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The deuteration-induced effect on the Fermi surface (FS) topology of V was investigated by the synchrotron-based Compton scattering technique with 115 keV X-rays. The three-dimensional occupation number density (OND) of α-VD 0.64 single crystal and V single crystal was reconstructed by the directional Compton profiles along 13–18 directions. The observed OND shows that the FS topology of α-VD 0.64 .differs from that of V at Γ, N, and H points in the crystal momentum space, which are hole position for V. This indicates that electrons that originate from deuterium modify the upper part of the host metal d bands (3rd and 4th bands) at the Fermi level E F , and forms the metal–hydrogen bonding states with the lower part of the host metal d band (1st band).

The magnetic and structural influences of hydrogen absorption were studied on the itinerant electron metamagnet YCo 2 . The system YCo 2 H x forms two crystalline hydrides of the α phase (0 x x ≤3.5). The hydride samples also include small amounts of the non-intrinsic ferromagnetic component due to the partial structural disordering upon hydrogen absorption. The high field susceptibility indicated that the α phase is an itinerant electron metamagnet in which the critical field is possibly lower than that of YCo 2 . The β phase has no metamagnetic transition. The hydrogen-induced changes in the magnetic properties are discussed on the basis of the itinerant electron magnetism.

We experimentally investigated the effects of magnetic fields on the heat of reaction by examining the metal–hydrogen system LaCo 5 –H 2 . The heat of reaction was measured in zero field and a magnetic field by the logarithmic pressure vs. inverse temperature method and the calorimetric one. Both the methods gave generally the same results that the magnetic field caused the absolute value of the heat of reaction to increase a little in the α+β region and decrease considerably in the β+γ region. These agree well with the calculation based on the general formulation of magneto-thermodynamic effects in chemical reactions. This is the first observation of the magnetic field effect on the heat of reaction.

The effects of magnetic fields were generally formulated on the thermodynamic properties of chemical reactions. The magnetic field-induced change in the heat of reaction was calculated for the metal–hydrogen system, especially for LaCo5-H2, on the basis of the general formulation. Independently of this, the heat of reaction under the influence of magnetic fields was estimated from the temperature dependence of the equilibrium hydrogen pressure. The agreement between both the results indicated that magnetic fields influence the heat of reaction in metal–hydrogen systems.

The magnetic properties of LaCo 5 hydrides and deuterides were investigated by (1) in situ magnetization at 77 K in 20 T which related magnetization with hydrogen/deuterium composition, and (2) high field magnetization at 4.2 K up to 44 T for well-characterized samples. The β-phase LaCo 5 H(D) 3.4 is recognized as a saturated ferromagnet, like the metal phase LaCo 5 . The γ-phase LaCo 5 H(D) 4.3 undergoes a metamagnetic transition from a low moment state to a high moment state at a critical field of 35 T (hydride) or 38 T (deuteride). These magnetic properties are discussed in relation to the electronic structure.

The electromotive force (emf) has been investigated for the hydride–hydride electrochemical cell under the influence of gradient magnetic fields. The thermodynamic consideration presumed the relationships between the emf and the gradient between the electrodes. The emf was measured for the array of the electrodes using the ferromagnetic metal hydride LaCo 5 H x in the magnetic fields with the maximum gradient of 106 T/m up to 15 T at 293 K. The emf was proportional to the averaged gradient of magnetic fields and the distance between two electrodes and depended on the magnetic properties of the electrode material, which agreed well with the thermodynamic consideration.

The three-dimensional electron momentum distribution (3D-EMD) of β -PdH0.84 were determined by the Compton scattering technique with 115 keV X-ray synchrotron radiation. The obtained 3D-EMD shows the existence of the neck around the [1 1 1] direction in the Fermi surface, which agrees well with the result of the band structure calculation by the LDA-FLAPW method. This result indicates that the synchrotron-based Compton scattering technique can be used for the Fermiology of metal hydrides.

Electrical resistivitiy of the unstable metal hydrides LaNi 5 H x and LaCo 5 H x has been measured in situ by a newly developed method where the samples are under the influence of mechanical stress. An activated sample does not give the bulk resistivity of the hydride, but the resistivity change with hydrogen pressure or temperature is correlated with the transformation of hydride phases. The resistivity of the bulk hydride is tolerably obtained in the first absorption of hydrogen for a virgin sample. The resistivity at 323 K is 1.3, 15.3 and 8.5 μΩm for the metal phase ( x = 0), the α phase ( x ⋍ 0.6) and the β phase ( x = 6.7) of LaNi 5 H x , and 1.5, 11.5 and 18.8 μΩm for the metal phase ( x = 0), the β phase ( x = 3.4) and the γ phase ( x = 4.3) of LaCo 5 H x , respectively. These resistivity changes with hydrogen composition are interpreted in relation to disordered-ordered arrangment of hydrogen.

γ-phase RCo3D∼4 deuterides with R ≡ Y and Er were studied by neutron diffraction analysis at room temperature in the paramagnetic state. Deuterium atoms are distributed over three types of interstitial site 18h2, 36i1 and 36i2 with occupancies of about 90%, 35% and 10% respectively. The deuterium site occupation is compared with that for β-phase RCo3 hydrides and with RCo2 and RCo5 structures from which the RCo3 lattice can be constructed.

The influence of hydrogen on the electronic structure of an intermetallic compound has been studied from magnetization measurements in ultrahigh magnetic fields up to 130 T for a prototype system YCo 3 H x . The dependence of the magnetic moment on hydrogen content and magnetic field lead to the following conclusions. The general structure of the density of states (DOS) in the upper part of the 3d band is still maintained in the hydride, but the curvature of the DOS curve is slightly modified in the β phase x = 1.0–1.9), and more strongly modified in the γ phase ( x = 3.2–4.0). The modification of the DOS is related to the occupation sites of hydrogen. The electronic population in the 3d band is hardly changed upon hydrogenation, and it is about +0.1 electron Co −1 per absorbed hydrogen atom. The electrons originating from hydrogen atoms are almost contained in the metal-hydrogen bonding band. The rest of them occupy the 3d band, causing the Fermi level to shift slightly upwards.

Magnetization measurements of single-crystal β phase hydrides RCo 5 H ∼3 with R ≡ La, Ce and Nd have been carried out in high magnetic fields up to 40 T. The uniaxial Co magnetocrystalline anisotropy becomes about half of that in the parent compounds upon hydrogenation, which is related to the decrease in the Co moment. The uniaxial contribution from the Ce sublattice in the parent compound almost vanishes upon hydrogenation. The magnetocrystalline anisotropy of the Nd sublattice and the NdCo intersublattice exchange interaction become lower than those in the parent compounds, resulting in a significant reduction of the spin reorientation temperature.

The equilibrium hydrogen pressures in the β I +γ region of the ferromagnetic hydride PrCo 5 H x have been investigated as a function of the hydrogen composition x under the influence of magnetic fields up to 5 T at 273.2 K. The logarithmic pressure change is increased linearly with increasing magnetic fields and depends on the hydrogen composition. The value Δ M S (the change in saturation magnetization per desorbed molH) is calculated from the relationship between the pressure change and the magnetic field. The results show a peculiar dependence of the Δ M S on the hydrogen composition. The maximum value of Δ M S of 25 J T −1 molH −1 at x =4.3 is much larger than that observed in other metal–hydrogen systems.

The magnetization of the metal hydrides GdFe3Hx (x=0∼3.6) and Dy6Fe23Hx (x=0∼20.3) was measured in magnetic fields up to 29 T at 77 K. For these measurements the hydrides were prepared in situ in a glass holder with gradually varying hydride composition x at room temperature. For each system the magnetization curve varied systematically with hydrogen content. The dependence of magnetization on hydrogen content is discussed.