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We report Cu, Fe, N‐doped carbon nanotubes, (Cu,Fe)−N−CNT, as electrocatalysts for the oxygen reduction reaction (ORR) in acidic media. (Cu,Fe)−N−CNT was prepared using a silica coating method in pyrolysis to minimize the formation of carbon‐coated metal oxide or carbide nanoparticles, which are known to be inactive for the H2O2 reduction. (Cu,Fe)−N−CNT shows a turnover frequency of 0.66 e− site−1 s−1 at +0.8 V vs. RHE and H2O2 yields of <1 % for the ORR with a utilization factor of active sites of 82 %. Kinetic analysis reveals that 4e− transfer rates for (Cu,Fe)−N−CNT are higher than those of a monometallic counterpart of Fe−N−CNT. In situ X‐ray absorption spectroscopy enables us to determine redox potentials: E°'(FeIII/FeII)=0.65 V vs. RHE and E°'(CuII/CuI)=0.45 V for (Cu,Fe)−N−CNT, and E°'(FeIII/FeII)=0.65 V for Fe−N−CNT. These results indicate that bimetallic doping into carbon nanotubes gives the effect on kinetic parameters but not on thermodynamic ones. In other words, there is no direct electronic interactions between the Cu and Fe active sites for (Cu,Fe)‐N‐CNT because such interactions should modulate their redox potentials. [ABSTRACT FROM AUTHOR]

The sub-nanometer amplitudes of the samples vibrated with high frequencies were measured by 57Fe Mössbauer spectroscopy using a nuclear Bragg monochromator and focusing optics. The Mössbauer spectra of the vibrated single-line absorber showed comb-like absorption peaks, and the amplitudes of the absorbers were determined by the sideband intensities. We used the stainless-steel foil glued on the quartz crystal or the polyvinylidene fluoride (PVDF) film as samples. The mean value and the variance of the amplitude in sub-nanometer order were obtained for the irradiated area of 100 μm diameter in the sample. We could obtain the sub-nanometer amplitude with almost zero variance, using the 57Fe synchrotron Mössbauer source and focusing optics for the PVDF film. The resonant absorber vibrated with high frequency and amplitude without variance is useful for the control of gamma rays. This work will advance X-ray quantum optics and quantum technology applications using a single gamma photon. [ABSTRACT FROM AUTHOR]

The temperature dependence of the spin-flip transition (SFT) on the Fe3BO6 crystal surface was studied by synchrotron-based conversion electron Mössbauer spectroscopy. Linearly polarized, highly directional synchrotron Mössbauer γ-rays enabled spin-orientation-sensitive measurements. Moreover, microscopic analysis using focused γ-rays reduced the influence of sample surface non-uniformity on the spectrum. The results revealed that the temperature dependence of the SFT within approximately 100 nm below the surface was bulk-like, in contrast to previously reported findings. [ABSTRACT FROM AUTHOR]

The pressure dependence of the 151Eu isomer shift of EuH2 in the Pnma structure was evaluated under the pressure below 5 GPa by 151Eu energy-domain synchrotron-radiation-based Mössbauer spectroscopy. The isomer shift gradually increased as the pressure increased and its pressure dependence was understood by the simple linear relation with the inverse of the lattice volume of the hydrides. Furthermore, the isomer shifts of Eu hydride under hydrogen pressure and those of EuH2 under helium pressure did not show systematic difference. These results suggested that there was no or little hydrogen absorption after the synthesis of Pnma EuH2 in the pressure range up to 5 GPa. The linear relation between the isomer shift and the inverse of the volume in this initial hydride phase would be applied to the 151Eu Mössbauer study on other Eu hydride phases under higher hydrogen pressure to estimate the presence or absence of additional hydrogen absorption. [ABSTRACT FROM AUTHOR]

The pressure dependence of the 151Eu isomer shift of EuH2 in the Pnma structure was evaluated under the pressure below 5 GPa by 151Eu energy-domain synchrotron-radiation-based Mössbauer spectroscopy. The isomer shift gradually increased as the pressure increased and its pressure dependence was understood by the simple linear relation with the inverse of the lattice volume of the hydrides. Furthermore, the isomer shifts of Eu hydride under hydrogen pressure and those of EuH2 under helium pressure did not show systematic difference. These results suggested that there was no or little hydrogen absorption after the synthesis of Pnma EuH2 in the pressure range up to 5 GPa. The linear relation between the isomer shift and the inverse of the volume in this initial hydride phase would be applied to the 151Eu Mössbauer study on other Eu hydride phases under higher hydrogen pressure to estimate the presence or absence of additional hydrogen absorption. [ABSTRACT FROM AUTHOR]

We conducted in situ high‐pressure acoustic‐wave velocity measurements of Fe2+‐bearing MgSiO3 glass up to 158 GPa by Brillouin scattering spectroscopy to clarify the effect of iron on the elasticity and structural evolution of silicate melts in the lower mantle. The change in trend of the VS profile, likely induced by the structural transition of Si‐O coordination number from 6 to 6+ proposed in previous studies of silicate glasses, was confirmed to be located at ∼106 GPa. Given the iron contents of partial melts derived from a pyrolitic or chondritic mantle, the transition pressure would be at around 84–97 GPa, which is well within the lowermost‐mantle pressure regime. Our data show the substitution of 12 mol% Fe in MgSiO3 glass decreases the VS by ∼5.5%. This implies that iron affects the buoyancy relations between melts/crystals and the melts at the lowermost mantle will have the higher coordination number than 6. Plain Language Summary: The physical property of silicate glasses under high‐pressures gives a clue to understand the nature of silicate melts possibly at the bottom of the Earth's mantle because silicate glasses have been considered as good analogs of silicate melts. According to our acoustic wave velocity measurements of Fe2+‐bearing MgSiO3 glass up to 158 GPa, we find the acoustic wave velocity in MgSiO3 glass decreases by ∼5.5% when it contains 12 mol% FeO. In addition, we observe the anomalous increase in the acoustic‐wave velocity profile at 106 GPa, which can be explained by the structural transition in Si‐O coordination number higher than 6. Our results imply that the possible melts at the bottom of the mantle have a denser structure. Key Points: We have conducted the acoustic wave velocity measurements of Fe2+‐bearing MgSiO3 glass up to 158 GPaThe change in trend of the VS profile, likely induced by the structural transition in Si‐O coordination number, is observed around 106 GPaOur data show the substitution of 12 mol% Fe in MgSiO3 glass decreases the VS by ∼5.5% [ABSTRACT FROM AUTHOR]