Electronic properties of the rare earth hydrides

Abstract The rare earth metal dihydrides may be divided into three groups based on their crystal structures. 1. La through Nd dihydrides all have the fluorite structure and form solid solutions with hydrogen up to the composition MH 3. 2. Sm through Lu (excluding Eu and Yb) dihydrides also have fluorite structures but they form hexagonal trihydrides at higher hydrogen contents. 3. Eu and Yb dihydrides have ortho‐rhombic structures. The electronic properties of all three classes of hydrides are reviewed. The class (3) hydrides appear to be ionic semiconductors. The class (1) and (2) dihydrides are metallic, but their resistivities increase with hydrogen content and they appear to become semiconductors at high hydrogen contents. The major portion of this paper deals with the class (1) hydrides, and more specifically with recent studies on cerium hydride. Resistivity measurements on single crystals of cerium hydride showed that it undergoes a composition‐dependent metal‐to‐semiconductor transition at a hydrogen content corresponding to CeH 2,8 . There is no apparent structural change associated with the transition. Recent measurements as a function of temperature revealed that cerium hydride in the composition range CeH 2,7 to CeH 2,8 undergoes an electronic transition at about 250°K which probably corresponds to the composition‐dependent metal‐to‐semiconductor transition. In this case, precise x‐ray diffractometry disclosed that the cubic fluorite structure became slightly tetragonal below the transition temperature. while neutron diffraction investigations showed that the distribution of hydrogen in the lattice remained random. The results of studies of electrical resistivity, thermoelectric power, magnetic susceptibility, and crystal structure are interpreted in terms of bonding and changes in the band structure of the class (1) rare earth hydrides.