X-ray diffraction study of Ba0.985Na0.015Ti0.985Nb0.015O3, Ba0.6Na0.4Ti0.6Nb0.4O3 and Ba0.3Na0.7Ti0.3Nb0.7O3 compositions

The Ba_0.985Na_0.015Ti_0.985Nb_0.015O₃, Ba_0.6Na_0.4Ti_0.6Nb_0.4O₃ and Ba_0.3Na_0.7Ti_0.3Nb_0.7O₃ compositions of the (1 − x) BaTiO₃–xNaNbO₃ (BTNNx) system have been studied by X-ray diffraction and by measurements of dielectric properties. The specimens with composition BTNN (x = 0.015, 0.40 and 0.70) have been refined by the JANA program from X-ray powder diffraction data. Ceramic samples with composition (1 − x) BaTiO₃ + xNaNbO₃ (where x = 0.015, 0.40 and 0.70) were prepared by calcinations from appropriate mixture of BaCO₃, TiO₂, Na₂CO₃ and Nb₂O₅. The calcined powder was sintered at temperature range 1200–1400 °C. As the composition x increased from 0.015 (and 0.70), the ferroelectric ceramics (x = 0.015, FE) with tetragonal phase changed to the ferroelectric relaxors (RFE, x = 0.40). RFE ceramics showed a peculiar diffuse phase transition and dielectric relaxation at the low temperature (down to 180 K) due to a frustration between RFE and FE state. These ceramics present the classical ferroelectric character when 0 ≤ x < 0.075 and 0.55 < x ≤ 1 and relaxor character when 0.075 ≤ x ≤ 0.55.     

Magnetic, electrical and plastic properties of Fe76Nb2Si13B9, Fe75Ag1Nb2Si13B9 and Fe75Cu1Nb2Si13B9 amorphous alloys

Influence of 1 h annealing in vacuum on magnetic, electrical and plastic properties of Fe₇₆Nb₂Si₁₃B₉, Fe₇₅Ag₁Nb₂Si₁₃B₉ and Fe₇₅Cu₁Nb₂Si₁₃B₉ melt spun ribbons were carefully investigated. It was shown that in all cases soft magnetic properties can be significantly enhanced by applying 1-h annealing at characteristic temperatures T_op. This optimization annealing causes that permeability increases more than 15-times and magnetic losses (tangent of loss angle) achieves a minimum in relation to the as quenched state. Using structural examinations (X-ray and HRTEM) it was shown that for the Fe₇₅Cu₁Nb₂Si₁₃B₉ alloy the optimized microstructure corresponds to a nanocrystalline αFe(Si) phase whereas in other alloys to a relaxed amorphous phase free of iron nanograins. As a consequence of this fact the Fe₇₆Nb₂Si₁₃B₉ and Fe₇₅Ag₁Nb₂Si₁₃B₉ alloys show higher plasticity in comparison to the nanocrystalline Fe₇₅Cu₁Nb₂Si₁₃B₉ alloy. Temperatures of the first stage of crystallization, and related diffusion parameters were determined using measurements of resistivity versus temperature with different heating rates.     

Synthesis and electrochemical properties of LiNi0.4Mn1.5Cr0.1O4 and Li4Ti5O12

Spinel compound LiNi_0.4Mn_1.5Cr_0.1O₄ (LNMCO) and Li₄Ti₅O₁₂ (LTO) were synthesized by the sol-gel method and the solid-state method, respectively. The particle sizes of the products LiNi_0.4Mn_1.5Cr_0.1O₄ and Li₄Ti₅O₁₂ were 0.5 to 2 um and 0.5 to 0.8 um, respectively. All samples exhibited excellent electrochemical properties. A LiNi_0.4Mn_1.5Cr_0.1O₄/Li₄Ti₅O₁₂ (LNMCO/LTO) cell was fabricated and was demonstrated to exhibit good electrochemical properties at the high current rate of 1 C. When the specific capacity was determined based on the mass of the LNMCO cathode, the LNMCO/LTO cell delivered 125 mAh g⁻¹ at 1 C and 77 mAh g⁻¹ at 5 C. The capacity retentions after 30 cycles were 94.4 % and 83.1 %, respectively.     

Zn_2SnO_4和Zn_2Sn_(0.8)Ti_(0.2)O_4/C的制备和电化学性能(英文)

以SnCl4.5H2O、TiCl4、ZnCl2和N2H4.H2O为原料,采用水热法制备Zn2Sn0.8Ti0.2O4纳米粉体。在此基础上,以葡萄糖和水热合成的Zn2Sn0.8Ti0.2O4为原料,以碳热还原法制备Zn2Sn0.8Ti0.2O4/C复合材料。利用XRD、XPS、TEM、恒电流充放电等方法分别研究Zn2SnO4和Zn2Sn0.8Ti0.2O4/C复合材料的结构、形貌和电化学性能。同时用非原位XRD、XPS和SEM分析Zn2Sn0.8Ti0.2O4/C复合材料电极在充放电过程中的结构和形貌变化。合成的纯Zn2SnO4的首次放电容量为1670.8mA.h/g,循环40次后放电容量迅速衰减为342.7mA.h/g。而Zn2Sn0.8Ti0.2O4/C复合材料的首次放电容量为1530.0mA.h/g,循环100次后容量还保持为479.1mA.h/g,与纯Zn2SnO4、Zn2Sn0.8Ti0.2O4和Zn2SnO4/C相比,电化学性能有较大的提高。     

New Zr6CoAs2-type compounds: Y6CoBi2, Ho6CoBi2 and Tm6CoBi2

Results of a powder X-ray diffraction investigation of new ternary compounds are reported. The compounds Y₆CoBi₂ [a=0.8312(1) nm, c=0.4144(1) nm], Ho₆CoBi₂ [a=0.8246(2) nm, c=0.4095(1) nm], and Tm₆CoBi₂ [a=0.8155(2) nm, c=0.4066(1) nm] crystallize in the hexagonal Zr₆CoAs₂-type structure (space group P6b2m No. 189). The Zr₆CoAs₂-type structure is a superstructure of the Fe₂P-type structure.     

Thermodynamics and phase diagrams in the binary systems Na2O-SiO2, Na2O-GeO2, Na2O-B2O3, Li2O-B2O3, CaO-B2O3, SrO-B2O3, and BaO-B2O3

We applied our model to the enthalpy of mixing data of the binary systems Na₂O-SiO₂, Na₂O-GeO₂, Na₂O-B₂O₃, Li₂O-B₂O₃, CaO-B₂O₃, SrO-B₂O₃, and BaO-B₂O₃. The most stable composition in the liquid, that is where the enthalpy of mixing is most negative, is with a metal-oxygen ratio of 4 to 3, for monovalent metals (Na and Li) and 3 to 4 for divalent metals (Ba and Ca) in liquid silicates or borates. The same applies to the CaO-SiO₂, CaO-Al₂O₃, PbO-B₂O₃, PbO-SiO₂, ZnO-B₂O₃, and ZnO-SiO₂ systems. The oxygen to metal ratio, its constant value in various types of systems, reflects and describes the structure of the liquid. Using the analyzed enthalpies of mixing data and the available phase diagrams, we calculated the enthalpies of formation of the various binary compounds. The results are in excellent agreement with data in the literature that were obtained from direct solid-solid calorimetry.     

Sinterability and dielectric properties of Ba0.55Sr0.4Ca0.05TiO3-CaTiSiO5-Mg2TiO4 composite ceramics

The composite ceramics of Ba_0.55Sr_0.4Ca_0.05TiO₃-CaTiSiO₅-Mg₂TiO₄ (BSCT-CTS-MT) were prepared by the conventional solid-state route. The sintering performance, phase structures, morphologies, and dielectric properties of the composite ceramics were investigated. The BSCT-CTS-MT ceramics were sintered at 1100 °C and possessed dense microstructure. The dielectric constant was tailored from 1196 to 141 as the amount of Mg₂TiO₄ increased from 0 to 50 wt%. The dielectric constant and dielectric loss of 40 wt% Ba_0.55Sr_0.4Ca_0.05TiO₃-10 wt% CaTiSiO₅-50 wt% Mg₂TiO₄ was 141 and 0.0020, respectively, and the tunability was 8.64% under a DC electric field of 8.0 kV/cm. The Curie peaks were broadened and depressed after the addition of CaTiSiO₅. The optimistic dielectric properties made it a promising candidate for the application of tunable capacitors and phase shifters.     

Syntheses and characterization of two new selenides Ba5Al2Se8 and Ba5Ga2Se8

Two new barium selenides Ba₅Al₂Se₈ and Ba₅Ga₂Se₈ have been synthesized by solid-state reactions. The structures of Ba₅Ga₂Se₈ and Ba₅Al₂Se₈ were determined by single-crystal X-ray diffraction method and the Rietveld method, respectively. The two isostructural compounds crystallize in space group Cmca of the orthorhombic system with isolated MSe₄ (M = Al, Ga) tetrahedra separated by Ba atoms. The optical band gap of 2.51(2) eV for Ba₅Ga₂Se₈ was deduced from the diffuse reflectance spectrum. Band structure calculation indicates that Ba₅Ga₂Se₈ is a direct-gap semiconductor. The valence band maximum is dominated by Se 4p orbitals, while the Ba 5d orbitals have the largest contribution to bottom of the conduction band.     

Electronic structures and Eu photoluminescence behaviors in Y2Si2O7 and La2Si2O7

The electronic structures and linear optical properties of Y₂Si₂O₇ (YSO) and La₂Si₂O₇ (LSO) are calculated by LDA method based on the theory of DFT. Both YSO and LSO are direct-gap materials with the direct band gap of 5.89 and 6.06 eV, respectively. The calculated total and partial density of states indicate that in both YSO and LSO the valence band (VB) is mainly constructed from O 2p and the conduction band (CB) is mostly formed from Y 4d or La 5d. Both the calculated VB and CB of YSO exhibit relatively wider dispersion than that of LSO. In addition, the CB of YSO presents more electronic states. Meanwhile, the VB of LSO shows narrower energy distribution with higher electronic states density. The theoretical absorption of YSO shows larger bandwidth and higher intensity than that of LSO. The results are compared with the experimental host excitations and impurity photoluminescence in Eu³⁺-doped YSO and LSO.     

Phase transition and piezoelectric properties of K0.48Na0.52NbO3-LiTa0.5Nb0.5O3-NaNbO3 lead-free ceramics

Plate-like NaNbO₃ (NN) particles were used as the raw material to fabricate (1 − x)[0.93 K_0.48Na_0.52Nb O₃-0.07Li(Ta_0.5Nb_0.5)O₃]-xNaNbO₃ lead-free piezoelectric ceramics using a conventional ceramic process. The effects of NN on the crystal structure and piezoelectric properties of the ceramics were investigated. The results of X-ray diffraction suggest that the perovskite phase coexists with the K₃Li₂Nb₅O₁₅ phase, and the tilting of the oxygen octahedron is probably responsible for the evolution of the tungsten-bronze-typed K₃Li₂Nb₅O₁₅ phase. The Curie temperature (T_C) is shifted to lower temperature with increasing NN content. (1 − x)[0.93 K_0.48Na_0.52NbO₃-0.07Li(Ta_0.5Nb_0.5)O₃]-xNaNbO₃ ceramics show obvious dielectric relaxor characteristics for x > 0.03, and the relaxor behavior of ceramics is strengthened by increasing NN content. Both the electromechanical coupling factor (k_p) and the piezoelectric constant (d₃₃) decrease with increasing amounts of NN. 0.01-0.03 mol of plate-like NaNbO₃ in 0.93 K_0.48Na_0.52NbO₃-0.07Li(Ta_0.5Nb_0.5)O₃ gives the optimum content for preparing textured ceramics by the RTGG method.     

Synthesis and properties of CaCd2Sb2 and EuCd2Sb2

High density polycrystalline CaCd₂Sb₂ and EuCd₂Sb₂ intermetallics are synthesized by Spark Plasma Sintering and their thermoelectric properties are investigated. X-ray diffraction measurements reveal both materials have a structure in space group, containing a small amount of CdSb as a second phase. Thermoelectric measurements indicate both are p-type conductive materials. The figure of merit value of CaCd₂Sb₂ is 0.04 at 600 K and that of EuCd₂Sb₂ is 0.60 at 617 K. Theoretical calculations show that CaCd₂Sb₂ is a degenerate semiconductor with a band gap of 0.63 eV, while EuCd₂Sb₂ is metallic with DOS of 13.02 electrons/eV. For deeper understanding of the better thermoelectric properties of EuCd₂Sb₂, its low temperature magnetic, transport and heat capacity properties are investigated. Its Nèel temperature is 7.22 K, convinced by heat capacity anomaly at 7.13 K. Hall effect convinced that it is a p-type conductive material. It has high Hall coefficient, high carrier concentration and high carrier mobility of +1.426 cm³/C, 4.38 × 10¹⁸/cm³ and 182.40 cm²/Vs, respectively. They are all in the magnitude of good thermoelectric materials. The Eu 4f level around Fermi energy and antiferromagnetic order may count for the better thermoelectric properties of EuCd₂Sb₂ than that of CaCd₂Sb₂.     

Crystal structures of the Ag4HgGe2S7 and Ag4CdGe2S7 compounds

The crystal structures of the Ag₄HgGe₂S₇ and Ag₄CdGe₂S₇ compounds were investigated using X-ray powder diffraction. These compounds crystallize in the monoclinic Cc space group with the lattice parameters a=1.74546(8), b=0.68093(2), c=1.05342(3) nm, β=93.398(3)° for Ag₄HgGe₂S₇ and a=1.74364(8), b=0.68334(3), c=1.05350(4) nm, β=93.589(3)° for Ag₄CdGe₂S₇. Atomic parameters were refined in the isotropic approximation (R_I=0.0761 and R_I=0.0727, respectively).     

Thermodynamic properties and phase diagrams of the binary systems B2O3–Ga2O3, B2O3–Al2O3 and B2O3–In2O3

We calculated the binary phase diagrams B₂O₃–Ga₂O₃, B₂O₃–In₂O₃ and B₂O₃–Al₂O₃, and the Gibbs energy of formation of the binary compounds, using experimental liquidus data. The B₂O₃–Ga₂O₃ system is of industrial importance, because liquid B₂O₃, in which Ga₂O₃ is not very soluble, is used to protect GaAs during growth of single crystals of GaAs. During recovery of noble metals B₂O₃ is added to slags containing Al₂O₃ to lower the melting point and the viscosity. The B₂O₃–In₂O₃ system is of much less importance to industry. In all three systems we have a liquid miscibility gap, and also solid binary compounds, none of which melt congruently. The miscibility gaps are not surprising, because even in the B₂O₃–Bi₂O₃ system where four congruently melting compounds are present, a liquid miscibility gap exists close to B₂O₃.     

Critical evaluation and optimization of the thermodynamic properties and phase diagrams of the CrO-Cr2O3-SiO2 and CrO-Cr2O3-SiO2-Al2O3 systems

Available thermodynamic and phase diagram data have been critically assessed for all phases in the CrO-Cr₂O₃-SiO₂ and CrO-Cr₂O₃-SiO₂-Al₂O₃ systems from 298 K to above the liquidus temperatures and for oxygen partial pressures ranging from equilibrium with metallic Cr to equilibrium with air. All reliable data have been simultaneously optimized to obtain one set of model equations for the Gibbs energy of the liquid slag and all solid phases as functions of composition and temperature. The modified quasi-chemical model was used for the slag. The models permit phase equilibria to be calculated for regions of composition, temperature, and oxygen potential where data are not available.     

Hydrogen diffusion in Sm2Fe17 and Sm2Fe14Ga3 compounds

The short-range and long-range diffusion of hydrogen in Sm₂Fe₁₇ and Sm₂Fe₁₄Ga₃ compounds was investigated by magnetic after-effect (MAE) and thermal desorption spectroscopy (TDS). The substitution of iron by gallium greatly reduces the uptake and diffusion of hydrogen in these compounds. After charging in a high-purity H₂ atmosphere of 0.13 MPa at 570 K for 1 h the H content was determined to be Sm₂Fe₁₇H₅ and Sm₂Fe₁₄Ga₃H_1.3. Sm₂Fe₁₇ shows a hydrogen-induced MAE relaxation maximum at 190 K with a mean activation enthalpy of Q=(0.48±0.02) eV and a pre-exponential factor of

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s. In the case of the Ga-substituted compound this maximum is replaced by a double maximum at 200 and 250 K corresponding to activation ethalpies of (0.52±0.02) eV and (0.64±0.02) eV with a pre-exponential factor of

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s. These relaxation maxima are attributed to the short-range diffusion of hydrogen, since they are absent in the degassed compounds. TDS measurements of Sm₂Fe₁₇H₅ and Sm₂Fe₁₄Ga₃H_1.3 were performed in the temperature range from 291 to 515 K yielding activation enthalpies of Q=(0.59±0.02) eV and Q=(0.75±0.03) eV, respectively. The results are explained by different interstitial sites for hydrogen and the impact on the hydrogen disproportionation process is discussed.     

Transport and ultrasonic properties in La0.67Sr0.33Mn0.87Fe0.13O3 perovskite

The longitudinal and transverse ultrasonic sound velocity and attenuation, as well as electric resistance have been carefully measured in single-phase polycrystalline giant magnetoresistance perovskite La_0.67Sr_0.3Mn_0.87Fe_0.13O₃ at a frequency of 10 MHz, from 20 to 300 K. A big electric resistance peak was observed at 85 K (T_C). At the temperature above T_C, the resistivity can be fitted well by Mott’s law ρ=exp (T₀/T)^1/4 and both the longitudinal and transverse sound velocities show a lattice softening, which was accompanied by an attenuation peak. This simultaneous occurrence of electron and lattice softening implies electron–phonon coupling, known to exist for the octahedrally coordinated d⁴ ion, originating in the Jahn–Teller distortion. Below 55 K, pronounced sound-velocity softening for both longitudinal and transverse waves was observed; this may correspond to the formation of a spin-glass state.     

Synthesis and superconductivity of Y2Ba1Cu1O5 and Pt added Nd1Ba2Cu3O7?y bulk superconductor

Nd₁Ba₂Cu₃O_7−y (Ndl23) bulk superconductor with T_c>77K was prepared by a melt process under 1 atm pure oxygen and reduced oxygen partial pressures. The addition of Y₂Ba₁Cu₁O₅ (Y211) particles suppressed the generation of microcracks during oxygenation in resulting in the dispersion of Y211 fine particles. The addition of a small amount of Pt resulted in the reduction of the size of Y211 particles. The oxygen partial pressure (Po₂) of 0.01 atm was the optimum atmosphere for the synthesis of Ndl23 in which Y211 particles were dispersed finely and homogeneously. The addition of Y211 and Pt enhanced the flux pinning force in a melt-processed Ndl23 specimen because the presence of Y atoms in a melt phase suppressed, to some extent, the substitution of Nd on Ba site in an Ndl23 crystal prepared by a melt process.     

Structure and surface energy of low-index surfaces of stoichiometric α-Al2O3 and α-Cr2O3

There is considerable interest in the low-temperature growth of the stable α-phase of alumina, α-Al₂O₃, due to its superior chemical and mechanical properties compared to the other forms of alumina. Conventional methods for achieving this phase (such as chemical vapour deposition) utilise high temperatures which limits the range of substrate materials that can be beneficially coated. Recently, it has been reported that α-Al₂O₃ can be grown on α-Cr₂O₃ templates at a much lower substrate temperature (around 400 °C) by the RF magnetron sputtering deposition technique. Previous studies showed that both α-Al₂O₃ and α-Cr₂O₃ (0001) free surfaces relax considerably in comparison with the corresponding bulk structures. From the experimental point of view, a knowledge of the surface structure and stability of both materials will be of great help in determining the facet of α-Cr₂O₃ which best facilitates the growth of α-Al₂O₃. Here we present the results of first-principles Hartree-Fock calculations on the surface energy of the low-index planes of both α-alumina and α-chromia. The suitability of possible α-Cr₂O₃ facets as templates for the growth of α-Al₂O₃ is discusssed.     

Microstructure studies of Zr65Cu17.5Al7.5Ni10 and Zr65Cu15Al10Ni10 glass forming alloys: Phase morphologies and undercooled melt solidification

Zr₆₅Cu_17.5Al_7.5Ni₁₀ (at.%) and Zr₆₅Cu₁₅Al₁₀Ni₁₀ (at.%) glass forming alloy microstructures have been investigated by means of optical and electron microscopies. They are composed of a fine eutectic matrix with eutectic dendrites (EDs) that have peculiar morphologies. Al and Cu concentrations, in these alloys, favour primary dendrites and determine the ED morphologies and compositions. Their locations within the microstructures suggest a two-step solidification process of the two undercooled melts. The identified crystalline phases indicate the occurrence of solid state phase transformations in agreement with the structural defects observed in the grains. The crystalline phases can be classified into Zr-rich, Cu-rich, Ni-rich and Al-rich compounds resulting from competing diffusion between Cu, Ni, and Al in the melts.