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Encyclopedia of Physical Science and Technology EN009A-426 July 6, 2001 20:44

Metal Hydrides 445

σ a . Neutron diffraction studies on compounds containing III. CRYSTAL STRUCTURES
these elements require either the use of the isotope effect, AND PROPERTIES
i.e., generally very expensive isotopic pure materials, or
1
the use of the wavelength dependence of σ a , which often Hydrogenwithitselectronconfiguration1s bondstomet-
makes it necessary to choose rather short wavelengths. als in different ways. Uptake of a further electron leads to
2
Because of the progress in structure solution from pow- a stable helium 1s configuration, which may be achieved
ders and in neutron diffraction, more than 300 crystal by combination with very electropositive metals. The re-
structures have already been fully determined, most of sulting hydride anion H − is extremely deformable and
them during the past two decades. Here, crystal structures less stable than He because of its charge. With less elec-
are only considered as fully characterized if all atomic po- tropositive metals hydrogen can donate its s-electron in
sitions have been determined and refined to an acceptable a covalent σ-bond. Finally, hydrogen can form intersti-
precision. The hydride fluoride database (HFD) provides tial metal hydrides of a variable composition and with a
a comprehensive, critical compilation of crystal structure metallic character. There are intermediates, and in ternary
data of metal hydrides. The power of the method de- hydrides different bonding patterns may be combined. The
scribed of combining X-ray and neutron powder diffrac- crystal structure and properties are largely determined by
tion may be illustrated by the example of Mg 6 Co 2 D 11 , the nature of the chemical bond in metal hydrides.
whose rather complex crystal structure, which contains
63 free positional parameters, was solved ab initio from
high-resolution synchrotron and neutron powder diffrac- A. Hydrides of Main Group Metals: From Ionic
tion data. Because of the main problems discussed in to Covalent
this section, the accuracy of metal hydride structural 1. Binary Main Group Metal Hydrides
parameters is generally lower than for other inorganic
compounds. Following the trend in electronegativities χ of the main
group metals, hydrogen (χ= 2.2) forms ionic hydrides
with alkaline metals such as K, Rb, Cs (χ= 0.9) and co-
D. Electronic Structure and Spectroscopy
valent compounds with group 4a and 5a metals such as
Quantum mechanical calculations are being increasingly Sn (χ = 1.7). Crystal structures, e.g., extended solid KH
used to determine the nature of the chemical bonding in (NaCl type) vs discrete SnH 4 molecules, and properties re-
metal hydrides, the site preferences of hydrogen, and the semble those of other typical ionic or covalent compounds.
factors limiting the hydrogen capacity of storage materi- However, there is a gradual transition between these two
als. Depending on the complexity of the crystal structure, extremes for the metals with medium electronegativities.
semiempirical methods such as Extended H¨uckel or self- All main group metal hydrides are colorless, dia-
consistent methods (LMTO, LAPW) are used. As for ex- magnetic, nonmetallic, stoichiometric compounds with
perimental techniques such as X-ray absorption near-edge a low mobility of hydrogen in the crystal structure and
structure spectroscopy (XANES) and photoelectron spec- a fixed hydrogen content determined by the metal va-
troscopy (PES), difficulties arise from the unavailability lence. On electrolysis of molten ionic hydrides, hydro-
of single crystals and the low thermodynamical stability gen is produced at the anode, proving the anionic char-
under experimental conditions (ultrahigh vacuum). acter of hydrogen. Ionic (often called saline or saltlike)
Diffraction methods provide a detailed picture of the hydrides are characterized by a high electron localiza-
crystal structure averaged over space and time, but fail to tion at the hydride anion, H . This has a huge polariz-
−
give information on local structures and coordinated dy- ability far surpassing that of all other anions because of
namics in materials. Here, spectroscopic methods such as the high charge ratio of 2/1 between valence shell and
inelastic neutron scattering (INS) are better suited for the nucleus, and this strongly influences its crystal chem-
study of metal–hydrogen and hydrogen-hydrogen interac- istry. In the series of the isotypical alkaline hydrides
tions, nuclear magnetic resonance (NMR), infrared (IR), LiH–NaH–KH–RbH–CsH(NaCltypestructure),theionic
Raman, M¨ossbauer, and muon spin rotation spectroscopy radius r(H ) varies as 128–142–148–150–152 pm, show-
−
(µSR) for local structure and dynamics of hydrogen. Be- ing the strong dependence of the polarizing effect of
1
cause of the high incoherent scattering of H, INS can the cation. Such radius values are close to that of F −
also be used to locate hydrogen positions in hydrides with (133 pm), causing some structural similarities between
very low H concentrations where diffraction techniques ionic hydrides and fluorides, known as the hydride flu-
fail. Many other characterization techniques such as elec- oride analogy. Some hydrides form solid solutions with
trical transport are often not feasible because of the low the corresponding fluorides, MH x−y F y . In comparison to
sample quality (powder, multiphase). the fluorides, metal hydrides are thermodynamically less