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It has been previously hypothesized that the enhanced rate capability of Li-Ni-PO4-treated xLi2MnO3·(1-x)LiMO2 positive electrodes (M = Mn, Ni, Co) in Li-ion batteries might be associated with a defect Ni-doped Li3PO4 surface structure [i.e., Li3-2yNiyPO4 (0 < y < 1)], thereby promoting fast Li+-ion conduction at the xLi2MnO3·(1-x)LiMO2 particle surface. In this paper, the solubility of divalent metals (Fe, Mn, Ni, Mg) in -Li3PO4 is predicted with the first-principles GGA+U method in an effort to understand the enhanced rate capability. The predicted solubility (x) is extremely small; this finding is consistent with experimental evidence: 1) X-ray diffraction data obtained from Li-Ni-PO4-treated xLi2MnO3·(1-x)LiMO2 electrodes that show that, after annealing at 550°C, a Li3PO4-like structure forms as a second phase at the electrode particle surface, and 2) X-ray absorption spectroscopy, which indicate that the nickel ions are accommodated in the transition metal layers of the Li2MnO3 component during the annealing process. However, electrochemical studies of Li3-2yNiyPO4-treated xLi2MnO3·(1-x)LiMO2 electrodes indicate that their rate capability increases as a function of y over the range y = 0 (Li3PO4) to y = 1 (LiNiPO4), strongly suggesting that, at some level, the nickel ions play a role in reducing electrochemical impedance and increasing electrode stability at the electrode particle surface. [ABSTRACT FROM AUTHOR]

We have measured the phonon density of states (PDOS) of isolated bcc and fcc FexAu1-x alloy nanoclusters (NCs) by nuclear resonant inelastic x-ray scattering. Drastic deviations were observed with respect to the PDOS of bulk Fe–Au alloys. Important information on the structure and thermodynamic properties of these NCs was obtained. [ABSTRACT FROM AUTHOR]

We have gained insight into the internal degree of atomic disorder in isolated size-selected Fe nanoparticles (NPs) (~2-6 ran in size) supported on Si02/Si(l 11) and Al2O3(0001) from precise measurements of the low-energy (low-£) part of the phonon density of states [PDOS, g(E)] via 57Fe nuclear resonant inelastic x-ray scattering (NRIXS) combined with transmission electron microscopy (TEM) measurements. An intriguing size-dependent trend was observed, namely, an increase of the low-£ excess density of phonon states (as compared to the PDOS of bulk bcc Fe) with increasing NP size. This is unexpected, since usually the enhancement of the density of low-E phonon modes is attributed to low-coordinated atoms at the NP surface, whose relative content increases with decreasing NP size due to the increase in the surface-to-volume ratio. Our NPs are covered by a Ti-coating layer, which essentially restores the local neighborhood of surface Fe atoms towards bulk-like coordination, reducing the surface effect. Our data can be qualitatively explained by the existence of low-coordinated Fe atoms located at grain boundaries or other defects with structural disorder in the interior of the large NPs (~3-6 nm), while our small NPs (~2 nm) are single grain and, therefore, characterized by a higher degree of structural order. This conclusion is corroborated by the observation of Debye behavior at low energy [g(E) ~ E" with n ~ 2] for the small NPs, but non-Debye behavior (with n ~ 1.4) for the large NPs. The PDOS was used to determine thermodynamic properties of the Fe NPs. Finally, our results demonstrate that, in combination with TEM, NRIXS is a suitable technique to investigate atomic disorder/defects in NPs. We anticipate that our findings are universal for similar NPs with bcc structure. [ABSTRACT FROM AUTHOR]

This study presents a systematic investigation of the thermodynamic properties of free and γ-Al2O3-supported size-controlled Pt nanoparticles (NPs) and their evolution with decreasing NP size. A combination of in situ extended x-ray absorption fine-structure spectroscopy (EXAFS), ex situ transmission electron microscopy (TEM) measurements, and NP shape modeling revealed (i) a cross over from positive to negative thermal expansion with decreasing particle size, (ii) size- and shape-dependent changes in the mean square bond-projected bond-length fluctuations, and (iii) enhanced Debye temperatures (ΘD, relative to bulk Pt) with a bimodal size-dependence for NPs in the size range of ~0.8-5.4 nm. For large NP sizes (diameter d > 1.5 nm) ΘD was found to decrease toward ΘD of bulk Pt with increasing NP size. For NPs ≤ 1 nm, a monotonic decrease of ΘD was observed with decreasing NP size and increasing number of low-coordinated surface atoms. Our density functional theory calculations confirm the size- and shape-dependence of the vibrational properties of our smallest NPs and show how their behavior may be tuned by H desorption from the NPs. The experimental results can be partly attributed to thermally induced changes in the coverage of the adsorbate (H2) used during the EXAFS measurements, bearing in mind that the interaction of the Pt NPs with the stiff, high-melting temperature γ-Al2O3 support may also play a role. The calculations also provide good qualitative agreement with the trends in the mean square bond-projected bond-length fluctuations measured via EXAFS. Furthermore, they revealed that part of the ΘD enhancement observed experimentally for the smallest NPs (d ≤ 1 nm) might be assigned to the specific sensitivity of EXAFS, which is intrinsically limited to bond-projected bond-length fluctuations. [ABSTRACT FROM AUTHOR]