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

444 Metal Hydrides

in general a H value of less than −38 kJ/mol is required gen shows the greatest isotope effect of all elements, but
to fulfill the necessary condition G = H − T S < 0 this mainly alters some physical and hardly any chemical
for hydride formation at room temperature ( G: free en- properties. Therefore, the deuterides are generally syn-
thalpy of hydride formation). This idealized thermody- thesized under the same experimental conditions as the
namic picture does not take into account kinetic effects. hydrides.
Thus, in real systems the plateaus are not completely flat
and a hysteresis is often observed on absorption and des-
orption of hydrogen indicating a nonequilibrium situation.
C. Chemical and Crystal Structure Analysis
The purity and homogeneity of the materials are critical
in this context. The determination of the exact composition of a metal
hydride phase requires precise chemical analysis of the
hydrogen content. This may be done by gravimetric or
volumetric methods on hydrogen absorption or desorp-
B. Methods of Synthesis
tion, heating of the hydride in oxygen atmosphere with
Many metal hydrides can be synthesized by a solid– subsequent gravimetrical analysis of the water produced,
gas reaction of hydrogen with a metal, an intermetal- neutron radiography, or interferometry. The analytically
lic compound, or mixtures of metals or binary hydrides found hydrogen content has to be consistent with the re-
and metals. Because of the limited thermal stability of sult of a structure refinement.
the resulting hydrides, the hydrogenation is usually car- Structure determination of metal hydrides is more dif-
ried out at moderate temperatures (<800 K). While many ficult than for most other inorganic compounds because
metals and intermetallic compounds easily take up hy- of two obstacles: (1) the unavailability of single crystals
drogen, others form hydrides only under high hydrogen in most cases, and (2) the difficulty using X-ray diffrac-
3
pressure, i.e., equilibrium pressures p eq vary from 10 to tion data of precisely locating hydrogen in the presence
9
10 Pa. Depending on the pressure required, silica appa- of heavy elements. Thus, most crystal structures of metal
ratus, standard steel, or special high-pressure autoclaves hydrides are solved by a combination of X-ray and neu-
(<25 MPa, <1 GPa) are used. High hydrostatic pressures tron powder diffraction using the complementary charac-
up to 10 GPa, as produced, for instance, in a belt-type ap- ter of the two techniques. Crucial steps for the structure
paratus or in a multianvil press, can be used for solid-state determination are the synthesis of a well-crystallized and
reactions between binary hydrides or binary hydrides and preferably single-phase sample and the collection of high-
metals. High-pressure syntheses allow the stabilization of quality diffraction data. Unit cell dimensions, space group,
new metal hydride phases with high coordination num- and metal atom positions are determined from X-ray data,
bers or high oxidation states of the metals, e.g., Sr 2 MgH 6 , and the hydrogen atoms are located from neutron data,
IV
K 2 Pt H 6 or FeH. both generally on powder samples. The space group de-
Solutionmethodsarerarelyapplied,butareinsomepar- termined from X-ray data has to be verified by the neutron
ticular cases the only successful route. The synthesis of data as sometimes pseudo-symmetry of the metal lattice
the first complex transition metal hydride, K 2 ReH 9 , from occurs. The deviation from more symmetrical structures
an aqueous solution is such a remarkable exception. Other can be exceedingly small, which makes the use of high-
examples are CuH, ZnH 2 , and the hydrides of groups 3a resolution X-ray (synchrotron) and neutron data indis-
and 4a. Alternative methods are ion implantation, ball- pensable. Pseudo-symmetry can cause microtwinning as
milling in a hydrogen atmosphere (hydrogen storage ma- seen by anisotropic line broadening, which is a further
terials), and electrochemical synthesis (e.g., CrH). Many problem for structure determination. For the refinement
metal hydrides are air sensitive and have to be kept in of the whole crystal structure including the D positions,
an inert gas atmosphere. Those containing heavy alka- joint refinements are often advantageous as this makes full
line metals are extremely reactive and have to be han- use of the complementary character of X-ray and neutron
dled with the utmost care. The synthesis of metal hydrides diffraction using the two data sets simultaneously.
yields samples that are often fine powders and contain by- In neutron diffraction deuterides are used instead of the
products. This causes difficulties for structure analysis and hydrides because of their more favorable coherent scatter-
the study of physical properties. The growth of single crys- ing and much less pronounced incoherent scattering. No
tals is rarely successful because metal hydrides are in gen- significant differences are found for their crystal structures
eral insoluble in common solvents, and high-temperature besides slightly smaller cell volumes of the deuterides
2
methods do not apply because of the low thermodynam- due to the lower thermal displacement of H compared
1
ical stability. In some cases the addition of LiH as a flux to H. Some elements, such as Cd, Eu, Sm, and Gd, ex-
agent has been helpful for single crystal growth. Hydro- hibit an excessively high neutron absorption cross section,