Gradual Structural Evolution Research Articles

Description of neutron-rich odd-mass krypton isotopes within the interacting boson-fermion model based on the Gogny energy density functional

The low-lying structure of neutron-rich odd-mass Kr isotopes is studied within the interacting boson-fermion model (IBFM) based on the Gogny-D1M energy density functional (EDF). The $(\beta,\gamma)$-deformation energy surfaces, spherical single-particle energies and occupation probabilities of the odd-mass systems are obtained using the constrained Hartree-Fock-Bogoliubov (HFB) approximation. Those quantities are used as a microscopic input to determine most of the parameters of the IBFM Hamiltonian. The remaining parameters are specifically tailored to the experimental spectrum for each of the studied odd-mass nuclei. A gradual structural evolution is predicted for the odd-mass isotopes $^{87-95}$Kr as a function of the nucleon number which, agrees well with the gradual growth of collectivity observed experimentally in the neighboring even-even isotopes.

Evidence of magnetoelectric coupling in 0.9BiFeO3-0.1Ba[Ti0.95(Yb0.5Nb0.5)0.05]O3 ceramic

The physical properties of the Multiferroïc lead-free 0.9BiFeO3-0.1Ba[Ti0.95(Yb0.5Nb0.5)0.05]O3 ceramic were investigated by in-situ Mössbauer spectroscopy, X-ray diffraction (XRD), dielectric measurements and Raman scattering. Both BiFeO3 and 0.9BiFeO3-0.1Ba[Ti0.95(Yb0.5Nb0.5)0.05]O3 ceramics underwent in-situ Mössbauer spectroscopy in the temperature range of 300 K to 653 K and the parameters of hyperfine interactions were determined. The magnetic transition temperature TN of our doped sample was of 568 K, which was lower than that of BiFeO3. This reduction of TN originated from the presence of different exchange constants resulting from the weakening of JFe-Fe. The deviation of the reduced magnetic hyperfine field H(T)/H(0) as a function of the reduced temperature T/TN from the Brillouin curve of Fe3+ (S = 5/2) for the three sextets and the determination of the critical exponent β, of 0.375 ± 0.022 in the range of 0.01 < 1-T/TN < 0.5 revealed a 3D magnetic long range order phase transition. The complexity of the spatial spin structure was a result of the canting of antiferromagnetically ordered spins of BiFeO3. Our findings revealed the effect of the non-magnetic substituents Ba and Yb/Nb/Ti in A and B sites, respectively on the magnetic properties. Around the magnetic transition temperature, both isomer shift evolution and quadruple splitting distribution showed pronounced anomalies associated with the onset of magnetic ordering. The Debye temperature (θD) was deduced to be of 257 ± 21 K. These results revealed 0.9BiFeO3-0.1Ba[Ti0.95(Yb0.5Nb0.5)0.05]O3 as a promising material for piezoelectric devices. XRD analyses were performed for temperatures from 300 K up to 750 K. Gradual structural evolution, as temperature increased, exhibited an isostructural phase transition and an anomaly in the evolution of lattice parameters and the unit cell volume as well as a significant shift in the atomic positions near the magnetic transition temperature TN. These behaviors indicate magnetoelastic coupling. The temperature dependence of the real part of dielectric permittivity and dielectric loss (tan δ) was investigated over a wide range of temperatures at 1 kHz. During its evolution, an anomaly was observed at 568 K; which corresponded to the magnetic transition temperature TN. Raman scattering spectra were measured in the temperature range from 303 K to 728 K. Drastic changes, especially that of the three two-phonon modes centered at around 1000-1500 cm−1, were the results of magnetic rearrangement and atomic displacement leading to both modulating magnetic exchange interaction and polarization state. All these results prove the magnetoelectric coupling of 0.9BiFeO3-0.1Ba[Ti0.95(Yb0.5Nb0.5)0.05]O3 ceramic.

Structure and Microwave Dielectric Behavior of A‐Site‐Doped Sr(1−1.5x)CexTiO3 Ceramics System

The ion valence state, phase composition, microstructure, and microwave dielectric properties of Sr(1−1.5x)CexTiO3 (x = 0.1–0.67, SCT) ceramics were systematically investigated. Sr(1−1.5x)CexTiO3 ceramics were produced with gradual structural evolution from a cubic to a tetragonal and turned to an orthorhombic structure in the range of 0.1 ≤ x ≤ 0.67. Above a critical Ce proportion (x = 0.4), microstructural changes and normal grain growth initially occurred. On the basis of chemical analysis results, the reduction of Ti4+ ions was hastened by tetravalent ions (Ce4+). By contrast, this reduction was inhibited by trivalent ions (Ce3+). The observed dielectric behavior was strongly influenced by phase composition, oxygen vacancies (), and defect dipoles, namely, () and (). Temperature stable ceramics sintered at 1350°C for 3 h in air yielded an intermediate value of dielectric constant (εr = 40), with the smallest reported value of temperature coefficient of resonant frequency (τf = +0.9 ppm/°C), and quality factor (Q × f = 5699 GHz) at x = 0.6.

Partially Fluorinated Graphene: Structural and Electrical Characterization.

Despite the number of existing studies that showcase the promising application of fluorinated graphene in nanoelectronics, the impact of the fluorine bonding nature on the relevant electrical behaviors of graphene devices, especially at low fluorine content, remains to be experimentally explored. Using CF4 as the fluorinating agent, we studied the gradual structural evolution of chemical vapor deposition graphene fluorinated by CF4 plasma at a working pressure of 700 mTorr using Raman and X-ray photoelectron spectroscopy (XPS). After 10 s of fluorination, our XPS analysis revealed a co-presence of covalently and ionically bonded fluorine components; the latter has been determined being a dominant contribution to the observation of two Dirac points in the relevant electrical measurement using graphene field effect transistor devices. Additionally, this ionic C-F component (ionic bonding characteristic charge sharing) is found to be present only at low fluorine content; continuous fluorination led to a complete transition to a covalently bonded C-F structure and a dramatic increase of graphene sheet resistance. Owing to the formation of these various C-F bonding components, our temperature-dependent Raman mapping studies show an inhomogeneous defluorination from annealing temperatures starting at ∼150 °C for low fluorine coverage, whereas fully fluorinated graphene is thermally stable up to ∼300 °C.

Transformation from pyrochlore to fluorite by diffraction and X-ray spectroscopy

We have studied the long-range average and local structures in a number of zirconium containing materials of the type A2B2O7 ( A = Ln or Y; B = Zr, Hf or Sn) using synchrotron X-ray and neutron powder diffraction and X-ray absorption spectroscopy. Studies of the system Gd2-xTbxZr2O7 include neutron diffraction data, obtained at λ ≍ 0.497 Å to minimise absorption, not only provide evidence for independent ordering of the anion and cation sublattices, but also suggest that the disorder transition across the pyrochlore-defect fluorite boundary of Ln2Zr2O7 is rather gradual. In general we observe that while the diffraction data indicate a clear phase transition from ordered pyrochlore to disordered defect-fluorite at specific compositions corresponding to a critical ionic radius ratio of the A and B cations (rA/rB) x ~ 1.0-1.2, X-ray absorption near-edge structure (XANES) results reveal a gradual structural evolution across the compositional range. These findings provide experimental evidence that the local disorder occurs long before the pyrochlore to defect-fluorite phase boundary as determined by X-ray diffraction, and the extent of disorder continues to develop throughout the defect-fluorite region. Where possible the experimental results were supplemented by ab initio atomic scale simulations, which provide a mechanism for disorder to initiate in the pyrochlore structure. Further, the coordination numbers of the cations in both the defect-fluorite and pyrochlore structures were predicted, and the trends agree well with the experimental XANES results. X-ray absorption measurements at the Zr L3-edge, which showed a gradual increase in the effective coordination number of the Zr from near 6-coordinate in the pyrochlore rich samples to near 7-coordinate in the defect fluorites.

Gradual Structural Evolution from Pyrochlore to Defect-Fluorite in Y2Sn2–xZrxO7: Average vs Local Structure

We have studied the long-range average and local structures in Y2Sn2–xZrxO7 (x = 0–2.0) using synchrotron X-ray powder diffraction and X-ray absorption spectroscopy, respectively, and by theoretical methods. While the diffraction data indicate a clear phase transition from ordered pyrochlore to disordered defect-fluorite at x ∼ 1.0–1.2, X-ray absorption near-edge structure (XANES) results at the Zr L3- and Y L2-edges reveal a gradual structural evolution across the whole compositional range. These findings provide experimental evidence that the local disorder occurs long before the pyrochlore to defect-fluorite phase boundary, as determined by X-ray diffraction, and the extent of disorder continues to develop throughout the defect-fluorite region. The Zr and Y L-edge spectra are very sensitive to changes in the local structure; such sensitivity enables us to reveal the progressive nature of the phase transition. Experimental results are supported by ab initio atomic scale simulations, which provide a mech...

Snowballs, quantum solvation and coordination: lead ions inside small helium droplets

Ab initio calculations are used to construct an analytical many-body potential for Pb(2+)He(n) and Pb(+)He(n) clusters which accounts for non pairwise additive interactions. The potential surface reproduces the global minima for cluster sizes ranging from n = 1 to n = 16 obtained from explicit ab initio calculations and found in a previous search for ultrahigh coordination numbers. Ground state energies and structures obtained by accurate diffusion quantum Monte Carlo calculations are used to investigate if quantum effects qualitatively affect the formation of coordination shells. For Pb(2+) doped clusters a first solvation shell is closed at n = 12 and gradually softened by additional helium atoms which start to form a distinct second shell only at n = 16. Spin-orbit coupling profoundly influences the structure of Pb(+)He(n) clusters and causes a gradual structural evolution without pronounced solvation shells.

Structural evolution of layered Li1.2Ni0.2Mn0.6O2 upon electrochemical cycling in a Li rechargeable battery

Recently Li1.2Ni0.2Mn0.6O2, one of the most promising cathode candidates for next generation Li rechargeable batteries, has been consistently investigated especially because of its high lithium storage capacity, which exceeds beyond the theoretical capacity based on conventional chemical concepts. Yet the mechanism and the origin of the overcapacity have not been clearly understood. Previous reports on simultaneous oxygen evolution during the first delithiation may only explain the high capacity of the first charge process, and not of the subsequent cycles. In this work, we report a clarified interpretation of the structural evolution of Li1.2Ni0.2Mn0.6O2 upon the electrochemical cycling, which is the key element in understanding its anomalously high capacity, through careful study of electrochemical profiles, exsitu X-ray diffraction, HR-TEM, Raman spectroscopy, and first principles calculation. Moreover, we successfully resolved the intermediate states of structural evolution upon electrochemical cycles by intentionally synthesizing sample with large particle size. All observations made through various tools lead to the result that spinel-like cation arrangement and lithium environment are gradually created and locally embedded in layered framework during repeated electrochemical cycling. Moreover, through analyzing the intermediate states of the structural transformation, this gradual structural evolution could explain the mechanism of the continuous development of the electrochemical activity below 3.5 V and over 4.25 V.

Magnetic Transition and Structural Evolution in NiCo/Ag Multilayers

The structures and magnetic properties of Ni77Co23 (1.8 nm)/Ag (tAg nm) multilayers, prepared by dc-magnetron sputtering, have been studied as a function of Ag layer thickness (tAg). The films progressively evolve from ferromagnetic to superparamagnetic with tAg increasing from 1.0 to 3.6 nm. This magnetic transition versus tAg is accompanied by a gradual structural evolution from layered multilayers to a granular system composed of NiCo giant granules. The sizes of giant superparamagnetic granules are approximately 40–80 and 100–140 nm for the samples with tAg=3.0 and 3.6 nm, respectively. Furthermore, the granules grow as metastable fcc NiCo phases with a lattice parameter of 0.4044 ±0.0002 nm. The mechanisms responsible for the phenomena and the relationship between magnetization and structural evolution in the films are discussed.