Elastic,electronic structure,and optical properties of orthorhombic Na<Subscript>3</Subscript>AlF<Subscript>6</Subscript>: a first-principles study

First-principles investigation of elastic, electronic, and optical properties of orthorhombic Na₃AlF₆ has been carried out by DFT using plane-wave pseudo-potentials within the LDA and GGA. Calculated lattice parameters agree well with experimental results. From calculated elastic constants, Na₃AlF₆ is a mechanically stable anisotropic and behaves in a ductile manner. Electronic structure analysis indicates that Na₃AlF₆ behaves as an insulator with a direct band gap of 6.065 eV in LDA and 5.868–5.949 eV in GGA. DOS, population analysis, and charge densities difference indicate that Al-F bonds are mainly ionic as well as partially covalent due to the hybridization of F-2p and Al-3s (3p) states. Moreover, the imaginary part of calculated dielectric function ε₂(ω) shows three prominent peaks due to the inter band transitions F 2p states→Na 3s states. From calculated ε (ω), other optical properties such as reflectivity and refractive index are also obtained up to the photon energy range of 40 eV.     

Improvement of electrochemical performance for AlF<Subscript>3</Subscript>-coated Li<Subscript>1.3</Subscript>Mn<Subscript>4/6</Subscript>Ni<Subscript>1/6</Subscript>Co<Subscript>1/6</Subscript>O<Subscript>2.40</Subscript> cathode materials for Li-ion batteries

Li-rich layered-layered-Spinel structure spherical Li_1.3Mn_4/6Ni_1/6Co_1/6O_2.40 particles was successfully prepared and coated with a uniform layer by a two-step co-precipitation method and evaluated in lithium cells. The structures and electrochemical properties of pristine Li_1.3Mn_4/6Ni_1/6Co_1/6O_2.40 and AlF₃-coated Li_1.3Mn_4/6Ni_1/6Co_1/6O_2.40 were characterized. When the coating amount was 2 wt%, the cathode showed the best cycling performance and rate capability compared to others. The AlF₃-coated Li_1.3Mn_4/6Ni_1/6Co_1/6O_2.40 Li-ion cell cathode had a capacity retention of 90.07 % after 50 cycles at 0.5 C over 2.0–4.8 V, while the pristine Li_1.3Mn_4/6Ni_1/6Co_1/6O_2.40 exhibited capacity retention of only 80.73 %. Moreover, the rate capability and cyclic performance also improved. Electrochemical impedance spectroscopy testing revealed that the improved electrochemical performance might attribute to the AlF₃ coating layer which can suppress the increase of impedance during the charging and discharging process by preventing direct contact between the highly delithiated active material and electrolyte.     

Memory effects in superparamagnetic La<Subscript>0.6</Subscript>Pb<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript> nanoparticles

Using different temperature and field protocols, the memory behaviors in the dc magnetization and magnetic relaxation are observed at temperature below blocking temperature T_B = 93 K in weakly interacting manganite La_0.6Pb_0.4MnO₃ nanoparticles. The results indicate that the magnetic dynamics of this nanoparticle system is strongly correlated with a wide distribution of particle relaxation times, which may arise from the particle weak interaction and distribution of the particle size.     

AlF<Subscript>3</Subscript> coating of LiNi<Subscript>0.5</Subscript>Mn<Subscript>1.5</Subscript>O<Subscript>4</Subscript> for high-performance Li-ion batteries

AlF₃-coating is attempted to improve the performance of LiNi_0.5Mn_1.5O₄ cathode materials for Li-ion batteries. The prepared powders are characterized by scanning electron microscope, powder X-ray diffraction, charge/discharge, and impedance. The coated LiNi_0.5Mn_1.5O₄ samples show higher discharge capacity, better rate capability, and higher capacity retention than the uncoated samples. Among the coated samples, 1.0 mol% AlF₃-coated sample shows highest capacity after charge–discharged at 30 mA/g for 3 cycles, but 4.0 mol% coated sample exhibits the highest capacity and cycling stability when cycled at high rate of 150 and 300 mA/g. The 40th cycle discharge capacity at 300 mA/g current still remains 114.8 mAh/g for 4.0 mol% AlF₃-coated LiNi_0.5Mn_1.5O₄, while only 84.3 mAh/g for the uncoated sample.     

Dipole ordering and ionic conductivity in NASICON-Type Na<Subscript>3</Subscript>Cr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> structures

The aspects of structure, dipole ordering, and ionic conductivity of the Na₃Cr₂(PO₄)₃ crystal with the four polymorphic phases (α, α', β, and γ) have been investigated. The features of the α-Na₃Cr₂(PO₄)₃ crystal structure and its dipole ordering and relaxation polarization in the low-temperature α and α' phases have been refined. The occurrence of Na₃Cr₂(PO₄)₃ dipole ordering in the α and α' phases and high ionic conductivity in the β and γ phases is attributed to the structural changes in the rhombohedral [Me₂(PO₄)₃]^–3_3∞ crystal frame upon phase transformations α → α', α' → β, and β → γ. A model for explaining the dipole ordering and ionic conductivity phenomena in Na₃Cr₂(PO₄)₃ is proposed.     

Surface modification of polycrystalline oxides (V<Subscript>2</Subscript>O<Subscript>5</Subscript>, MoO<Subscript>3</Subscript>, and WO<Subscript>3</Subscript>) irradiated with high-power ion beams

The influence of a high-power ion beam on polycrystalline oxides (V₂O₅, MoO₃, and WO₃) is investigated. Oxide irradiation with ion beams with current densities of greater than ～30 A/cm² is established to initiate changes in the color of irradiated layers and lead to surface-layer particle melting. It is demonstrated that a distinctive feature of the interaction between a high-power ion beam and V₂O₅ is the formation of surface nanosheets and nanowires whose characteristic cross-sectional size and thickness are ～1 μm and up to ～40 nm, respectively. The nanosheets are generated near emerging surface cracks if the beam current density is ～100 A/cm². Possible mechanisms of surface nanostructures formation under the action of pulsed ion beams are discussed.     

Synthesis and luminescent properties of Eu<Superscript>3+</Superscript>- and Eu<Superscript>2+</Superscript>-activated sodium pyrophosphate Na<Subscript>4</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript>

The luminescent properties of Eu³⁺ and Eu²⁺ ions in sodium pyrophosphate, Na₄P₂O₇, have been studied. The excitation spectrum of the Eu³⁺ emission in Na₄P₂O₇ consists of several sets of bands in the range 280–535 nm due to 4f–4f transitions of Eu³⁺ ions and a broad band with a maximum at about 240 nm interpreted to be due to a charge transfer (CT) transition from oxygen 2p states to empty states of the Eu³⁺ 4f⁶-configuration. Although the CT band energy is large enough, the quantum efficiency (η) of the Eu³⁺ emission in Na₄P₂O₇ under CT excitation was estimated to be very low (η ≤ 0.01). In terms of a configurational coordinate model, this fact is interpreted as a result of the high efficiency of a radiationless relaxation from the CT state to the ⁷F₀ ground state of Eu³⁺ ions occupying sodium sites in Na₄P₂O₇. A strong reducing agent is required in order to stabilize Eu²⁺ ions in Na₄P₂O₇ during the synthesis. Several nonequivalent Eu²⁺ luminescence centers in Na₄P₂O₇ were found.     

Growth and spectral properties of self-frequency doubling crystal,Nd:Ca<Subscript>9.03</Subscript>Na<Subscript>1.08</Subscript>La<Subscript>0.62</Subscript>(VO<Subscript>4</Subscript>)<Subscript>7</Subscript>

The non-doped and doped Nd³⁺ of Ca_9.03Na_1.08La_0.62(VO₄)₇ crystals were grown by the Czochralski technique. The effective segregation coefficients of Na⁺ and Nd³⁺ ions in the crystal were measured to be about 0.5 and 1.1, respectively. The XPS analysis of Ca_9.03Na_1.08La_0.62(VO₄)₇ crystal indicates that the vanadium in the crystal is a mixture of V⁴⁺ (1.46 at. %) and V⁵⁺ (98.54 at. %). The hardness of Nd:Ca_9.03Na_1.08La_0.62(VO₄)₇ crystal is about 383.1 VDH. Nd:Ca_9.03Na_1.08La_0.62(VO₄)₇ crystal exhibits similar thermal expansion coefficients along the a (11.2×10^-6 K^-1) and c (13.7×10^-6 K^-1) axes, indicating a low thermal expansion anisotropy (α_c/α_a≈1.2). The qualitative frequency-doubling experiment shows that the doping of Na⁺ ion can help reduce the scattering of frequency-doubling light, and the intensity of SHG for Ca_9.03Na_1.08La_0.62(VO₄)₇ crystal is found to be about 3.5 times as large as that of KDP. The polarized absorption and fluorescence spectra are analyzed based on Judd–Ofelt theory, which exhibits that the π-polarized absorption and stimulated emission cross sections are 6.07×10^-20 cm² with an FWHM 12.0 nm at 810 nm and 1.42×10^-19 cm² at 1069 nm, respectively. The fluorescence lifetime is 115 μs at room temperature. All the results indicate that Nd:Ca_9.03Na_1.08La_0.62(VO₄)₇ crystal is a candidate of self-frequency doubling laser material. PACS 42.62.Fi; 42.70.Mp; 81.10.Fq     

Acoustic attenuation in ferroelectric Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript> crystals

The temperature and frequency dependencies of sound attenuation for the proper uniaxial ferroelectric Sn₂P₂S₆, which has a strong nonlinear interaction of the polar soft optic and fully symmetrical optic modes that is related to the triple well potential, were studied by Brillouin spectroscopy. It was found that the sound velocity anomaly is described in the Landau-Khalatnikov approximation with one relaxation time. For explanation of the observed temperature and frequency dependencies of the sound attenuation in the ferroelectrric phase, the accounting of several relaxation times is needed and, for quantitative calculations, the mode Gruneisen coefficients are more appropriate as interacting parameters than are the electrostrictive coefficients. Relaxational sound attenuation by domain walls also appears in the ferroelectric phase of Sn₂P₂S₆ crystals.     

Electrical properties of a lead-free perovskite ceramic: (Na<Subscript>0.5</Subscript>Sb<Subscript>0.5</Subscript>)TiO<Subscript>3</Subscript>

Polycrystalline (Na_0.5Sb_0.5)TiO₃ was prepared using a high-temperature solid-state reaction method. An XRD analysis indicated the formation of a single-phase monoclinic structure. Complex impedance studies revealed the presence of grain boundary effects from 300 °C onwards. Also, the dielectric relaxation in the system was found to be of a non-Debye type. The ac conductivity data were used to evaluate the density of states at the Fermi level, the minimum hopping length and activation energy of the compound. The dc electrical and thermal conductivities of grain and grain boundary have been assessed. The correlated barrier hopping model was found to successfully explain the mechanism of charge transport in (Na_0.5Sb_0.5)TiO₃. PACS 72.20.Ee; 77.22.Ch; 77.22.Gm; 77.84.Dy; 81.05.Je     

Transient hole-polaron optical absorption in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript> crystals

Results of a study of transient optical absorption (TOA) and luminescence of lithium gadolinium orthoborate Li₆Gd(BO₃)₃ (LGBO) in the visible and UV spectral regions are presented. As revealed by absorption optical spectroscopy with nanosecond time resolution, the LGBO TOA derives from optical transitions in hole centers, with the optical density relaxation kinetics being mediated by interdefect tunneling recombination involving these centers and neutral lithium atoms acting as electronic Li⁰ centers. At 290 K, the Li⁰ centers are involved in thermostimulated migration, which is not accompanied by carrier transfer to the conduction or valence band. The slow components of the TOA decay kinetics, with characteristic times ranging from a few milliseconds to seconds, have been assigned to diffusion-limited annihilation of lithium interstitials with vacancies. The mechanisms responsible for the creation and relaxation of short-lived Frenkel defect pairs in the LGBO cation sublattice have been analyzed.     

Spin dynamics in LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript> and NaCu<Subscript>2</Subscript>O<Subscript>2</Subscript> low-dimensional helical magnets

Comprehensive NMR investigation of low-frequency spin dynamics of LiCu₂O₂ (LCO) and NaCu₂O₂ (NCO) low-dimensional helical magnets in the paramagnetic state has been carried out for the first time. Temperature dependences of the spin–lattice relaxation rate and anisotropy on various LCO/NCO nuclei have been determined at various orientations of single crystals in an external magnetic field. The spatial asymmetry of spin fluctuations in LCO multiferroic has been discovered. The quantitative analysis of the anisotropy of spin–lattice relaxation in LCO/NCO has allowed estimating the contributions of individual neighboring Cu²⁺ ions to the transferred hyperfine field on Li⁺(Na⁺) ions.     

Synthesis and enhanced acetone gas-sensing performance of ZnSnO<Subscript>3</Subscript>/SnO<Subscript>2</Subscript> hollow urchin nanostructures

A kind of novel ZnSnO₃/SnO₂ hollow urchin nanostructure was synthesized by a facile, eco-friendly two-step liquid-phase process. The structure, morphology, and composition of samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption–desorption techniques. The results revealed that many tiny needle-like SnO₂ nanowires with the average diameter of 5 nm uniformly grew on the surface of the ZnSnO₃ hollow microspheres and the ZnSnO₃/SnO₂ hollow urchin nanostructures with different SnO₂ content also were successfully prepared. In order to comprehend the evolution process of the ZnSnO₃/SnO₂ hollow urchin nanostructures, the possible growth mechanism of samples was illustrated via several experiments in different reaction conditions. Moreover, the gas-sensing performance of as-prepared samples was investigated. The results showed that ZnSnO₃/SnO₂ hollow urchin nanostructures with high response to various concentration levels of acetone enhanced selectivity, satisfying repeatability, and good long-term stability for acetone detection. Specially, the 10 wt% ZnSnO₃/SnO₂ hollow urchin nanostructure exhibited the best gas sensitivity (17.03 for 50 ppm acetone) may be a reliable biomarker for the diabetes patients, which could be ascribed to its large specific surface area, complete pore permeability, and increase of chemisorbed oxygen due to the doping of SnO₂.     

Synthesis of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles with tunable magnetism by the modified hydrothermal method

Cobalt ferrite (CoFe₂O₄) nanoparticles were synthesized by using the hydrothermal route with the addition of trisodium citrate dihydrate (Na₃CA·2H₂O). The formation of CoFe₂O₄ nanoparticles with size ranging from 13 to 19 nm was confirmed by X-ray diffraction, energy dispersive X-ray analysis, and Fourier transform infrared spectroscopy; the clear-cut sharp of the nanoparticles was observed by transmission electron microscopy. By these characterization methods, the evolution of lattice constant and morphologies of the nanoparticles with the addition of Na₃CA·2H₂O is observed. Furthermore, the magnetic hysteresis loops measured at room temperature indicate that the magnetic properties of the products also show clear relationship with the masses of Na₃CA·2H₂O. For example, coercivity and high-field paramagnetic susceptibility increase with the increasing masses of Na₃CA·2H₂O, whereas the saturation magnetization and the effective magnetic anisotropy constant have the maximum values as the mass of Na₃CA·2H₂O is 1 g. This change of magnetic properties is related with the expanded lattice and the varied size and shape because of the addition of Na₃CA·2H₂O.     

Anomalous magnetic relaxation in thin Pd<Subscript>0.99</Subscript>Fe<Subscript>0.01</Subscript> films

The magnetic relaxation in Pd_0.99Fe_0.01 films, which have the thicknesses that are practically important for cryoelectronics (25 and 40 nm), is detected and experimentally studied. The relaxation is shown to be substantial only in thin films. The magnetization relaxation is found to be well described by the sum of two exponential functions with characteristic times that differ by an order of magnitude from each other. The characteristic relaxation time and the ratio of the contributions of two relaxations depend on temperature. The activation energies of the relaxation processes are determined. The activation volume is shown to correspond to a 20-nm ferromagnetic cluster. The results obtained agree with the model of two-component magnetization in thin PdFe films [6].     

Dielectric investigations in Sr<Subscript>0.75</Subscript>Ba<Subscript>0.25</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> relaxor ferroelectric thin films

The dielectric properties of Sr_0.75Ba_0.25Nb₂O₆ relaxor ferroelectric thin films were carefully analyzed. In contrast to bulk samples which present three distinct dielectric relaxation phenomena Sr_0.75Ba_0.25Nb₂O₆ thin films present only two of them. The suppression of the third anomaly can be mainly attributed to the narrow grain size distribution of nanograins and weak tensile strains imposed to the film from the substrate. The whole set of results point to the interpretation of a dielectric response characteristic of mesoscopic structure, which is composed of clusters and nanodomains.     

Multi-hierarchical nanosheet-assembled chrysanthemum-structured Na<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C as electrode materials for high-performance sodium-ion batteries

The structure and morphology of sodium vanadium phosphate (Na₃V₂(PO₄)₃) play a vital role in enhancing the electrochemical performance of sodium-ion batteries due to the inherent poor electronic conductivity of the phosphate framework. In order to improve this drawback, a new chrysanthemum-structured Na₃V₂(PO₄)₃/C material has been successfully assembled with multi-hierarchical nanosheets via a hydrothermal method. Continuous scattering nanosheets in chrysanthemum petals are beneficial in reducing energy consumption during the process of sodium ion diffusion, on which the carbon-coated surface can significantly increase overall conductivity. The as-prepared sample exhibits outstanding electrochemical performance due to its unique structure. It rendered a high initial specific capacity of 117.4?mAh?g^?1 at a current density of 0.05 C. Further increasing the current density to 10 C, the initial specific capacity still achieves 101.3?mAh?g^?1 and remains at 87.5?mAh?g^?1 after 1000 cycles. In addition, a symmetrical sodium-ion full battery using the chrysanthemum-structured Na₃V₂(PO₄)₃/C materials as both the cathode and anode has been successfully fabricated, delivering the capacity of 62?mAh?g^?1 at 1?C and achieving the coulombic efficiency at an average of 96.4% within 100 cycles. These results indicate that the new chrysanthemum-structured Na₃V₂(PO₄)₃/C can provide a new idea for the development of high-performance sodium-ion batteries.     

Recovery of electrical-resistivity and viscoelasticity relaxation in thermally aged bulk Pd<Subscript>40</Subscript>Cu<Subscript>30</Subscript>Ni<Subscript>10</Subscript>P<Subscript>20</Subscript> metallic glass

The structural relaxation of a bulk Pd₄₀Cu₃₀Ni₁₀P₂₀ metallic glass is studied by measuring the electrical resistivity and infralow-frequency (0.05 Hz) internal friction. It is demonstrated that the structural relaxation in thermally aged samples can be restored by quenching them from a supercooled liquid state. It is found that the degree of relaxation after quenching can exceed the initial one by several times.     

AC conductivity evolution in bulk and grain boundary response of sodium tungstate Na<Subscript>2</Subscript>WO<Subscript>4</Subscript>

The Na₂WO₄ compound has been obtained by the conventional solid-state reaction and characterized by X -ay powder diffraction. The title material crystallizes in the cubic system with Fd-3m space group. The electrical properties of the compound have been studied using complex impedance spectroscopy in the frequency range 200 Hz–5 MHz and temperature range 586–679 K. Two semicircles are observed in impedance plot indicating the presence of two relaxation processes in the compound associated with the grain and grain boundary. The relaxation behavior of the grain and grain boundary of the Na₂WO₄ are also obtained from the analyzed electrical modulus data. AC conductivity measured follows the power-law dependence σ_AC～ω^s typical for charge transport. Therefore, the experimental results are analyzed with various theoretical models. Temperature dependence of the power law exponent s strongly suggests that tunneling of large polarons is the dominant transport process. The mechanism of conduction is probably due from the displacements of the Na+ ion in the tunnel-type cavities along [111] direction.     

EPR Study of Sc<Subscript>2</Subscript>SiO<Subscript>5</Subscript>:Nd<Superscript>143</Superscript> Isotopically Pure Impurity Crystals

The Sc₂SiO₅ single crystals doped with 0.001 at.% of the ¹⁴³Nd³⁺ ion were studied by continuous-wave and pulse electron paramagnetic resonance methods. The g-tensors and hyperfine structure tensors for two magnetically non-equivalent Nd ions were obtained. The spin–spin and spin–lattice relaxation times were measured at 9.82 GHz in the temperature range from 4 to 10 K. It was established that three relaxation processes contribute to the spin–lattice relaxation processes. There are one-phonon spin–phonon interaction, two-phonon Raman interaction and two-phonon Orbach–Aminov relaxation processes. It was established that spin–spin relaxation time is of the same magnitude for neodymium ion doped in Sc₂SiO₅ and in Y₂SiO₅.