Spin and orbital magnetic moments and spin anisotropy energies of light rare earth atoms embedded in graphene: A first-principles study

The geometries, electronic structures, spin magnetic moments (SMMs), orbital magnetic moments (OMMs) and spin anisotropy energies (SAEs) of light rare earth atoms (La, Ce, Pr, Nd, Pm, Sm, Eu, and Gd) embedded in graphene were studied by using first-principles calculations based on Density Functional Theory (DFT). The spin-orbital coupling effect was taken into account and GGA+U method was adopted to describe the strongly localized and correlated 4f electrons. There is a significant deformation of the graphene plane after doping and optimization. The deformation of Gd doped graphene is the largest, while Eu the smallest. The results show that the valence is +3 for La, Ce, Pr, Nd, Pm, Sm and Gd, and +2 for Eu. Except Eu and Gd, there are obvious OMMs. When the spin is in the Z direction, the OMMs are −0.941 μ_B, −1.663 μ_B, −3.239 μ_B, −3.276 μ_B and −3.337 μ_B for Ce, Pr, Nd, Pm and Sm, respectively, and point the opposite direction of SMMs. All the doped systems except Gd show considerable SAEs. For Ce, Pr, Nd, Pm, Sm, and Eu, the SAEs are −0.928 meV, 20.941 meV, −8.848 meV, 7.855 meV, 75.070 meV and 0.810 meV, respectively. When the spin orientation is different, different orbital angular moments lead to apparent charge density difference of the 4f atoms, which can also explain the origin of SAEs.     

First principle study of structural,phase stabilization and oxygen-ion diffusion properties of β-La2 − xLxMo2O9 (L = Gd,Sm, Nd and Bi) and β-La2Mo2 − yMyO9 (M = Cr,W)

We have performed a first principle study of structural and phase stabilization of β-La_2 ? xL_xMo₂O₉ (L = Gd, Sm, Nd and Bi) and β-La₂Mo_2 ? yM_yO₉ (M = Cr, W). The substitutional-site properties were discussed in terms of the empirical parameter, bond valence sums (BVS), which characterizes the interactions between atoms and its nearest-neighbor atoms and correlates well with the stability of the structure. We found that Gd, Sm and Nd atoms prefer the crystallographic sites with largest BVS values. The nonlinear dependence of cell parameter on W content in W-doped systems results from the nonlinear change in Mo/W–O bond length with W content. The decrease of cohesive energy and the deviation of BVS values from the expected values upon the Gd, Sm, Nd and W-doped concentration help us understand the experimentally observed stabilization of the β phase to lower temperatures in these doped system. The O ion diffusion properties in W-doped systems have been studied using the nudged elastic band method and the dimer method. We found that, W-doping leads to the obvious increase in the energy barriers of O ion concerted diffusion. In addition, there is a remarkable decrease in the difference of energy barriers between two diffusion channels involving O(1) ion, which sheds light on only one relaxation peak in the mechanical relaxation measurement in W-doped system, compared to undoped system.     

Flux pinning and superconducting properties of melt-textured NEG-123 superconductor with TiO2 addition

We studied the effect of TiO₂ doping on flux pinning and superconducting properties of a melt-grown (Nd_0.33Eu_0.33Gd_0.33) Ba₂Cu₃O_y + 35 mol% Gd₂BaCuO₅ (70 nm in size) composite (NEG-123) processed in Ar–1% O₂ atmosphere. As indicated by similar, sharp superconducting transitions, the small quantities of TiO₂ used in our experiments did not deteriorate superconducting properties of the NEG material. Transmission electron microscopy (TEM) analysis found 20–50 nm Ti-based particles in the NEG-123 matrix. However, we have not observed the clouds of <10 nm sized particles in the NEG-123 matrix, as in the case of recently reported NEG-123 composites doped by Mo and Nb nanoparticles. Nevertheless, quite a good J_c–B performance in the 0.1 mol% Ti-doped sample, namely 550 kA/cm² at the self-field and at the secondary peak field (4.5 T) was achieved at 65 K, while 320 kA/cm² was obtained at zero-field at 77 K, and 50 kA/cm² at 90.2 K. The pinning effectiveness decreased with increasing Ti content above 0.2 mol%. The analysis of the pinning force showed that higher concentration of Ti (>0.2 mol%) increased the amount of normal pins (δl pinning), indicated by the Fp(h) peak shift from h = 0.42–0.36. The maximum pinning effect in a broad field range could be achieved by optimizing Ti content and adding sub-micron Gd-211 particles.     

Synthesis of ammonia at atmospheric pressure with Ce0.8M0.2O2−δ (M = La,Y, Gd,Sm) and their proton conduction at intermediate temperature

Ionic conduction in fluorite-type structure oxide ceramics Ce_0.8M_0.2O_2−δ (M = La, Y, Gd, Sm) at temperature 400–800 °C was systematically studied under wet hydrogen/dry nitrogen atmosphere. On the sintered complex oxides as solid electrolyte, ammonia was synthesized from nitrogen and hydrogen at atmospheric pressure in the solid states proton conducting cell reactor by electrochemical methods, which directly evidenced the protonic conduction in those oxides at intermediate temperature. The rate of evolution of ammonia in Ce_0.8M_0.2O_2−δ (M = La, Y, Gd, Sm) is up to 7.2 × 10^− 9, 7.5 × 10^− 9, 7.7 × 10^− 9, 8.2 × 10^− 9 mol s^− 1 cm^− 2, respectively.     

Photocatalytic properties of lanthanide tungstates Ln2W2O9 (Ln=La,Pr, Nd,Sm, and Gd)

Lanthanide tungstates, Ln₂W₂O₉ (Ln=La, Pr, Nd, Sm, and Gd), were prepared via the polymerized complex method at 1273 K for 2 h, and their photocatalytic activities for hydrogen and oxygen evolution were investigated. Pt-loaded Gd₂W₂O₉ exhibited activity for H₂ evolution from an aqueous methanol solution under light irradiation (λ>300 nm). The remaining Ln₂W₂O₉ were inactive for H₂ evolution due to the influence of the Ln elements and their crystal structures. All Ln₂W₂O₉ were inactive for O₂ evolution from an aqueous AgNO₃ solution due to the lack of O₂ evolution sites on the surface.     

Conductivity and oxygen permeability of a novel oxide Pr2Ni0.8 − x Cu0.2FexO4 and its application to partial oxidation of CH4

Iron-doped Pr₂Ni_0.8Cu_0.2O₄ was studied as a new mixed electronic and oxide-ionic conductor for use as an oxygen-permeating membrane. An X-ray diffraction analysis suggested that a single phase K₂NiF₄-type structure was obtained in the composition range from x = 0 to 0.05 in Pr₂Ni_0.8 − xCu_0.2Fe_xO₄. It is considered that the doped Fe is partially substituted at the Ni position in Pr₂NiO₄. The prepared Pr₂NiO₄-based oxide exhibited a dominant hole conduction in the P_O2 range from 1 to 10^− 21 atm. The electrical conductivity of Pr₂Ni_0.8−xCu_0.2Fe_xO₄ is as high as 10² S cm^− 1 in the temperature range of 873–1223 K and it gradually decreased with the increasing amount of Fe substituted for Ni. The oxygen permeation rate was significantly enhanced by the Fe doping and it was found that the highest oxygen permeation rate (60 μmol min^− 1 cm^− 2) from air to He was achieved for x = 0.05 in Pr₂Ni_0.8 − xCu_0.2Fe_xO₄. Since the chemical stability of the Pr₂NiO₄-based oxide is high, Pr₂Ni_0.75Cu_0.2Fe_0.05O₄ can be used as the oxygen-separating membrane for the partial oxidation of CH₄. It was observed that the oxygen permeation rate was significantly improved by changing from He to CH₄ and the observed permeation rate reached a value of 225 μmol min^− 1 cm^− 2 at 1273 K for the CH₄ partial oxidation.     

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.     

Synthesis under ambient pressure and tri-axial magnetic orientation in REBa2Cu4O8 (RE = Y,Sm, Eu,Gd, Dy,Ho, Er)

We report the rare-earth (RE)-dependent magnetization axes of REBa₂Cu₄O₈, which was synthesized by a flux method under ambient pressure, using powder samples tri-axially oriented in a modulated rotating magnetic field of 10 T. By optimizing the growth temperature and cooling rate, RE124 crystals were successfully grown for RE = Y, Sm, Eu, Gd, Dy, Ho, and Er. From the X-ray diffraction measurement, the magnetically oriented directions were largely dependent on the type of RE ions of RE124. However, the tri-axial magnetic anisotropies of RE124 could be qualitatively understood in terms of the magnitude relation between the single-ion magnetic anisotropy of RE³⁺ ions and the magnetic anisotropy generated by the CuO₂ plane and Cu–O chain. For the practical use of this magneto-scientific process, the control of magnetization axes and tri-axial magnetic anisotropies through crystallochemical control is indispensable.     

Effect of the composition on the superconducting properties of IBAD-MgO SmBCO coated conductors with superconducting film directly deposited on epi-MgO layer

Sm_1+xBa_2?xCu_3+yO_7?δ (SmBCO) films were directly deposited on the epi-MgO/IBAD-MgO/Y₂O₃/Al₂O₃/Hastelloy template by co-evaporation using the evaporation using drum in dual chambers (EDDC) system without the buffer layer in order to investigate the effect of the composition ratios on superconducting property, microstructure and texture of SmBCO film. The films with gradient composition ratios of Sm:Ba:Cu were deposited using a shield with an opening which was placed between the substrate and the boats. The highest I_c of 52 A (corresponding to J_c = 1.6 MA/cm² and a thickness of 800 nm) was observed at 77 K in self field at a composition x = 0.01–0.05 and y = ?0.23 to ?0.46. When the composition ratio is outside this range, the I_c value rapidly decreased. The superconducting critical current was highly dependent on the composition ratio. As the composition ratio is farther away from that of the highest I_c, the SmBCO (1 0 3) peak intensity increased and the amount of a-axis oriented parts increased. A dense microstructure with round-shape grains was observed in the region showing the highest I_c. The optimum composition ratio can be found by analyzing films deposited with variable deposition rates of each depositing element.     

Anisotropic properties of rare-earth dibismites

We report measurements of magnetic, thermal and transport properties of single crystals of rare-earth dibismites RBi₂ (R=La–Nd, Sm), grown via self-flux method. All compounds are good metals, and those with magnetic ions order antiferromagnetically at low temperatures. Ce, Pr and Sm members of the series show single magnetic transition whereas NdBi₂ most likely exhibits two magnetic transitions. Significant magnetic anisotropy and a series of metamagnetic transitions in fields up to 55 kG are found in PrBi₂. Ordering temperatures range from 3 K to just above 16 K and they scale well with the de Gennes factor.     

Characterization of Sr2.7Ln0.3Fe1.4Co0.6O7 (Ln = La,Nd, Sm,Gd) intergrowth oxides as cathodes for solid oxide fuel cells

Perovskite-related intergrowth oxides Sr_2.7Ln_0.3Fe_1.4Co_0.6O_7 ? δ (Ln = La, Nd, Sm, and Gd) have been investigated as cathode materials for solid oxide fuel cells (SOFC). With decreasing size of the Ln³⁺ ions, the unit cell volume, oxygen content, thermal expansion coefficient (TEC), and total electrical conductivity decrease from Ln = La to Gd. The decreasing unit cell volume and oxygen content is attributed to the decreasing size of Ln³⁺ ions from Ln = La to Gd and a consequent preference for lower coordination numbers. While the decrease in the ionicity of the Ln–O bonds from Ln = La to Gd causes a decrease in the TEC, the increasing amount of oxygen vacancies leads to a decrease in electrical conductivity arising from a thermally activated semiconducting behavior. The cathode polarization conductance (R_p^? 1) measured using the ac-impedance spectroscopy and the catalytic activity for the oxygen reduction reaction in SOFC decrease from Ln = La to Gd partly due to the decrease in electrical conductivity.     

Anionic conductors Ln2/3[Bi12O14](MoO4)5 with Ln = La,Nd, Gd,Ho, Yb. Synthesis–spark plasma sintering–structure–electric properties

A new series of columnar phases Ln_2/3□_1/3[Bi₁₂O₁₄](MoO₄)₅ (□ vacancy) with Ln = La, Nd, Gd, Ho and Yb have been synthesized and structurally characterized. They feature the same formula as the molybdate phase Bi_2/3□_1/3[Bi₁₂O₁₄](MoO₄)₅ and crystallize in the monoclinic system, space group P2/c. These phases are isostructural with the prototype structure Bi[Bi₁₂O₁₄](MoO₄)₄(VO₄). Pellets of this rare-earth series obtained by spark plasma sintering and measured by impedance spectroscopy show a good anionic conductivity with a parabolic evolution whose maximum is raised by the Gd species at a determined value σ = 6.6 × 10^− 3 S cm^− 1 at 980 K.     

Electrical and magnetic properties of Nd3+-substituted GdBaSrCu3O7 superconductor

The compounds GdBaSr_1?xNd_xCu₃O_y (where x = 0, 0.1, 0.2 and 0.3) were prepared using the solid-state reaction technique. The single phase and microrange crystalline nature of the compounds were studied by powder X-ray diffraction and SEM analysis. The electrical resistivity and magnetic properties of the compounds were studied as a function of temperature. A decrease in superconducting transition temperature for an increase in Nd substitution was observed. A decreasing trend of diamagnetic strength and an increasing trend of magnetic moment of the samples with Nd substitution were observed from magnetization measurement. The optimal sintering temperature of the compounds was found to be 950 °C.     

Critical temperature and orthorhombicity of Y2Ba4Cu7O15−δ

Superconducting properties have been systematically studied for oxygen content controlled Y₂Ba₄O₇O_15?δ (Y247) polycrystalline bulks sintered under various conditions, 940–975 °C and P(O₂) = 4–16 atm. T_c of oxygen annealed Y247 samples at 400 °C varied largely depending on the sintering conditions from ～60 to ～90 K. Samples sintered under higher P(O₂) exhibited lower T_c, whereas they were confirmed to be of Y247 single phase by powder X-ray analyses. Strong relationship between orthorhombicity defined as 1000(b ? a)/(a + b) and T_c was found in Y247 as in the cases of REBa₂Cu₃O_7?δ with light rare-earth elements, such as La, Nd, Sm, Eu and Gd. In addition, T_c‘s of samples with nominal compositions of Y_2+xBa_4?xO₇O_15?δ systematically decreased with an increase of x. These results indicated that partial substitution of Y for Ba occurred in the Y247 compounds possibly due to high occupancy ratio of oxygen at the Cu–O_1-δ chain during the sintering process.     

Ionic and electronic conductivities,stability and thermal expansion of La10 − x(Si,Al)6O26 ± δ solid electrolytes

Apatite-type La_10 − xSi_6 − yAl_yO_{27 − 3x/2 − y/2} (x = 0–0.33; y = 0.5–1.5) exhibit predominant oxygen ionic conductivity in a wide range of oxygen partial pressures. The conductivity of silicates containing 26.50–26.75 oxygen atoms per formula unit is comparable to that of gadolinia-doped ceria at 770–870 K. The average thermal expansion coefficients are (8.7–10.8) × 10^− 6 K^− 1 at 373–1273 K. At temperatures above 1100 K, silicon oxide volatilization from the surface layers of apatite ceramics and a moderate degradation of the ionic transport with time are observed under reducing conditions, thus limiting the operation temperature of Si-containing solid electrolytes.     

Diffusion of Fe,Co, Nd,and Dy in R2(Fe1−xCox)14B where R=Nd or Dy

Transition metal and rare earth diffusion coefficients at 1323 K in Dy_2−yNd_y(Fe_1−xCo_x)₁₄B were determined by field emission energy dispersive spectroscopy compositional analysis of diffusion couple specimens. Various arrangements of component materials and temperatures were examined in order to understand the mechanisms affecting diffusion of the components and to predict the stability of functionally graded microstructures consisting of a dysprosium-rich (Dy_2−yNd_y(Fe_1−xCo_x)₁₄B) outer layer and a neodymium-rich (Nd₂(Fe_1−xCo_x)₁₄B) interior. Estimates of the mutual interdiffusion coefficients of Dy, Nd, Fe, and Co in this system were obtained from the preparation of arc melted and annealed polycrystalline specimens, assuming that the diffusion coefficients were independent of concentration (Grube solution). Fifteen diffusion couples were prepared and heat treated at 1323 K for various times in order to provide data for calculation of the diffusion coefficients. The results indicate that the diffusion coefficients of Fe and Co (D_Fe=3.28×10⁻¹⁰ cm²/s and D_Co=7.63×10⁻¹⁰ cm²/s) were significantly higher at 1323 K in this system than those for Dy and Nd (D_Nd=2.3×10⁻¹² cm²/s and D_Dy=2.9×10⁻¹² cm²/s).     

Diode-pumped doubly Q-switched Nd:Lu0.33Y0.37Gd0.3VO4 laser with an electro-optic modulator and a single-walled carbon nanotube saturable absorber

By simultaneously using an electro-optic (EO) modulator and a single-walled carbon nanotube saturable absorber (SWCNT-SA) in the cavity, a diode-pumped doubly Q-switched Nd:Lu_0.33Y_0.37Gd_0.3VO₄ (Nd:LuYGdVO₄) laser is demonstrated. At the incident pump power 11.43 W and f=2 kHz, the minimum pulse width 17.6 ns and the maximum pulse peak power 19,886 W can be obtained. The experimental results show that this doubly Q-switched Nd:LuYGdVO₄ laser can generate shorter pulse width and higher peak power compared to the singly Q-switched Nd:LuYGdVO₄ laser with only EO or SWCNT-SA.     

High performance batch production of LREBa2Cu3Oy using novel thin film Nd-123 seed

A batch production for fabrication of LREBa₂Cu₃O_y (LRE: Sm, Gd, NEG) “LRE-123” pellets are developed in air and Ar-1% O₂ using a novel thin film Nd-123 seeds grown on MgO crystals. The SEM and XRD results conformed that the quality and orientation of the seed crystals are excellent. On the other hand, new seeds can withstand temperatures >1100 °C, as a result, the cold seeding process was applied even to grow Sm-123 material in Air. The trapped field observed in the best 45 mm single-grain puck of Gd-123 was in the range of 1.35 T and 0.35 T at 77.3 K and 87.3 K, respectively. The average trapped field at 77.3 K in the 24 mm diameter NEG-123 samples batch lies between 0.9 and 1 T. The maximum trapped field of 1.2 T was recorded at the sample surface. Further, the maximum trapped field of 0.23 T at 77 K was recorded in a sample with 16 mm diameter of Sm-123 with 3 mol% BaO₂ addition. As a result we made more then 130 single grain pucks within a couple of months. Taking advantage of the single grain batch processed material, we constructed self-made chilled levitation disk, which was used on the open day of railway technical research Institute. More then 150 children stood on the levitation disk and revel the experience of levitation. The present results prove that a high-performance good-quality class of LREBa₂Cu₃O_y material can be made by using a novel thin film Nd-123 seeds.     

Magnetic tri-axial orientation in (Y1−xErx)2Ba4Cu7O15−y superconductors

We report the tri-axial grain-orientation effects under a modulated rotation magnetic field for (Y_1?xEr_x)₂Ba₄Cu₇O_y [(Y, Er)247]. The magnetic easy axis at room temperature was drastically changed around x ～ 0.1; however, the Er-doping levels for the conversion of magnetic easy axes from the c-axis to the ab-direction and from the a- to b-axes were quite different. Tri-axial single-ion magnetic anisotropy of Er³⁺ was roughly 10 times greater than tri-axial magnetic anisotropy generated by both the superconducting CuO₂ plane and the blocking Cu–O chain layer. An appropriate choice of rare-earth (RE) ions in RE-based cuprate superconductors enables the reduction of the required magnetic field for the production of bulks and thick films based on the magnetic orientation technique.