元素在常压下具有最简单的晶体结构和物理性质。随着压强增加,氢单质发生相变,由绝缘体转变为金属,被称为金属氢。数值模拟表明,金属氢具有高温超导电性,因此,金属氢研究也被称为高压物理领域的“圣杯”课题。利用基于密度泛函理论的第一性原理计算方法,对固体氢在极端高压(0.5～5.0 TPa)下的结构和超导电性开展了系统研究。研究结果表明：固体氢的高压相变序列为I4₁/amd→oC12→cI16;对于同一种结构,随着压强增加,电声耦合系数减小,费米面处电子态密度减小,特征振动频率增加,超导转变温度发生小幅变化;在2.0 TPa压强下,固体氢的超导转变温度高达418 K(库伦赝势经验值μ^*=0.10)。研究工作将为金属氢及其超导电性的后续理论和实验研究提供参考。     

偏硼酸钡低温相晶体在高压下的电学性质与相变

 在金刚石压砧装置上，采用电容和电阻测量方法研究了偏硼酸钡低温相晶体（β-BaB₂O₄）在室温下和16 GPa内的电容、电阻与压力的关系。实验结果表明，它的电容在2.1、4.6、6.4、8、9.5、10.7 GPa左右都有一个突变。这说明β-BaB₂O₄内部的结构状态在这些压力下都发生了变化，可能发生了相变。还发现β-BaB₂O₄样品在较高压力下已发生了非晶化转变，而且是不可逆的，在卸压后被保留下来。这个非晶化转变的压力大约在11~12 GPa。     

正辛烷的高压拉曼光谱研究

 采用金刚石对顶砧高压装置,在室温下对正辛烷（C₈H₁₈）进行了原位高压拉曼光谱研究,实验的最高压力为13GPa。在实验的压力范围内,正辛烷的拉曼峰位随压力的升高均向高频移动,峰强逐渐减弱,峰形变宽。常态为液态的正辛烷在0.8 GPa时,拉曼频移随压力的变化曲线出现了拐点,发生了液－固相变;在6.8 GPa时,伴随着原拉曼峰的消失或劈裂,以及新拉曼峰的出现,此时正辛烷可能发生了固－固相变。该相变压力低于已有的低碳数正烃烷的压致相变结果。正烃烷的压致相变压力点,具有随着结构链长的增加,其相变压力降低的规律。     

静态超高压下氢的晶体结构实验研究

在极端条件下,固态氢会经历一系列相变,理论预测其在足够高的压力下会演变为金属。由于金属氢被预测具有室温超导和超流等特性,其研究受到了学界的极大关注。然而,研究金属氢存在巨大的技术挑战:一方面,达到氢金属化的压力条件极为苛刻,至今对冷压下是否已制备出金属氢仍未达成共识;另一方面,超高压下氢的精确表征十分困难,特别是表征固态氢晶体结构的技术手段更是严重滞后。晶体结构作为了解一种材料的最基本信息,对其认知的匮乏阻碍了理解氢在高压下如何逐步演化为金属氢的过程。为此,着眼于超高压氢的晶体结构测量,发展了一套先进同步辐射X射线衍射方法,在室温下将氢的晶体结构测量扩展至254 GPa,将相关压力记录提高了一倍。介绍了相关的技术突破,探讨了在超高压下对氢进行晶体结构测量的方法以及存在的问题,以期为在更高压力条件下测量氢的结构信息做好铺垫。     

金属氢研究新进展

简要介绍了金属氢的研究意义和应用前景 ,详细评述了有关的高压实验方法和最近的研究成果及进展 ,特别是固体氢的相图、结构和相变 .近十多年来 ,随着超高压技术的发展 ,已能在金刚石对顶砧 (DAC)上产生30 0GPa的静态压力 ,并可进行高压原位实验研究 .对固体氢进行了高压拉曼、同步辐射X射线、光反射和吸收、同步辐射红外光谱等一系列高压物性和相变研究 .从而确定了固体氢的三个相 ,并提出了可能的相结构 .     

Absence of metallization in solid molecular hydrogen

Being the simplest element with just one electron and proton the electronic structure of a single Hydrogen atom is known exactly. However, this does not hold for the complex interplay between them in a solid and in particular not at high pressure that is known to alter the crystal as well as the electronic structure and eventually causes solid hydrogen to become metallic. In spite of intense research efforts the experimental realization of metallic hydrogen, as well as the theoretical determination of the crystal structure has remained elusive. Here we present a computational study showing that the distorted hexagonal P6₃/m structure is the most likely candidate for Phase III of solid hydrogen. We find that the pairing structure is very persistent and insulating over the whole pressure range, which suggests that metallization due to dissociation may precede eventual bandgap closure. Due to the fact that this not only resolve one of major disagreement between theory and experiment, but also excludes the conjectured existence of phonon-driven superconductivity in solid molecular hydrogen, our results involve a complete revision of the zero-temperature phase diagram of Phase III.     

Evolution of metallization and superconductivity in solid hydrogen

Inspired by the recent experimental reports of metallic hydrogen [Science 355 (2017) 715, Nature, 577 (2020) 631], we have reexamined the metallization and superconductivity of solid hydrogen in the pressure range of interest. Based on high quality calculations with zero-point vibrations and van der Waals (vdW) corrections, hydrogen is disclosed to metallize at about 485 GPa via the phase transition from insulate molecular C2/c-24 to metallic molecular Cmca-4, then dissociate into atomic phase with increasing pressure to 600 GPa. Further analyses demonstrate that vdW interaction can reduce the H-H distances in the metallic molecular Cmca-4, thus pulling down the contributions from phonon frequencies and electronic structures to electron-phonon coupling λ, and resulting in the declining superconducting $T_{\mathrm{c}}$. Meanwhile, slightly influence on these cases can be found in metallic atomic I4₁/amd by vdW correction. Our results indicate that aspirational room-temperature superconductivity in solid hydrogen requires a high pressure beyond 600 GPa.     

高压下FCC相金属氢结构稳定性和非谐效应的理论研究

利用第一性原理分子动力学方法研究金属氢体系的非简谐效应, 给出金属氢的声子谱, 讨论金属氢声子谱的温度效应。计算得到氢的同位素氕、氘和氚的FCC相在非零温下的声子谱, 不同温度下的声子谱对比发现零温下3.6 TPa为热力学稳定的临界压强点, 而有限温度下(100 K)临界压强点降到2.8 TPa, 非简谐效应显著地改变了体系的结构稳定性和声子振动性质。     

Metallization of fluid hydrogen at 140 GPa (1.4Mbar) by shock compression

Abstract

The minimum electrical conductivity of a metal was produced in dense hydrogen using shock compression. Metallization occurs at 140 GPa (1.4 Mbar), 0.6 g/cm³ (ninefold compression of initial liquid-H₂ density), and 3000 K. The relatively modest temperature generated by a reverberating shock wave produced the metallic state in a warm quantum fluid at a lower pressure than expected previously for the crystallographically ordered solid at low temperatures. Future research directions are discussed. Possible scientific and technological uses of metastable solid metallic hydrogen are speculated upon in the unlikely event that the metallic fluid can be quenched to this state at ambient pressure and temperature.     

Solid hydrogen. Ground-state energy,pressure, compressibility and phase transition at high densities

The ground-state energy, the pressure and the compressibility of solid molecular hydrogen is calculated by means of a modified Brueckner theory. The Bethe-Gold-stone equation is solved to give the reaction matrix or the effective interaction in coordinate space, and the ground-state energy for hcp and fcp hydrogen is calculated. Also, the pressure and the compressibility is estimated from the dependence of the ground-state energy on density or molar volume. The possibility of a phase transition from solid molecular hydrogen into a metallic atomic phase is also considered. The ground-state energy and pressure for bcc atomic hydrogen is calculated, and a phase transition is found to occur at a pressure of 1.2·10⁶ atm.     

Mössbauer study of Mg–Ni(Fe) alloys processed as materials for solid state hydrogen storage

Mg–Ni–Fe magnesium-rich intermetallic compounds were prepared following two distinct routes. A Mg₈₈Ni₁₁Fe₁ sample (A) was prepared by melt spinning Mg–Ni–Fe pellets and then by high-energy ball milling for 6 h the obtained ribbons. A (MgH₂)₈₈Ni₁₁Fe₁ sample (B) was obtained by high-energy ball milling for 20 h a mixture of Ni, Fe and MgH₂ powders in the due proportions. A SPEX8000 shaker mill with a 10:1 ball to powder ratio was used for milling in argon atmosphere. The samples were submitted to repeated hydrogen absorption/desorption cycles in a Sievert type gas–solid reaction controller at temperatures in the range 520?÷?590 K and a maximum pressure of 2.5 MPa during absorption. The samples were analysed before and after the hydrogen absorption/desorption cycles by X-ray diffraction and Mössbauer spectroscopy. The results concerning the hydrogen storage properties of the studied compounds are discussed in connection with the micro-structural characteristics found by means of the used analytical techniques. The improved kinetics of hydrogen desorption for sample A, in comparison to sample B, has been ascribed to the different behaviour of iron atoms in the two cases, as proved by Mössbauer spectroscopy. In fact, iron results homogeneously distributed in sample A, partly at the Mg₂Ni grain boundaries, with catalytic effect on the gas–solid reaction; in sample B, instead, iron is dispersed inside the hydride powder as metallic iron or superparamagnetic iron.     

基于路径积分分子动力学与热力学积分方法的高压氢自由能计算

在相图研究中, 严格计算一个真实系统在特定温度、压强下的自由能是近年来该领域理论方法发展的前沿. 自Mermin提出有限温度密度泛函理论后, 在电子结构层面, 弱关联系统中人们就其在对自由能贡献的描述已相对完善, 但在原子核运动的描述上, 热运动与量子运动的非简谐项却总被忽视. 本文将路径积分分子动力学与热力学积分结合, 对300 GPa下氢晶体Cmca 结构中原子核热涨落与量子涨落对自由能的影响进行了分析. 发现在100 K核量子涨落非简谐项的贡献约为15 meV每原子, 远大于不同结构间静态焓的差别. 该研究提醒人们简谐近似在核量子效应描述中可能存在的不准确性(即使在低温下). 同时, 我们采取的方法 也为人们进行自由能的准确计算提供了一个简单有效的手段.     

Solid hydrogen. Transition into a metallic phase at high densities

The ground-state energy, the pressure and the compressibility of solid molecular hydrogen is calculated by means of a modified Brueckner theory. The possibility of a phase transition into a metallic atomic phase is then considered, and a phase transition is found to occur at a pressure of 1.9 Mbar