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              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
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