High-temperature (high-T c ) superconductivity appears as a consequence of the carrier-doping of an undoped parent compound exhibiting antiferromagnetic order; thereby, ground-state properties of the parent compound are closely relevant to the superconducting state 1,2 . On the basis of the concept, a spin-fluctuation has been addressed as an origin of pairing of the superconducting electrons in cuprates 1 . Whereas, there is growing interest in the pairing mechanism such as an unconventional spin-fluctuation or an advanced orbital-fluctuation due to the characteristic multi-orbital system in iron-pnictides 3-6 . Here, we report the discovery of an antiferromagnetic order as well as a unique structural transition in electron-overdoped
Fast ionic conductors have considerable potential to enable technological development for energy storage and conversion. Hydride (H
−
) ions are a unique species because of their natural abundance, light mass, and large polarizability. Herein, we investigate characteristic H
−
conduction, i.e., fast ionic conduction controlled by a pre-exponential factor. Oxygen-doped LaH
3
(LaH
3
−2
x
O
x
) has an optimum ionic conductivity of 2.6 × 10
−2
S cm
−1
, which to the best of our knowledge is the highest H
−
conductivity reported to date at intermediate temperatures. With increasing oxygen content, the relatively high activation energy remains unchanged, whereas the pre-exponential factor decreases dramatically. This extraordinarily large pre-exponential factor is explained by introducing temperature-dependent enthalpy, derived from H
−
trapped by lanthanum ions bonded to oxygen ions. Consequently, light mass and large polarizability of H
−
, and the framework comprising densely packed H
−
in LaH
3
−
2
x
O
x
are crucial factors that impose significant temperature dependence on the potential energy and implement characteristic fast H
−
conduction.
Nanometer-sized materials attract much attention because their physical and chemical properties are substantially different from those of bulk materials owing to their size and surface effects. In this work, neutron powder diffraction experiments on the nanoparticles of palladium hydride, which is the most popular metal hydride, have been performed at 300, 150, and 44 K to investigate the positions of the hydrogen atoms in the face-centered cubic (fcc) lattice of palladium. We used high-quality PdD0.363 nanocrystals with a diameter of 8.0 ± 0.9 nm. The Rietveld analysis revealed that 30% of D atoms are located at the tetrahedral (T) sites and 70% at the octahedral (O) sites. In contrast, only the O sites are occupied in bulk palladium hydride and in most fcc metal hydrides. The temperature dependence of the T-site occupancy suggested that the T-sites are occupied only in a limited part, probably in the subsurface region, of the nanoparticles. This is the first study to determine the hydrogen sites in metal nanoparticles.
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