关注：

1） 关注氢化物近10年了，可是对氢化物的认识到底有多少？

2） 氢在金属或合金表面的离解方式如何？ 氢如何进入金属的晶格当中？以什么样的形态(阴离子还是质子形式？ 金属是阳离子？)存在于金属中的什么位置？

3） 金属的表面状态对氢气在上面的解离及氢原子的复合有何影响？

4） 升温和加压形成氢化物的方式的差异；

5） 氢致相变、温度相变和压力相变的异同；

6） 加压下氢在晶格中存在位置的变化，在高压下H2单元能否存在于晶格当中？

7） 压力升高，氢在晶格中的振动频率有何变化趋势？

8） 氢化物中虚频出现的原因；

9)  什么是氢的量子效应呢？ 压力升高，量子效应如何变化？

......

http://www.science.uwaterloo.ca/~cchieh/cact/c120/bondel.html

01 2A规则 Search for metal hydrides with short hydrogen–hydrogen separation: Ab initio calculations

   Numerous studies have been carried out to explain observed stabilities, stoichiometries, and preferred hydrogen sites in various kinds of hydrides.6,7 It has been suggested that limiting factors in terms of minimum hole sizes0.40 Åd and HuH distance s2.1 Åd in stable hydrides should be prime selection criteria for guidance about hydrogen absorption capacity.7–10 Hence, reduction in HuH separation may be one means to enhance the hydrogen content at minimum matrix volume and mass. From experimental and theoretical

studies it has been established that the separation between

hydrogen atoms in interstitial matrix sites is ruled by repulsive

H-to-H interactions, which in practice is assumed to

prevent the hydrogen atoms to approach one another closer

than 2.1 Å (Switendick’s criterion).7,9,10

   The interaction between a pair of hydrogen (H) atoms embedded in a metal matrix has been studied by using the self-consistent-field molecular-orbital method.

   The total energy of the cluster as a function of H-H separation reveals two minima corresponding to the bond lengths of molecular hydrogen and metal-hydrogen dimer.

    It is suggested that the stronger metal-hydrogen bonding may be responsible for the 2.1 Å minimum H-H distance observed by Switendick in stable metal hydrides

0.Summary – Rules of hydrogen bonding

RULE 1: The greater the charges, the stronger the hydrogen bond.

RULE 2: The shorter the distance the stronger the hydrogen bond.

  Hydrogen bond length is traditionally measured by the distance between the donor atom and the acceptor atom. By definition we say a hydrogen bond occurs if the distance between the donor and the acceptor atoms is shorter than the sum of the atomic radius of the acceptor atom (~1.5Å), the atomic radius of the hydrogen (1.2Å) and bond length between the donor atom and the hydrogen (~1Å). 

   So the longest hydrogen bonds are ~3.5 Å. Anything longer would be considered a pure dipole-dipole interaction.

  Good hydrogen bonds have a distance of ~2.8 Å and some ultra-short hydrogen bonds have been reported with donor to acceptor distances of 2Å.

RULE 3: Bonding angles matter, the more ideal the bonding angle, the stronger the hydrogen bond. This is an indication that hydrogen bonds have a partial covalent character.

  Think about where the lone pairs sit.

   The bond is ideal if the donor atom, the hydrogen the lone pair and the acceptor atom all lie on a straight line.

Rule 4: Linear Networks of hydrogen bonds increase the Dipole moment and lead to stronger hydrogen bonds. The dipoles in hydrogen bonds are induced dipoles.

   Formation of a hydrogen bond further polarized the bonds

RULE 5: Hydrogen bonds contribute little to overall protein stability, but they align molecular groups in a specific orientation giving proteins a defined structure.

RULE6: Unsatisfied Hydrogen bonds in the protein interior are quite rare.

1. 从温度相图说起

From IOP2013:First-principles calculations of niobium hydride formation in superconducting radio-frequency cavities

http://iopscience.iop.org/0953-2048/26/9/095002/article

The phases that hydrogen forms with niobium have been studied extensively experimentally. The phase diagram, given in a literature review by Manchester and Pitre [29] and summarized by Ricker and Myneni [1], is displayed in figure 1. Disordered interstitial hydrogen in bcc niobium is denoted α (for H concentrations <24 at.%) and α' (for greater H concentrations). Ordered interstitial hydrogen in face-centered niobium occurs at higher H concentrations and lower temperatures, and consists of the β (orthorhombic, H/Nb ≈ 1), δ (cubic, H/Nb ≈ 2), and ε (orthorhombic, H/Nb ≈ 3/4) phases. The phases, λ and λ_c, and others, which are indicated by dashed lines, are not well established. It is possible for multiple hydride phases to occur simultaneously [26].

  Reset

Figure 1. Nb–H phase diagram [29]. The α and α' niobium hydride phases consist of interstitial hydrogen atoms dispersed in body-centered cubic (bcc) niobium at differing low hydrogen concentrations, the β phase consists of face-centered orthorhombic (fco) niobium with ordered hydrogen interstitials and with an ~NbH stoichiometry, the δ phase has the fluorite structure with hydrogen in the tetrahedral sites of face-centered cubic (fcc) niobium and an ~NbH₂ stoichiometry, the ε phase is structured like the β phase with the ~Nb₄H₃ stoichiometry, and λ and λ_c phases are experimentally unconfirmed niobium hydride phases. Reprinted with permission of ASM International

2.Properties of Scandium

   Scandium is a chemical element with symbol Sc and atomic number 21.

        A silvery-white metallic transition metal, it has historically been sometimes classified as a rare earth element, together with yttrium and the lanthanoids.

   It was discovered in 1879 by spectral analysis of the minerals euxenite and gadolinite from Scandinavia.

     Scandium is present in most of the deposits of rare earth and uranium compounds, but it is extracted from these ores in only a few mines worldwide.

    Because of the low availability and the difficulties in the preparation of metallic scandium, which was first done in 1937, it took until the 1970s before applications for scandium were developed.

     The positive effects of scandium on aluminium alloys were discovered in the 1970s, and its use in such alloys remains its only major application. The global trade of the pure metal is around a hundred pounds a year on average.^[3]

   The properties of scandium compounds are intermediate between those of aluminium and yttrium.

   A diagonal relationship exists between the behavior of magnesium and scandium, just as there is between beryllium and aluminium. In the chemical compounds of the elements shown as group 3, above, the predominant oxidation state is +3.

Chemical characteristics of the element[edit]

  Scandium is a soft metal with a silvery appearance. It develops a slightly yellowish or pinkish cast when oxidized by air.

  It is susceptible to weathering and dissolves slowly in most dilute acids. It does not react with a 1:1 mixture of nitric acid (HNO₃) and 48%hydrofluoric acid (HF), possibly due to the formation of an impermeable passive layer. Scandium turnings ignite in air with a brilliant yellow flame to form scandium(III) oxide.^[4]

Compounds[edit]

   The chemistry is almost completely dominated by the trivalent ion, Sc³⁺. The radii of M³⁺ ions in the table below indicate that in terms of chemical properties, scandium ions are more similar to those of yttrium than to those of aluminium. In part for this similarity, scandium is often classified as a lanthanide-like element.

Oxides and hydroxides[edit]

The oxide Sc₂O₃ and the hydroxide Sc(OH)₃ are amphoteric:^[13]

Sc(OH)₃ + 3 OH⁻ → Sc(OH)3−
6Sc(OH)₃ + 3 H⁺ + 3 H₂O → [Sc(H₂O)₆]³⁺

   The α- and γ- forms of scandium oxide hydroxide (ScO(OH)), are isostructural with their aluminium oxide hydroxide counterparts.^[14] Solutions of Sc³⁺ in water are acidic because of hydrolysis.

Halides and pseudohalides[edit]

The halides ScX₃ (X = Cl, Br, I) are very soluble in water, but ScF₃ is insoluble. In all four halides the scandium is 6-coordinated. The halides are Lewis acids; for example, ScF₃ dissolves a solution containing excess fluoride to form [ScF₆]³⁻. The coordination number 6 is typical of Sc(III). In the larger Y³⁺ and La³⁺ ions,coordination numbers of 8 and 9 are common. Scandium(III) triflate is sometimes used as a Lewis acid catalyst in organic chemistry.

Organic derivatives[edit]

   Scandium forms a series of organometallic compounds with cyclopentadienyl 【环戊二烯基】ligands (Cp), similar to the behavior of the lanthanides. One example is the chlorine-bridged dimer, [ScCp₂Cl]₂ and related derivatives of pentamethylcyclopentadienyl ligands.^[15]

Uncommon oxidation states[edit]

   Compounds that feature scandium in the oxidation state other than +3 are rare but well characterized. 

  The blue-black compound CsScCl₃is one of the simplest. This material adopts a sheet-like structure that exhibits extensive bonding between the scandium(II) centers.^[16] 

     Scandium hydride is not well understood, although it appears not to be a saline【含盐的，咸的】 hydride of Sc(II).^[2] 

   As is observed for most elements, a diatomic scandium hydride has been observed spectroscopically 【
分光镜地(spectroscopic的变形)】at high temperatures in the gas phase.^[1] S

  candium borides and carbides are non-stoichiometric, as is typical for neighboring elements.^[17]

3. Scandium hydrides from wikipeida

    Scandium hydride, also known as scandium–hydrogen alloy, is an alloy made by combining scandium and hydrogen.

    Hydrogen acts as a hardening agent, preventing dislocations in the scandium atom crystal lattice from sliding past one another. Varying the amount of hydrogen controls qualities such as the hardness of the resulting scandium hydride. Scandium hydride with increased hydrogen content can be made harder than scandium.

   It can be formed by progressive hydrogenation of scandium foil with hydrogen.^[1]

     In the narrow range of concentrations which make up scandium hydride, mixtures of hydrogen and scandium can form two different structures. At room temperature, the most stable form of scandium is the hexagonal close-packed (HCP) structure α-scandium.^[2]It is a fairly soft metallic material that can dissolve a moderate concentration of hydrogen, no more than 0.89 wt% at 22 °C.

    If scandium hydride contains more than 0.89% hydrogen at room temperature then it transforms into a face-centred cubic (FCC) structure, the δ-phase. It can dissolve considerably more hydrogen, as much as 4.29%, which reflects the upper hydrogen content of scandium hydride.

   Research indicates the existence of a third phase created under extreme conditions termed the η-phase. This phase can dissolve as much as 6.30% hydrogen.

   Concentration dependent activation-energies are observed for hydrogen diffusion in scandium metal.^[3]