Superconductivity in CeO1−xFxFeAs with upper critical field of 94 T

We have successfully synthesized Ce based oxypnictide superconductors with fluorine doping (CeO_1?xF_xFeAs) by a two step solid state reaction method. Detailed XRD and EDX confirm the crystal structure and chemical compositions. We observe that an extremely high H_c2(0) of 94 T can be achieved in the x = 0.1 composition. This increase in H_c2(0) is accompanied by a decrease in transition temperature (38.4 K in x = 0.1 composition) from 42.5 K for the x = 0.2 phase. The in-plane Ginzburg–Landau coherence length is estimated to be ～27 Å at x = 0.2 suggesting a moderate anisotropy in this class of superconductors. The Seebeck coefficient confirms the majority carrier to be electrons and strong dominance of electron–electron correlations in this multiband superconductor.     

Exploring the role of samarium in the modification of rhodium catalysts through surface science approach

In this paper we review the preparation and reaction properties of ordered SmRh surface alloys and SmO_x/Rh(1 0 0) model catalyst which have been systematically investigated by low energy electron diffraction (LEED), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS) and temperature desorption spectroscopy (TDS). The growth of Sm on Rh(1 0 0) at room temperature follows the Stranski-Krastanov mode. Thermal treatment of the Sm films on Rh(1 0 0) leads to the formation of ordered SmRh surface alloys. An “inverse” SmO_x/Rh(1 0 0) model catalyst is produced under controlled oxidation of the SmRh surface alloy. CO adsorption on the SmRh alloy and SmO_x/Rh(1 0 0) surfaces gives rise to five TDS characteristic features originating from different adsorption sites. Both the site blocking of SmO_x and the electron transfer between SmO_x and Rh substrate significantly affect the CO adsorption. Acetate decomposition on both Rh(1 0 0) and the SmO_x/Rh(1 0 0) surfaces are found to undergo two competitive pathways that yields either (i) CO(a) and O(a) or (ii) CO₂(g) and H₂(g) at high temperature. The reactive desorption of acetic acid on SmO_x/Rh(1 0 0) is dramatically different from that on Rh(1 0 0). A rapid decomposition of acetic acid to produce CO(g) and CO₂(g) can be observed only on SmO_x/Rh(1 0 0), where CO(g) becomes the predominant product at 225 K, indicating a strong promotional effect of SmO_x in the selective decomposition of acetate. Finally, we briefly describe the future outlook of research on rare earth oxide/metal model catalysts.     

Suppression of superconductivity in Zn-doped SmFeAsO0.8F0.2 system

A series of SmFe_1?xZn_xAsO_0.8F_0.2 samples with x = 0, 0.05, 0.1, 0.2 and 0.4 have been successfully synthesized using a solid state method. The lattice parameters are found to increase with increasing Zn doping content. The superconductivity has been definitely suppressed by Zn doping at Fe site with the transition temperature T_c being reduced from 52.5 K to 23.3 K for the sample of x = 0.05, and to 18.2 K for the sample of x = 0.1. For the samples with x > 0.1, the superconducting transition vanishes, and, at the meantime, the spin-density-wave anomaly recovers at 140 K. The metal to semiconductor transition is also observed in the SmFe_1?xZn_xAsO_0.8F_0.2 system. The behavior of SmFe_1?xZn_xAsO_0.8F_0.2 is very different from that of R_EFeAsO (R_E = rare earth metal), which reveals a very strong electron correlation in SmFe_1?xZn_xAsO_0.8F_0.2.     

Oxygen-rich Ti1−xO2 pillar growth at a gold nanoparticle–TiO2 contact by O2 exposure

In-situ gas-injection transmission electron microscopy revealed that a pillar grew at the edge of the interface of a gold nanoparticle and a TiO₂ substrate during exposure to O₂ gas at 100 Pa. The pillar was found to have a titanium-deficient chemical composition of Ti_1 ? xO₂ (x > 0) by electron energy loss spectroscopy (EELS). The spectra showed a chemical shift of oxygen and titanium ions to have ionic states of Ti³⁺ and O^y? (y < 3/2). The formation of the Ti_1 ? xO₂ at the contact edge of gold–Ti_1 ? xO₂ interface is discussed from the perspective of an O₂ affinity, which plays an important role in CO oxidation process of supported gold particle.     

Band structure of a cylindrical GaAs/AlxGa1 − xAs superwire

We investigate the existence of a band structure in GaAs/Al_xGa_1 − xsuperlattices with cylindrical symmetry, namely GaAs/Al_xGa_1 − xAs cylindrical superwires. These systems consists of a large number of concentric GaAs and Al_xGa_1 − xAs alternate cylindrical shells around a central GaAs cylindrical wire. Despite the radial configuration (that breaks the translational symmetry) and the electron confinement in the central three-dimensional well, a band structure can emerge depending on the number and thickness of the cylindrical shells.     

Tuneability of Sm(1 − x)CexFeO3 ± λ perovskites: Thermal stability and electrical conductivity

Trimetallic perovskite oxides, Sm_(1 ? x)Ce_xFeO_3 ± λ (x = 0–0.05), were prepared by thermal decomposition of amorphous citrate precursors followed by calcinations. The material properties of the substituted perovskites were characterized by X-ray diffraction (XRD), X-ray florescence spectroscopy (XRF), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The doped materials exhibited a single perovskite phase in air up to 1350 °C and have specific surface areas in the range of 2.696–8.665 m²/g. In reducing atmosphere (5%v/vH₂/N₂), the unsubstituted perovskite (x = 0) decomposed into two phases while the ceria stabilized materials (x = 0.01, x = 0.03, x = 0.05) remained in a single phase as revealed by XRD analysis. Their conductivities were measured by the four point probe method in air and in dilute hydrogen (5%v/vH₂/N₂) separately. The ceria substituted materials show increased stability versus reduction and phase separation for a wide temperature range (up to 1000 °C). Although undoped SmFeO₃ has higher conductivity under oxidizing conditions than ceria doped SmFeO₃ due its p-type nature, the situation is reversed under reducing conditions. The ceria substituted perovskites (Sm_(1 ? x)Ce_xFeO_3 ± λ, x = 0–0.05) showed higher conductivity in reducing than in oxidizing conditions, suggesting that ceria doping at the A-site has changed the SmFeO₃ from p-type to n-type semi-conducting behavior.     

Electrical conductivity of (Ba0.3Sr0.2La0.5)(In1−xFex)O3−δ

We have synthesized the perovskite oxides of the (Ba_0.3Sr_0.2La_0.5)(In_1−xFe_x)O_3−δ system and measured the total electrical conductivity as a function of temperature and oxygen partial pressure. It was found that the single-phase composition region extended from x = 0.0 to x = 1.0, and that the Fe valence increased from 3.06 to 3.50 in that region. The electrical conductivity was semiconducting from x = 0.0 to x = 0.40 and metallic from x = 0.50 to x = 1.0. The total electrical conductivity at 800 °C also increased with the Fe content and achieved a maximum value of 140 (S/cm) at x = 1.0. From the dependence of the electrical conductivity on the oxygen partial pressure, we conclude that above x = 0.50, the majority carriers are holes. The estimated hole conductivity increased exponentially with the amount of Fe⁴⁺ cation present. The oxide ion conductivity was dependent on the oxygen vacancy content.     

Sonochemical preparation of SbS1−xSexI nanowires

A sonochemical method for direct preparation of nanowires of SbS_1?xSe_xI solid solution has been established. The SbS_1?xSe_xI gel was synthesized using elemental Sb, S, Se and I in the presence of ethanol under ultrasonic irradiation (35 kHz, 2 W/cm²) at 50 °C for 2 h. The product was characterized by using techniques such as powder X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, energy dispersive X-ray analysis, selected area electron diffraction, and optical diffuse reflection spectroscopy. The SEM and HRTEM investigations exhibit that the as-prepared samples are made up of large quantity nanowires with lateral dimensions of about 10–50 nm and lengths reaching up to several micrometers and single-crystalline in nature. The increase of molar composition of Se affects linear decrease of the indirect forbidden optical energy gap as well as the distance between (121) planes of the SbS_1?xSe_xI nanowires.     

Oxygen nonstoichiometry and defect equilibrium in La2 − xSrxNiO4 + δ

Nonstoichiometric variation of oxygen content in La_2 ? xSr_xNiO_4 + δ (x = 0, 0.1, 0.2, 0.3, 0.4) and decomposition P(O₂) were determined by means of high temperature gravimetry and coulometric titration. The measurements were carried out in the temperature range between 873 and 1173 K and the P(O₂) range between 10^? 20 and 1 bar. La_2 ? xSr_xNiO_4 + δ showed the oxygen excess and the oxygen deficient compositions depending on P(O₂), temperature, and the Sr content. The value of partial molar enthalpy of oxygen approaches zero as δ increases in the oxygen excess region, which indicate that the interstitial oxygen formation reaction is suppressed as δ increase. The relationship between δ and logP(O₂) were analyzed by two types of defect equilibrium models. One is a localized electron model, and the other is a delocalized electron model. Both models can well explain the oxygen nonstoichiometry of La_2 ? xSr_xNiO_4 + δ with a regular solution approximation.     

An oxygen permeable membrane La0.8Sr0.2(Ga0.8Mg0.2)1 − xCrxO3 − δ for a reduced atmosphere

Mixed electron hole and oxide ion conducting perovskite-type oxides, La_0.8Sr_0.2(Ga_0.8Mg_0.2)_1 ? xCr_xO_3 ? δ (0 ≤ x ≤ 1.0)_, were prepared by solid state reaction. The phase stability and the oxygen permeation properties of the oxides were examined as a function of the content of Cr. La_0.8Sr_0.2(Ga_0.8Mg_0.2)_1 ? xCr_xO_3 ? δ has a perovskite related tetragonal phase with x = 0.1 to 0.8. The total electrical conductivity of La_0.8Sr_0.2(Ga_0.8Mg_0.2)_1 ? xCr_xO_3 ? δ increases with increasing x. The oxygen permeation flux across the La_0.8Sr_0.2(Ga_0.8Mg_0.2)_1 ? xCr_xO_3 ? δ membranes at higher temperatures increases with x up to x = 04. The maximum oxygen permeation flux of 1.6 × 10^? 7 mol^? 1 cm^? 2 at 1100 °C in a oxygen activity gradient of air/10^? 2 Pa is observed in La_0.8Sr_0.2(Ga_0.8Mg_0.2)_0.6Cr_0.4O_3 ? δ. This perovskite-type oxide is stable under an oxygen partial pressure of 7 × 10^? 10 Pa at 1000 °C.     

Ternary effects on the optical interface phonons in onion-like GaN/AlxGa1 − xN quantum dots

The interface optical phonons and its ternary effects in onion-like quantum dots are studied by using dielectric continuum model and the modified random-element isodisplacement model. The dispersion relations, the electron–phonon interactions and ternary effects on the interface optical phonons are calculated in the GaN/Al_xGa_1 − xN onion-like quantum dots. The results show that aluminium concentration has important influence on the interface optical phonons and electron–phonon interactions in GaN/Al_xGa_1 − xN onion-like quantum dots. The frequencies of interface optical phonons and electron–phonon coupling strengths change linearly with increase of aluminium concentration in high frequency range, and do not change linearly with increasing aluminium concentration in low frequency range.     

Synthesis and electrical properties of nanocrystalline Ca1 − xEuxMnO3 ± δ (0.1 ≤ x ≤ 0.4) powders prepared at low temperature using citrate gel method

Nanopowders of Ca_1 − xEu_xMnO₃ (0.1 ≤ x ≤ 0.4) manganites were synthesized as a single phase using the auto gel-combustion method. The citrate method shows to be simple and appropriate to obtain single phases avoiding segregation or contamination. The Ca_1 − xEu_xMnO₃ system has been synthesized at 800 °C during 18 h, against the conventional method of mixing oxides used to obtain these materials at higher temperatures of synthesis. The formation reaction was monitored by X-ray diffraction (XRD) analysis and an infrared absorption technique (FTIR). The polycrystalline powders are characterised by nanometric particle size, ∼ 48 nm as determined from X-ray line broadening analysis using the Scherrer equation. Morphological analysis of the powders, using the scanning electron microscope (SEM), revealed that all phases are homogeneous and the europium-substituted samples exhibit a significant decrease in the grain size when compared with the undoped samples. The structure refinement by using the Rietveld method indicates that the partial calcium substitution by europium (for x ≥ 0.3) modifies the orthorhombic structure of the CaMnO₃ perovskite towards a monoclinic phase. All manganites show two active IR vibrational modes around 400 and 600 cm^− 1. The high temperature dependence of electrical resistivity (between 25 and 600 °C) allows us to conclude that all the samples exhibit a semiconductor behaviour and the europium causes a decrease in the electrical resistivity by more than one order of magnitude. The results can be well attributed to the Mn⁴⁺/Mn³⁺ ratio.     

Electronic property and structure of double-doping Y1−2xPrxCaxBa2Cu3O7−δ with 0 ⩽ x ⩽ 0.14

Equal amount Pr and Ca double-doping Y_1?2xPr_xCa_xBa₂Cu₃O_7?δ with 0 ? x ? 0.14 have been investigated by X-ray diffraction, resistivity, and X-ray photoemission spectroscopy (XPS). The deviation of the linearly decreasing of T_c vs. x curve was observed when x < 0.10. The resistivity and the temperature coefficient of resistivity also exhibit abnormal behaviors around x = 0.10. It is revealed that the conductivity behavior of Y_1?2xPr_xCa_xBa₂Cu₃O_7?δ with low Pr content (x < 0.10) is different from that of the relative high Pr content (x > 0.10), which suggests a change of Pr valence with the Pr content. XPS measurement shows that the relative amount of Pr³⁺ and Pr⁴⁺ is closely related to the total Pr content x. The valence of Pr is close to +3 when x < 0.10 and increases towards +4 when x > 0.10, which implies a different mechanism for depression of superconductivity of Pr content x < 0.10 from that of Pr content x > 0.10 in Pr doping Y-123.     

Radiation stability of M1 − xPrxF2 + x (M = Ca,Sr, Ba) crystals

The radiation stability of the mixed crystals M_1 ? xR_xF_2 + x (M = Ca, Sr, Ba) depends on types of the alkaline-earth and rare-earth ions. Different to Eu- and Ce-containing systems, M_1 ? xPr_xF_2 + x solid solutions have a low radiation resistance, which may be associated with hole trapping on praseodymium ion according to the reaction Pr³⁺ → Pr⁴⁺ which is typical for praseodymium. The coloration efficiency of M_1 ? xPr_xF_2 + x crystals grows in the row Ca → Sr → Ba, which is explained satisfactorily within the model of rare-earth clusters, the structure of which is determined by the ratio of the base alkaline-earth cation to the praseodymium ion radii.     

The interfacial effect on ionic conduction of AgI–anatase TiO2 composites

AgI–anatase TiO₂ nanoparticle composites, (x)AgI–(1 ? x)anatase, with different porosities were fabricated over a wide range of 0–1 of AgI content. The electrical conductivity was measured at room temperature as function of AgI content (x) and porosity (p). The conductivity varies considerably with both x and p. In the vicinity of x = 0.4 and p = 0.31, the conductivity attains a maximum (2.5 × 10^? 3 S/cm). The conductivity is enhanced by three orders of magnitude in comparison with that of pristine AgI. The mechanism of the observed conductivity enhancement is discussed in the light of the scanning electron microscope images and X-ray diffraction patterns of the different (x)AgI–(1 ? x)anatase composites.     

Dielectric and piezoelectric properties of Li-substituted lead-free (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–BaTiO3 ceramics

Lead-free 0.79(Bi_0.5Na_0.5)TiO₃–0.14[Bi_0.5(K_0.5−xLi_x)]TiO₃–0.07BaTiO₃ (BNBK79 + xLi, x = 0.0, 0.1, 0.2, 0.25, 0.3, and 0.4) ceramics were prepared by conventional solid state reaction process. The crystalline structures and surface morphologies are investigated by X-ray diffraction method and scanning electron microscopy. Dielectric and piezoelectric properties were measured. With increasing of lithium substitution, the Curie temperatures of BNBK79 + xLi ceramics increase, but the maximum value of the dielectric constant decreases. And a relatively large remnant polarization of 17.6 μC/cm² and 157 pC/N of d₃₃ has been obtained when x = 0.3.     

Synthesis and electrical transport properties of Lu2+xTi2−xO7−x/2 oxide-ion conductors

The Lu_2+xTi_2−xO_7−x/2 (x = 0; 0.052; 0.096; 0.286; 0.44; 0.63; 33.3–49 mol% Lu₂O₃) nanoceramics with partly disordered pyrochlore-type structure are prepared by sintering freeze-dried powders obtained by a co-precipitation technique with 1600 °C annealing. Similar to pyrochlore-like compositions in the zirconate system, some of the new titanates are good oxide-ion conductors in air. The new solid-state electrolytes have oxide-ion conductivity in the interval of 1.0 × 10^− 3 – 2.5 × 10^− 3 S/cm at 740 °C in air. This value of conductivity is comparable with that of ZrO₂/Y₂O₃ ceramics. The conductivity of Lu_2+xTi_2−xO_7−x/2 depends on the chemical composition. The highest ionic conductivity is exhibited by nearly stoichiometric Lu_2+xTi_2−xO_7−x/2 (x = 0.096; 35.5 mol% Lu₂O₃) material containing ∼ 4.8 at.% Lu_Ti anti-site defects.     

Synthesis and characterization of La4Ni3−xCoxO10±δ (0.0 ≤ x ≤ 3.0, Δx = 0.2) for solid oxide fuel cell cathodes

The cobalt-doped lanthanum–nickel oxide system, La₄Ni_(3−x)Co_xO_10±δ (0.0 ≤ x ≤ 3.0, Δx = 0.2), was investigated as possible cathode materials for intermediate-temperature solid-oxide fuel cells. X-ray diffraction shows the presence of two structural phases in the series belonging to Bmab for 0.0 ≤ x ≤ 0.2, 0.8 ≤ x ≤ 2.0 and 2.6 ≤ x ≤ 3.0 and Fmmm for 0.4 ≤ x ≤ 0.6 and 2.2 ≤ x ≤ 2.4. All compositions are oxygen-deficient (δ < 0). Electrical conductivity measurements show a systematic decrease in the conductivity as cobalt content increases from x = 0.0 to 2.0, and reverses for x > 2.0. AC impedance measurements of the x = 0.4 composition in symmetrical cells with LSGM as an electrolyte show improved electrode performance over the parent nickelate La₄Ni₃O_9.78. Long-term thermal stability studies show the x = 0.4 composition to be more stable than the x = 3.0 phase after annealing at 1173 K in air for 1 week making this material a viable candidate for cathodes in solid oxide fuel cells.     

A single-crystal study of the electrochemically Li-ion intercalated spinel-type Li4Ti5O12

Single crystals of Li_4 + xTi₅O₁₂ were prepared by means of electrochemical Li-ion intercalation technique using parent Li₄Ti₅O₁₂ single crystals. The obtained Li_4 + xTi₅O₁₂ (x = 1.35) crystallizes in the cubic spinel-related type structure, space group Fd3?m, and lattice parameters of a = 8.346(2) Å and V = 581.3(5) Å³ and Z = 8. The Li-ion intercalated sites were successfully determined to be both the 8a and 16c sites by using the difference Fourier synthesis map. The structure was determined by single-crystal X-ray structure analysis and refined to the conventional value of R = 3.7% for 132 independent observed reflections. The chemical composition has been determined to be Li_5.35Ti₅O₁₂ from the result of site-population refinements. In addition, theoretical electron density distributions and total energy were calculated for three postulated compounds of “Li_4.5Ti_4.5O₁₂” and “Li_4.5 + xTi_4.5O₁₂” with x = 1.5 and 3.0.     

Application of pseudopotential theory to magnetic semiconductor heterostructures

The results of empirical pseudopotential calculations for the semiconductor compound Cd_1 − xMn_xTe are presented. The effective electron and hole masses obtained from the pseudopotential calculations are then employed in an envelope function approximation, using two different effective mass Hamiltonians to evaluate the transition energies of the excitonic ground state in CdTe– Cd_1 − xMn_xTe quantum wells of variable width. It is shown that in non-magnetic systems it is not possible to utilize exciton energies alone to either distinguish between different model Hamiltonians or to quantify the interface roughness. However, it is shown that the latter can be quantified in magnetic systems via the resulting Zeeman effect.