This article reports the recent studies of the pressure-induced phase separation of rare-earth metal dihydrides, which have a face-centered-cubic (fcc) metal lattice with tetrahedral interstitial sites fully occupied. Synchrotron radiation x-ray diffraction patterns of LaH2.3 show that additional Bragg reflections appeared just outside of the original ones around 11 GPa, indicating the formation of the small fcc metal lattice. The coexistence state of two fcc metal lattices thus observed is interpreted in terms of phase separation from the dihydride toward lower and higher concentration phases. The formation of insulating LaH2+δ (δ≥0.7) is confirmed by infrared spectroscopy, which shows decrease in reflection intensity. These results indicate that the interstitial hydrogen atoms transfer from T-sites and the domains of solid solution and higher concentration hydride are formed. Pressure-induced phase separation of metal hydrides would develop a new research field, that is, the dynamics of hydrogen transfer in a metal lattice.
X-ray magnetic circular dichroism (XMCD) study of 3d transition metal hydrides FeH, CoH, and NiH is presented. The hydrogen effects on the electronic structure and magnetic states have been investigated. This study reveals that ferromagnetic Ni transforms to nonmagnetic NiH, whereas FeH and CoH remain ferromagnetic after the hydrogenation. The XMCD profiles indicate that the electronic states of 3d electron near the Fermi energy are modified as a result of the hydrogenation. In this article, the stability of the ferromagnetic state in FeH and CoH is also discussed.
Raman and visible absorption spectra of rare earth trihydrides have been measured at high pressures, in order to investigate the structural and electronic phase transitions. The successive phase transitions of hcp-intermediate-fcc phase were confirmed by Raman measurements. From the visible absorption experiments, the band gap energy was estimated. The gap closure was found to occur on the transition from the intermediate to the fcc phase.
We present the newly developed electrical resistance measurement technique for metal hydrides compressed in high-pressure H2 and the first successful in-situ simultaneous measurements of electrical resistance and X-ray diffraction of FeH at high pressures and low temperatures. The electrical resistivity ρ showed a sharp increase with the formation of iron-hydride FeHx (x∼1) at 3.5 GPa. The ε′-phase of FeH was found to be metallic up to 25.5 GPa. The ρ vs. T curves up to 16.5 GPa approximately follow Fermi-liquid law below 25 K. However, T5 was found to be better fitting at 25.5 GPa. This change can be related to the previously reported ferromagnetism collapse at corresponding pressures.
Hydrogenation of iron-light element alloys, such as Fe-Si and Fe-S, under high pressure is important to understand the composition and the thermal structure of the planetary cores. Here, we report the hydrogenation pressures of FeSi and FeS, hydrogen solubilities into these alloys and the effect of hydrogen on their phase relations based on in situ X-ray diffraction experiment of FeSi-H and FeS-H systems up to 17 GPa and 2123 K. Hydrogenation of FeSi and FeS occurs at the pressures more than 10 GPa and 3 GPa, respectively. The H solubilities (x) are estimated to be x∼0.2-0.3, which are well below compared to the H solubility into pure iron (x∼1.0). This small amount of H solubility into the alloys causes only small depressions (150-250 K) of their melting temperaure whereas dissolution of H into Fe decreases its melting temperature significantly.
Interstitial metal hydrides for hydrogen storage were studied by in situ powder X-ray and neutron diffractions. This paper reports structural change and distribution of hydrogen atoms in La2Ni7- and La5Ni19-based hydrides with stacking structures consisting of La2Ni4 cells and LaNi5 cells. La2Ni7 formed two hydrides, La2Ni7H7.1 and La2Ni7H10.8. During formation of La2Ni7H10.8, La2Ni4 and LaNi5 cells expanded by 69.8% and 12.2%, respectively, indicating that most of absorbed hydrogen was located in La2Ni4 cells. In contrast, La5Ni19-based La4MgNi19 formed only one hydride, La4MgNi19H∼24. Hydrogen was distributed almost evenly in LaMgNi4 and LaNi5 cells (0.8-1.0 H/M) in La4MgNi19D∼24 from Rietveld refinement of in situ neutron diffraction data. This result indicated that Mg substitution for La changes preference of hydrogen occupation in the La2Ni4 and LaNi5 cells, which leads to difference in hydrogenation properties.
An x-ray diffraction measurement and a density functional theory (DFT) calculation of the pressure-induced transformation in LiBH4 were performed. The structure of its first high pressure phase at room temperature (phase-III) was determined to be I41/acd structure, whose unit cell was a √ 2 ×√ 2 ×2 supercell of Ama2 structure proposed previously. Pressure-induced transformation from phase-III took place at 16 GPa and the structure of the second high-pressure phase was analyzed as I4/mmm structure (phase-V′), in which hydrogen atoms were disordered. However, an annealing treatment for phase-V′ under high-pressure suggested that it was a metastable phase. With the pressure elevated up to 30 GPa, the tetragonal I4/mmm structure was gradually transformed to a cubic Fm3m structure, which has been reported as a stable phase of phase-V previously. The P-T diagram was examined using high-pressure/high-temperature Raman scattering, and pressure/temperature dependence of the relative ionic conductivity was observed across the phase boundaries.
High-pressure experimental techniques using sintered diamond anvils in Kawai-type multianvil apparatuses have been developed for the last 20 years toward Mbar region to reproduce deep lower mantle conditions. In this article, some tips in the recent technical developments and results of the application studies, phase equilibria and equations of state among the major lower mantle minerals, (Mg,Fe)SiO3 perovskite and (Mg,Fe)O magnesiowustite, are reviewed along with a P-V-T equation of state of MgO determined by the scale-free unified analysis. These fundamental properties should contribute to modeling of the composition, dynamics, and evolution of the lower mantle.