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Encyclopedia of Physical Science and Technology EN009A-426 July 6, 2001 20:44
Metal Hydrides 453
hydrogensubstructureshaveaconfigurationalentropydif- metallic compound remain unchanged on hydrogenation.
ferent from zero and thus are likely to be unstable at low This leads either to the protonic model for hydrogen in
temperatures. As a consequence a further transition oc- case the H 1s band lies above the conduction band in the
−
curs that can be described as a crystallization of the lattice metal or intermetallic compound (H as e donator) or to
−
liquid. Here, the repulsive interaction of neighboring H the anionic model if it lies below (H as e acceptor). Ac-
atoms comes into play, and a long-range order in the hy- cordingly, the free electron carrier concentration should
drogen sublattice is introduced. The “liquid-like” peak in increase or decrease, respectively. A more realistic ap-
the neutron diffraction patterns vanishes on ordering of proach accounts for the H-induced changes of the con-
the H substructure. Hydrogen ordering is accompanied by duction band (Switendick’s hybrid band model). Quantum
a reduction in symmetry and often a drastic drop in H mechanical calculations and photoelectron spectroscopy
mobility. have indeed shown that hydrogen strongly influences the
The lattice gas approach is valid within certain limits metal conduction band and induces new low-lying energy
for typical metallic hydrides, binaries as well as ternar- states some electron volts below the Fermi level E F (M–
ies. Deviation from this idealized picture indicates that H bonding). From electronic structure considerations, the
metallic hydrides are not pure host–guest systems, but real maximum hydrogen content of hydrides can often be ex-
chemical compounds. An important difference between plained. The prediction that Pd can accommodate 0.76
the model of hydrogen as a lattice gas, liquid, or solid additional electrons corresponds nicely to the fact that Pd
and real metal hydrides lies in the nature of the phase readily takes up hydrogen to form PdH 0.7 . In contrast to
transitions. Whereas the crystallization of a material is the H s, metal d overlap in transition-metal and rare-earth
a first-order transition according to Landau’s theory, an hydrides, the M–H bonding in actinide hydrides seems
order–disorder transition in a hydride can be of first or to be dominated by interaction between H 1s andM 5 f
secondorder.Thestructuralrelationshipsbetweenordered electrons (M = actinide metal). β-UH 3 may also be con-
and disordered phases of metal hydrides have been proven sidered as a heavy fermion compound (γ = 28.7 mJ/mol K
in many cases by crystallographic group–subgroup rela- as compared to γ = 9.88 mJ/mol K for U). Examples for
tionships, which suggests the possibility of second-order superconducting hydrides are PdH (T c = 9.5 K) and PdD
(continuous) phase transitions. However, in many cases (T c = 11 K) with a reverse isotope effect, and Th 4 H 15
hints for a transition of first order were found due to a sur- (T c = 8 K). As superconductivity is based on a strong
face contamination of the sample that kinetically hinders electron–phonon coupling, both altering the phonon
the transition to proceed. modes and the density of states (DOS) at the Fermi level
E F on introducing hydrogen in a metal or intermetallic
compound are critical. Because of the additional electrons
4. Electronic, Magnetic, and
of hydrogen, E F in the hydride is often shifted to regions of
Mechanical Properties
lower DOS as compared to the metal or intermetallic, and
Hydrogen entering the crystal structure of a metal T c of superconductors often drops on forming the hydride.
or an intermetallic obviously influences its electronic, Because of the complex interplay of volume expan-
magnetic, and phonon structure. The main effects are sion (variation of interatomic distances) and the DOS at
that of the generally observed lattice expansion, the E F (Stoner criterion), the changes of magnetic bulk prop-
electronic interaction between hydrogen and the neigh- erties on hydride formation are manifold. Ferrimagnetic
boring metal atoms (M–H bonding), and the H–H in- Y 6 Mn 23 and Pauli-paramagnetic Th 6 Mn 23 crystallize with
teractions. Hydrogen may influence the electronic and the same cubic structure (Th 6 Mn 23 type). On hydrogena-
magnetic properties in many ways: On hydrogenation, tion the former loses its magnetic order while the latter
metal–semiconductor transitions may occur (YH 2 –YH 3 ), becomes a ferromagnet. This different behavior was ex-
ferromagnetism may appear (Th 6 Mn 23 –Th 6 Mn 23 H 30 ) or plained by a structural transformation (cubic–tetragonal)
change into antiferromagnetism (Gd–GdH 2 ), paramag- that takes place in Y 6 Mn 23 H 30 (Table III), but not in the
netic metals may become diamagnetic (Pd–PdH 0.6 ), an- homologous Th compound. Ce often changes its valency
tiferromagnetic metals may become ferromagnetic semi- in compounds from IV+ to III+ on formation of hydrides,
conductors (Eu–EuH 2 ), metal valences may change causing for example the appearance of ferromagnetism in
III
II
IV
III
(Ce Ru 2 /Ce Ru 2 H x , Eu Rh 2 /Eu Rh 2 H 5.5 ), or heavy Pauli-paramagnetic CeNi 3 by hydrogenation.
fermion behavior may appear (CeH 2.6 with a Sommerfeld The formation of metal hydrides deteriorates mechani-
coefficient γ = 110 mJ/mol K compared to γ = 10 mJ/ cal properties of materials, which is a serious problem in
mol K for γ -Ce). engineering. The precipitation of H 2 in voids and cracks
As for the electronic structure, the ridid band approxi- of a material causes high internal pressure and the hydride
mation assumes that the energy bands in the metal or inter- formation in areas of high stress lowers the cohesion