Investigation of electronic states of infinite-layer SrFeO2 epitaxial thin films by X-ray photoemission and absorption spectroscopies

We investigated the electronic states of a single-crystal SrFeO₂ epitaxial thin film in the valence-band and conduction-band regions using synchrotron-radiation X-ray photoemission and absorption spectroscopies. Fe 2p–3d resonant photoemission measurements revealed that the Fe 3d states have higher densities of states at binding energies of 3–5 eV and 5–8.5 eV in the valence-band region. The O K-edge X-ray absorption spectrum exhibited three peaks in the Fe 3d-derived conduction band hybridized with O 2p states; these can be assigned to Fe 3d_xy, 3d_xz + 3d_yz, and 3d_x²_–y². In addition, the indirect bandgap value of the SrFeO₂ film was determined to be 1.3 eV by transmission and absorption spectroscopies.     

Synthesis and visible light photocatalysis water splitting property of chromium-doped Bi4Ti3O12

Photocatalysts Bi₄Ti_3 ? xCr_xO₁₂(x = 0.00, 0.06, 0.15, 0.30, 0.40, and 0.50) with perovskite structure were synthesized by sol–gel method and their electronic structures and photocatalytic activities were investigated. The Bi₄Ti_2.6Cr_0.4O₁₂ photocatalyst exhibited the highest performance of H₂ evolution in methanol aqueous solution (58.1 μmol h^? 1 g^? 1) under visible light irradiation (λ > 400 nm) without a co-catalyst, whereas no H₂ evolution is observed for Bi₄Ti₃O₁₂ under the same conditions. The UV–vis spectra indicated that the Bi₄Ti_2.6Cr_0.4O₁₂ had strong photoabsorption in the visible light region. The results of density functional theory (DFT) calculation illuminate that the conduction bands of Bi₄Ti₃O₁₂ are mainly attributable to the Ti 3d + Bi 6p orbitals, and the valence bands are composed of O 2p + Bi 6s hybrid orbitals, while the conduction bands of chromium-doped Bi₄Ti₃O₁₂ are mainly attributable to the Ti 3d + Bi 2p + Cr 3d orbitals, and the O 2p + Cr 3d hybrid obitals are the main contribution to the valence band.     

Reduction of thin-film ceria on Pt(111) by supported Pd nanoparticles probed with resonant photoemission

Local defects present in CeO_2 ? x films result in a mixture of Ce³⁺ and Ce⁴⁺ oxidation states. Previous studies of the Ce 3d region with XPS have shown that depositing metal nanoparticles on ceria films causes further reduction, with an increase in Ce³⁺ concentration. Here, we compare the use of XPS and resonant photoemission spectroscopy (RESPES) to estimate the concentration of Ce³⁺ and Ce⁴⁺ in CeO_2 ? x films grown on Pt (111), and the variation of this concentration as a function of Pd deposition. Due to the nature of the electronic structure of CeO_2 ? x, resonant peaks are observed for the 4d–4f transitions when the photon energy matches the resonant energy; (hν = 121.0 eV) for Ce³⁺ and (hν = 124.5 eV) for Ce⁴⁺. This results in two discrete resonant photoemission peaks in valence band spectra. The ratio of the difference of these peaks with off-resonance scans gives an indication of the relative contribution of Ce³⁺. Results from RESPES indicate reduction of CeO_2 ? x on deposition of Pd, confirming earlier findings from XPS studies.     

Proton conduction of Ba6Mn24O48 tunnel manganite

A protonated Ba₆Mn₂₄O₄₈ phase with an original tunnel crystal structure and mixed-valent manganese has been prepared in the form of powders and millimeter-long fibrous crystals (whiskers). A combined study either by impedance spectroscopy or transport measurements revealed that the Ba₆Mn₂₄O₄₈ phase demonstrates mixed conductivity with both proton and electronic components reaching ～ 10^? 3 Ω^? 1?cm^? 1 at room temperature.     

Room-temperature photoluminescence in MnF2 under high pressure

A novel two-color photoluminescence (PL) is found in MnF₂ at room temperature (RT) under high pressure. In contrast to the low-temperature PL, the observation of room-temperature PL is unusual in transition-metal concentrated materials like MnF₂ since the de-excitation process at RT is fully governed by energy transfer to non-radiative centers. We show that room-temperature MnF₂ emissions originate in the pressure-induced cotunnite phase. Both the nine-fold Mn²⁺ coordination and the Mn–F–Mn exchange pathway inhibit exciton migration among Mn²⁺, favoring two-band PL at RT. The electronic structure and the excited-state dynamics are investigated by time-resolved emission and excitation spectroscopies under pressure. The two PL bands at 2.34 and 1.87 eV above 15 GPa are assigned to Mn²⁺ emissions arising from two distinct Mn²⁺ centers formed in the MnF₂ high-pressure phase. The microscopic origin of the two-color PL is analyzed in terms of exciton dynamics in the MnF₂ cotunnite structure.     

Mechanisms for Ce3+ excitation at energies below the zero-phonon line in YAG crystals and nanocrystals

Excitation of YAG:Ce³⁺ crystals and nanocrystals was performed at λ_exc=473–584 nm over a wide temperature range. It was observed that the luminescence of both nano- and single-crystal YAG:Ce samples is efficiently excited with photon energies well below the Ce³⁺ absorption band and at least 1650 cm^?1 below the ZPL of the 4f¹(²F_5/2)?4f⁰5d¹ transition, located at 489 nm. The studies of Ce³⁺ fluorescence spectra as a function of temperature and excitation wavelength and of their temporal evolution point to the role of phonon-assisted nonradiative energy transfer between different groups of Ce³⁺ centers in the excitation mechanism.     

Synthesis and crystallographic studies of garnet-related lithium-ion conductors Li6CaLa2Ta2O12 and Li6BaLa2Ta2O12

High-purity specimens of Li₆CaLa₂Ta₂O₁₂ and Li₆BaLa₂Ta₂O₁₂ have been successfully synthesized by solid-state reactions. The analytical chemical compositions of these samples were in good agreement with the nominal compositions of Li₆CaLa₂Ta₂O₁₂ and Li₆BaLa₂Ta₂O₁₂. The Rietveld refinements verified that these compounds have the garnet-type framework structure with the lattice constants of a = 12.725(2) Å for Li₆CaLa₂Ta₂O₁₂ and a = 13.001(4) Å for Li₆BaLa₂Ta₂O₁₂. All of the diffraction peaks of X-ray powder diffraction patterns were well indexed on the basis of cubic symmetry with space group Ia-3d. To make a search for Li sites, the electron density distributions were precisely examined by using the maximum entropy method. Li⁺ ions occupy partially two types of crystallographic site in these compounds: (i) tetrahedral 24d sites, and (ii) distorted octahedral 96h sites, the latter of which are the vacant sites of the ideal garnet-type structure. The present Li₆CaLa₂Ta₂O₁₂ and Li₆BaLa₂Ta₂O₁₂ samples exhibit the conductivity σ = 2.2 × 10^? 6 S cm^? 1 at 27 °C (E_a = 0.50 eV) and σ = 1.3 × 10^? 5 S cm^? 1 at 25 °C (E_a = 0.44 eV), respectively.     

LiMn2−xTixO4 spinel-type compounds (x ≤ 1): Structural,electrical and magnetic properties

LiMn_2−xTi_xO₄ compounds with 0 ≤ x ≤ 1 were prepared by solid state reaction and Pechini technique. Powder X-ray diffraction showed that all samples crystallize with the spinel crystal structure (S.G. Fd3¯m). The cubic unit-cell parameter increases with the Ti content. The influence of the Ti content and cationic distribution on the magnetic properties of the compounds was studied by measuring the temperature and magnetic field dependences of the magnetization: substitution by non-magnetic d⁰ Ti⁴⁺ ions appeared to weaken the magnetic interactions between the manganese ions. The electrical properties of LiMnTiO₄ were studied by AC impedance spectroscopy and DC polarisation measurements, which revealed the electronic character of the conduction process.     

Large photonic band gap and strong attenuation of multiconnected Peano network

In this paper, by means of the network equation and generalized eigenfunction method we investigate the optical transmission spectra and attenuation behavior of electromagnetic (EM) waves in multiconnected Peano networks composed of one-dimensional (1D) waveguides. It is found that for some two-segment-connected networks a very large photonic band gap (PBG) can be created in the middle of a frequency period and the width of the large PBG can be controlled by adjusting the matching ratio of waveguide length, d₂ : d₁. When d₂ : d₁ = 2 : 1, the width of the large PBG is bigger than half of frequency period. The influence of generation on the width and attenuation of the large PBG are also studied and the numerical results demonstrate that the first-generation Peano network with d₂ : d₁ = 2 : 1 is a good selectable structure for the designing of optical devices with large PBG and strong attenuation.     

Superconductivity of the platinum doped 122 iron arsenide SrFe2−xPtxAs2

Pt doped 122 iron arsenide SrFe_1?xPt_xAs₂ (0 ? x ? 0.4) was successfully synthesized. The tetragonal unit-cell volume and the lattice constant a increase with increasing the Pt content, while c decreases, suggesting that the Fe ions are indeed replaced by Pt ions. By the Pt doping, the magnetic order of the parent phase is suppressed, and superconductivity emerges at approximately x = 0.15. T_c reaches the maximum of 16 K at x = 0.2. The compounds series can be a suitable subject to investigate role of the doped 5d state in the superconducting 3d Fe–As layer.     

Structural and magnetic properties of the double perovskite Sr2MnWO6

The nuclear and magnetic structure and the magnetic properties of the polycrystalline double perovskite Sr₂MnWO₆ have been studied. Rietveld analysis of neutron powder diffraction (NPD) data at T=295 K shows that the sample is tetragonal (space group P4₂/n, a=8.0119(4) Å, c=8.0141(8) Å). Some additional magnetic diffraction peaks were found in the NPD pattern at 10 K, which can be accounted for by antiferromagnetic ordering of spins at the Mn sites. The magnetic unit cell is doubled in all three unit axes directions (a=b=15.9984(8) Å, c=16.012(2) Å) and the manganese moments are coupled antiferromagnetically along the unit cell axes. The total magnetic moment of Mn²⁺ is found to be 2.27(7) μ_B. The antiferromagnetic behaviour was confirmed from magnetisation measurements. The transition from a paramagnetic to an antiferromagnetic state takes place at 13.0±0.1 K.     

Effect of W-addition on pinning property of MgB2

The 5d transition metal W was added into the MgB₂ superconductor. The Mg, B and W were sintered at 1173 K for 30 min under H₂/Ar atmosphere in the electric furnace. The W_x(MgB₂)_1?x samples were prepared in the W concentration range of 0 ? x ? 0.05. Temperature and field dependences of magnetization were measured by the SQUID magneto-meter. The field and x dependences of J_c at 20 K were analyzed by the extended critical state model. The enhancement of J_c became maximum for the x = 0.02 sample.     

Interface capacitance of nano-patterned electrodes

By employing numerical solutions of the Poisson–Boltzmann equation we have studied the interface capacitance of flat electrodes with stripes of different potentials of zero charge ?_pzc. The results depend on the ratio of the width of the stripes l to the dielectric screening length in the electrolyte, the Debye length d_Debye, as well as on the difference Δ?_pzc in relation k_BT/e. As expected, the capacitance of a striped surface has its minimum at the mean potential of the surface if l/d_Debye << 1 and displays two minima if l/d_Debye >> 1. An unexpected result is that for Δ?_pzc ? 0.2V, the transition between the two extreme cases does not occur when l ? d_Debye, but rather when l > 10d_Debye. As a consequence, a single minimum in the capacitance is observed for dilute electrolytes even for 100 nm wide stripes. The capacitance at the minimum is however higher than for homogeneous surfaces. Furthermore, the potential at the minimum deviates significantly from the potential of zero mean charge on the surface if l > 3d_Debye and Δ?_pzc is larger than about 4k_BT/e. The capacitance of stepped, partially reconstructed Au(11n) surfaces is discussed as an example. Consequences for Parsons–Zobel-plots of the capacitances of inhomogeneous surfaces are likewise discussed.     

Hump structure below Tc in the thermal conductivity of MgB2 superconductor

A reasonable cause of absence of hump structure in thermal conductivity of MgB₂ below the superconducting transition temperature (T_c) lies in the appearance of multigap structure. The gaps of lower magnitude can be suppressed by defects so that this system becomes effectively a single-gap superconductor. When such a situation is created, it is hoped that thermal conductivity (κ) will show hump below T_c. Proceeding along these lines, a sample of MgB₂ with a relatively higher residual resistivity ρ_o = 33.8 μΩ cm has been found to show a hump structure below T_c. The actual electronic thermal conductivity κ_el of this sample is less than that expected from the Wiedeman–Franz law by more than a factor of 2.6 in the considered temperature range.Modifying the Wiedeman–Franz law for the electronic contribution by replacing the Lorenz number L₀ = 2.45 × 10^?8 W Ω K^?2 by an effective Lorenz number L_eff (<L₀) we have obtained two sets of κ_el, namely those with L_eff = 0.1L₀ and 0.2L₀. Corresponding to these two sets of κ_el, two sets of the phonon thermal conductivity κ_ph are obtained. κ_ph has been analyzed in terms of an extended Bardeen–Rickayzen–Tewordt theory. The main result of this analysis is that the hump structure corresponds to a gap ratio of 3.5, and that large electron-point defect scattering is the main source of drastic reduction of the electronic thermal conductivity from that given by the usual Wiedeman–Franz law.     

Structure sensitivity of propene oxidation over Co-Mn spinels

Single-phase cobalt–manganese spinel oxides (Co_3?nMn_nO₄, CMO) were studied for the catalytic oxidation of propene in a systematic optimization strategy. CMO films were synthesized by pulsed-spray evaporation chemical vapor deposition (PSE–CVD) and characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman and Ultraviolet–Visible (UV–Vis) spectroscopy. The effect of Co/Mn ratio in the mixed oxide systems on their catalytic activity was investigated in a fixed-bed reactor at T = 100–800 °C, with a space velocity of 90,000 mL/g_cat h and a feed of 2% C₃H₆/20% O₂/78% Ar. XRD patterns, FTIR and Raman spectroscopy reveal that a cubic single-phase spinel structure is obtained for n ? 1.23, while a tetragonal spinel structure is observed for n > 1.23. With increasing of the manganese content, the temperature–programmed analysis demonstrates a lower reducibility, a general decrease of the temperature required for the reduced samples to be re-oxidized and increasing thermal stability. The catalytic tests show that the involvement of cobalt–manganese oxides in propene oxidation suppresses the formation of reaction intermediates, favoring the selectivity toward CO₂ at low temperatures. Co_2.35Mn_0.65O₄ exhibits the best catalytic performance, which follows in line with its better reducibility compared with the other compositions in the series of CMO oxides. These results show the great potential of CMO for future industrial application as a low-temperature catalytic system which does not rely on precious metals.     

Magnetic properties of GdCo12B6 compound under high pressures

The effects of hydrostatic pressure up to 10 kbar on Curie temperature T_C, compensation temperature T_COMP and spontaneous magnetization M_S of ferrimagnetic GdCo₁₂B₆ compound have been studied. Two antiferromagnetically coupled sublattices that are carrying magnetization of typically 0.42 μ_B/Co atom and 7 μ_B/Gd cancel out at compensation temperature at about 50 K and magnetic ordering temperature T_C=163±2 K. The volume dependence of intrinsic magnetic properties of the GdCo₁₂B₆ compound has been determined by studying it under hydrostatic pressure. The observed increase of M_S with pressure (dM_S/dp=+0.005 μ_B kbar^?1 at 5 K) is attributed predominantly to the pressure induced decrease of Co magnetic moments. The crucial role of Co in this behavior is confirmed by the change of sign of the pressure slope at temperatures above T_COMP and by the fact that the estimated decrease of m_Co is also quite comparable with pressure induced decrease of M_S in YCo₁₂B₆ (dM_S/dp=?0.007 μ_B kbar^?1). The decrease of m_Co is also responsible for the increase of T_COMP with pressure (dT_COMP/dp=+0.06 K kbar^?1). The decrease of T_C with pressure (dT_C/dp=?0.55 K kbar^?1) is comparable to the decrease observed on RCo₁₂B₆ compounds with non-magnetic R and can be attributed to the volume dependence of Co–Co exchange interactions. The remarkable role of the hybridization as a consequence of small distances between Co and B atoms could be a background of this rather unexpected volume stability of magnetic properties.     

Electrical properties of yttrium doped strontium titanate with A-site deficiency as potential anode materials for solid oxide fuel cells

Yttrium doped strontium titanate with A-site deficiency ((Y_0.08Sr_0.92)_1 ? xTiO_3 ? δ) was synthesized by conventional solid state reaction. The deficiency limit of A-site in (Y_0.08Sr_0.92)_1 ? xTiO_3 ? δ is below 6 mol% in Ar/H₂ (5%) at 1500 °C. The sinterability of (Y_0.08Sr_0.92)_1 ? xTiO_3 ? δ samples decreases slightly with increasing A-site deficiency level (x). The ionic conductivity of (Y_0.08Sr_0.92)_1 ? xTiO_3 ? δ samples increases while the electronic conductivity decreases with increasing A-site deficient amount. The defect chemistry analysis indicates that the introduction of A-site deficiency results in not only the increase of oxygen vacancy concentration but also the decrease of Ti³⁺-ion concentration. The latter plays the main role in the electrical conduction. (Y_0.08Sr_0.92)_1 ? xTiO_3 ? δ shows good thermal-cyclic performance in electrical conductivity and has an excellent chemical compatibility with YSZ electrolyte below 1500 °C.     

Band overlap via chemical pressure control in double perovskite (Sr2−xCax)FeMoO6 (0 ⩽ x ⩽ 2.0) with TMR effect

The chemical pressure control in (Sr_2−xCa_x)FeMoO₆ (0 ⩽ x ⩽ 2.0) with double perovskite structure has been investigated systematically. We have performed first-principles total energy and electronic structure calculations for x = 0 and x = 2.0. The increasing Ca content in (Sr_2−xCa_x)FeMoO₆ samples increases the magnetic moment close to the theoretical value due to reduction of Fe/Mo anti-site disorder. An increasing Ca content results in increasing (Fe²⁺ + Mo⁶⁺)/(Fe³⁺ + Mo⁵⁺) band overlap rather than bandwidth changes. This is explained from simple ionic size arguments and is supported by X-ray absorption near edge structure (XANES) spectra and band structure calculations.     

Characterization of doped La0.7A0.3Cr0.5Mn0.5O3 − δ (A = Ca,Sr, Ba) electrodes for solid oxide fuel cells

Doped lanthanum manganese chromite based perovskite, La_0.7A_0.3Cr_0.5Mn_0.5O_3 ? δ (LACM, A = Ca, Sr, Ba), on yttria-stabilized zirconia (YSZ) electrolyte is investigated as potential electrode materials for solid oxide fuel cells (SOFCs). The electrical conductivity and electrochemical activity of LACM depend on the A-site dopant. The best electrochemical activity is obtained on the La_0.7Ca_0.3Cr_0.5Mn_0.5O_3 ? δ/YSZ (LCCM/YSZ) composite electrodes. The conductivity of LCCM is 29.9 S cm^? 1 at 800 °C in air, and the electrode polarization resistance (R_E) of the LCCM/YSZ composite cathode for the O₂ reduction reaction is 0.5 Ω cm² at 900 °C. The effect of Gd-doped ceria (GDC) impregnation on the LCCM cathode polarization resistances is also studied. GDC impregnation significantly enhances the electrochemical activity of the LCCM cathode. In the case of the 6.02 mg cm^? 2 GDC-impregnated LCCM cathode, R_E is 0.4 Ω cm² at 800 °C, ~ 60 times smaller than 24.4 Ω cm² measured on a LCCM cathode without the GDC impregnation. Finally the electrochemical activities of the doped lanthanum manganese chromites for the H₂ oxidation reaction are also investigated.     

Ion conduction in proton- and related defect (super) ionic conductors: Mechanical,electronic and structure parameters

We find the relationships among optical dielectric constant ε_∞, activation energy E_ac, averaged atomic mass per a formula unit, ∑_jm_j / N, volume V and transition temperature T_c for various type ion conductors with forms of E_ac = α / (ε_∞ ? β), E_ac = A₀ + δ / [(∑_jm_j / N) ? d], E_ac = A_v / V^2/3, and ln(T_c) = g ? hln(∑_jm_j), where α, β, δ, A₀, d, A_v, g and h are constants depending on the kinds of conduction elements. We derive those proportional forms from a simple equation of motion under the assumption of ion hopping assisted by enhanced vibration displacement of host lattice. The enhancement is induced from the large fourth-order term of the host lattice potential originating from the electronic shielding effect of Coulomb force, heavy atomic mass of constituent ion, and volume expansion under the long-range periodicity of crystal structure. This mechanism is ascertained from characteristic phenomena of various kinds of conduction elements. For impurity-type H⁺-ion or defect conductor, the proportional form is shifted from that of superionic conductor because of weakened effect of host lattice vibration mode on H⁺-ion or O-ion defect. Photo-induced spectra of mobile ion in AgCl are understood, and a small quantum effect of H⁺ -ion is suggested.