关注：

1) 镧系与锕系金属氢化物

2) 计算方法摘录

3) 高压研究意义

研究意义摘录

摘录1：

The study of structural phase transition in solids at highpressure has been a challenging area of research for the last few decades. In thecase of rare earth compounds, both lanthanides and actinides,such a study howeverbecomes more interesting due to the presence of f electrons, which are responsible for itinerant behavior or non-integral valance[1–11]. 【稀土化合物更有趣，由于f电子的存在】

From the point of view of electronic properties, the actinide compounds maybe classified into two categories: onebelonging to localized5f electrons and other having itinerant5f electrons.The degree of delocalization or itinerancy largely depends on the actinide-actinidedistance under pressure [12].

It is well known that with increasingatomic number the f-shells becomes maller and thus are more localized, implyinga smaller bandwidth [8].【bandwidth概念不清楚】. This localization isalso favoured on going from lighter (e.g.P) to heavier anions(e.g.Bi).

In thiscontext it will be interesting as well to investigate thestructural phase transitions in NpP and correlate the same with electronic propertiesfor a few selected actinide compounds,with increasing atomic number but keepinganion mass fixed.

摘录2：

http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8005618

Investigations of ActinideMetals and Compounds under Pressure Provide Important Insights into Bonding and Chemistry.

One effect of pressure on elements and compounds is todecease their interatomic distances, which can bringabout dramatic perturbations in their electronic nature and bonding,which can be reflected in changes in physical and/or chemical properties.

One important issue inthe actinide series of elements is the effect of pressure on the 5f-electrons.

We have probed changes in electronic behavior with pressureby monitoring structure by X-ray diffraction, and have studied several actinidemetals and compounds from thorium through einsteinium. These studies have employed angle dispersive diffraction using synchrotronradiation, and energy dispersive techniques via conventional X-ray sources.

The 5f-electrons of actinide metals and theiralloys are oftenaffected significantly by pressure, while with compounds, the structuralchanges are often not linked to the involvementof 5 f-electron. We shall present some of our more recent findingsfrom studies of selected actinide metals, alloys andcompounds under pressure. A discussion of the results in terms of thechanges in electronic configurations and bonding with regard to the element'sposition in the series is also addressed.

参考网址：

http://www.baike.com/wiki/%E4%BA%8C%E5%85%83%E6%B0%A2%E5%8C%96%E7%89%A9

　　Sc+H2→ScH2（>450℃）

　　Y+H2→YH2（正心立方体结构） / YH3（六方晶体）

　　La+H2→LaH3（300℃）（密度5.36，黑色，易燃，固体，面心立方结构，抗磁性）

　　Ce+H2 →CeH2（300℃，H2）

　　CeH2+H2O （沸）→Ce（OH）3+H2

　　CeO2+H2→CeH3+H2O （H2中加热）

　　CeN+H2→CeH3+NH3 （加热）

　　Ce+CH3Cl→CeH3...（500~600℃）

　　CeH3+O2→CeO2+H2O （自燃）

　　CeH3+HCl→CeCl3+H2

　　Nd（粉）+H2→NdH3 （220℃，石英管中）

　　Ac+(1+x/2)H2=AcH2(+x) 分子式尚未很确定（350℃）（面心立方结构，密度8.35）

　　K（蒸汽）+AcCl3 +H2（痕量）→Ac+AcH2+KCl

　　Th+H2→ThH2 （500~800℃）

　　Th+H2→Th4H15 氢化钍（350℃）(黑色 粉末，立方晶系，密度4，放射性，剧毒）

　　Pa+H2→PaH3 （250~300℃）

　　U+H2→UH3 氢化铀（250℃）

　　U+H2O （沸）→UH3...

　　UH3+HF→UF4 四氟化铀 +H2

　　UH3+BrH→UBr4 四溴化铀 +H2

　　UH3+BrH→UBr3 三溴化铀 +H2 （加热）

　　UH3+HI→UI4 四碘化铀 +H2

　　UH3+HI→UI3 三碘化铀 +H2

　　UH3+H2S→US 一硫化铀 +H2 （400~500℃，24h小时）

　　Np+H2→NpH3 （50℃）（黑色，片状）

　　Pu+H2→PuH2

　　Pu+H2 →PuH3 (25~50℃)

　　Pu+H2O→PuH2 + PuO （慢）

　　Pu+H2O→PuH3+Pu2O3（慢）

　　Pu+H2O→PuH2+Pu4O10

　　Am+H2→AmH3 氢化镅（六方晶，密度9.76）（71kpa氢气压下，100℃，40h）

参考网址：

http://www.ijser.org/icrapid2014-conference-papers.aspx

金属氢化物/金属间氢化合物形成条件

摘自：2013 Structure andmagnetic properties of f-electron compounds and their hydrides

In the case ofintermetallic hydrides【金属间】, certain positions for hydrogen atomsare preferred.

Fig. 2.2 presents some favorable positions onthe example of three principal crystal structures (fcc, hcp, and bcc).Only two types of interstitial sites –octahedral and tetrahedral sites arepractically the only ones that are occupied by hydrogenatoms.

Hydride formation criteria

In order to make a predictionwhether particular material would form a hydride it is necessary to take intoaccount various conditions:

geometry of the atomic arrangementin the unit cell,

electronic factors,

diffusive kinetics,

surface properties, etc.

One of the possible approaches[8] is based on the comparison of the contributions to theformation energy of the hydride.

(1) The first contribution is theenergy to convert the crystal structure of the parent compound to the crystalstructure formed in the hydride.

(2) A second contribution, whichfor some materials is dominant, is the loss of cohesiveenergy when the structure is expanded to form a hydride. This expansion lowersthe cohesive energy.

(3) The final contribution to thehydride formation energy is the chemical bondingbetween the hydrogen and other elements in the compound. This is the onlycontribution which is negative and hence favorable to hydride formation.

These contributions are relativelyhigh competing with each other. The more negative the total enthalpy of hydrideformation is, the more probable the hydrogenation is. These calculations arenot easy for systems more complicated than pure metal hydrides. Easierpredictions can be made when examining the crystal structure of the parentcompound.

Some geometrical criteria have tobe fulfilled to make the hydride formation possible. Geometrical requirementsinclude sufficient size for the interstitial positions and their arrangement inspace.

1. Westlake’s criterion states that availableinterstitial sites must have a spherical volume with the radius ≥ 40 pm [9-13].

2. The minimum H-H distanceshould be 210 pm.

3. According to the “Shoemaker’sexclusion rule” two tetrahedra sharing the

same face cannot beoccupied simultaneously [14-16].

Some of the sites that do notsatisfy these criteria in the parent compound may become suitable for fillingwith hydrogen due to the lattice expansion afterthe first stage of the hydrogenation. Despite the simplicity of these criteriathey are very useful as the first estimate of hydrogen absorption possibility.However one should keep in mind that there are always some exceptions fromthese rules due to the fact that the stability of thehydride is determined by many factors and none of them predominates in allcases.