Synthesis and characterization of layered perovskite-type oxides LnSr2MnFeO7 (Ln = La, Nd, Gd, and Dy)

Layered perovskite-type oxides LnSr₂MnFeO₇, with Ln?=?La, Nd, Gd, and Dy, have been synthesized by conventional solid-state reaction method. Rietveld refinement shows that all the phases were found to crystallize with tetragonal unit cell in the space group I4/mmm. The unit cell parameters a and c decrease monotonically with decreasing effective ionic radius of the lanthanide ion. The phases show insulating behavior with no anomalous features in the entire temperature range of 150–350 K and the electrical conduction occurs by Mott’s variable range hopping mechanism. The magnetic studies suggest that the phases are antiferromagnetic and the Weiss constant (θ) increases with decreasing ionic radius of lanthanide ion.     

Electronic structure and magnetic properties of the quaternary perovskites LnMn3V4O12 (Ln = La,Nd and Gd)

ABSTRACT

Structural, electronic and magnetic properties of the quaternary perovskites LnMn₃V₄O₁₂ (Ln?=?La, Nd and Gd) are calculated using generalised gradient approximation along with Hubbard potential (GGA?+?U) within the frame work of density functional theory (DFT). It is observed that the calculated lattice constants are decreases as going from La→Gd due to the lanthanide’s contraction and found consistent with the experiments. The electron charge densities show that the Ln—O bond is ionic whereas the bond between Mn/V—O is covalent. The metallic nature of these compounds is confirmed by the overall band structures. The magnetic susceptibility curves show that all these compounds are anti-ferromagnetic and the exchange interactions play vital role in the magnetic properties of these perovskites. Furthermore, the electrical resistivities calculated by BoltzTrap code also verify the metallic nature of all these compounds. This study also confirms that in these compounds Mn 3d-states electrons are responsible for the magnetic behaviour whereas V 3d-states electrons for the electronic behaviour.     

Preparation,characterization, and conductivity of YLnTiZrO7 (Ln = La,Nd, Sm,and Eu)

The pyrochlore oxide of composition YLnTiZrO₇ (Ln?=?La, Nd, Sm, and Eu) was prepared by sol–gel method. All the samples were characterized by powder X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), diffused reflectance spectroscopy, and impedance spectroscopy. The powder XRD and Raman studies reveal that these samples were crystallized in cubic lattice with pyrochlore structure. The Rietveld analysis of the samples was carried out to obtain the unit cell parameters and reliability factors. The broad Raman bands observed for all the samples are due to cation/anion disorder in the lattice and nanosize. The XPS analysis of the samples shows the characteristic peaks belonging to Y³⁺, Ln³⁺ (Ln?=?La, Nd, Sm, and Eu), Ti⁴⁺, and Zr⁴⁺. Electrical conductivity of YLaTiZrO₇ (YLTZ) and YEuTiZrO₇ (YETZ) samples was calculated from the impedance as a function of frequency and temperature. These samples have shown conductivity of the order of 10^?5 scm^?1 at 500 °C.     

Synthesis and characterization of layered perovskite oxides La1 + x Sr2 − x MnCrO7 (x = 0.2, 0.5)

Intergrowth perovskite type complex oxides of composition La_1.2Sr_1.8MnCrO₇ and La_1.5Sr_1.5MnCrO₇ have been synthesized by ceramic method. Rietveld profile analysis shows that the phases crystallize with tetragonal unit cell in the space group I4/mmm. Both the phases behave as insulators in the high temperature region and the linearity of log ρ versus T ^?1/4 plot in the temperature range 150–300 K shows that the electronic conduction occurs by a 3D variable range hopping mechanism. The phases show insulator-metal transition at low temperature which could be due to the mixed valence state of Mn³⁺/Mn⁴⁺ by double exchange mechanism. The ferromagnetic interactions observed for the samples arises from double exchange interaction between Mn ³⁺ and Mn⁴⁺ and Cr³⁺ and Mn³⁺ ions.     

Synthesis and characterization of layered perovskite oxides (La, Sr) n + 1(Ti, Cr) n O3n + 1 (n = 1, 2)

We report here the synthesis of layered perovskite oxides of the composition La_0.5Sr_1.5Ti_0.5Cr_0.5O₄ and LaSr₂TiCrO₇ by conventional solid-state reaction method. Results of XRD analysis show that the phases crystallize with tetragonal unit cell in the space group I4/mmm. Both phases behave as insulators and the linearity of log ρ versus T ^?1/4 plot in the temperature range 150–350 K suggests that the electronic conduction occurs by Mott’s variable range hopping mechanism. The antiferromagnetic interactions observed for the samples arise from Cr³⁺–O–Cr³⁺ superexchange interaction.     

Structural and magnetic properties of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O4-δ spinels： A first-principles study

<正>Structural and magnetic properties of LiNi_0.5Mn_1.5O₄ and LiNi_0.5Mn_1.5O_4-δ are investigated using densityfunctional theory calculations.Results indicate that nonstoichiometric LiNi_0.5Mn_1.5O_4-δ and stoichiometric LiNi_0.5Mn_1.5O₄ exhibit two different structures,i.e.,the face-centred cubic（Fd-3m） and primitive,or simple,cubic （P4₃32） space groups,respectively.It is found that the magnetic ground state of LiNi_0.5Mn_1.5O₄（P4₃32 and Fd-3m） is a ferrimagnetic state in which the Ni and Mn sublattices are ferromagnetically ordered along the[110]direction whereas they are antiferromagnetic with respect to each other.We demonstrate that it is the presence of an O-vacancy in LiNi_0.5Mn_1.5O_4-δ with the Fd-3m space group that results in its superior electronic conductivity compared with LiNi_0.5Mn_1.5O₄ with the P4₃32 space group.     

Nonlinear refractive index of RECOB (RE = Gd and La) crystals

The Z-scan technique is employed to obtain the nonlinear refractive index (n ₂) of the Ca₄REO(BO₃)₃ (RECOB, where RE = Gd and La) single crystals using 30 fs laser pulses centered at 780 nm for the two orthogonal orientations determined by the optical axes (X and Z) relative to the direction of propagation of the laser beam (k//Y// crystallographic b-axis). The large values of n ₂ indicate that both GdCOB and LaCOB are potential hosts for Yb:RECOB lasers operating in the Kerr-lens mode locking (KLM) regime.     

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).     

Structural analysis of nonlinearities of Ca4ReO(BO3)3 (Re = La, Nd, Sm, Gd, Er, Y)

4 ReO(BO₃)₃ (CReOB, Re=La, Nd, Sm, Gd, Er, and Y) have been theoretically predicted. The d₁₁ tensor coefficient of CReOB is predicted to be -11 d₃₆(KDP), which is the largest d_ij tensor that has been found in borate crystals. From the structural characteristic of CReOB, we find the isolated BO₃ ^3- clusters play a dominant role in contributions to the total nonlinearity, and the largest d₁₁ tensor of CReOB-type crystals is also ascribed to these BO₃ ^3- clusters. We also find the NLO property of this family does not change dramatically for different rare-earth elements. The details of the calculation of CGdOB only are presented. Received: 3 March 1998/Accepted: 22 June 1998     

155Gd and 161Dy Mössbauer study of R1+εFe4B4 alloys (R = Gd,Dy)

Tetragonal R_1+εFe₄B₄ alloys with R = Gd and Dy have been investigated by Mössbauer spectroscopy, using the ¹⁵⁵Gd and ¹⁶¹Dy resonances, respectively. The Gd quadrupolar interaction e²qQ = 12.65(5)mm/s is the largest observed to date in metallic compounds of Gd. For Dy this interaction is e²qQ = 74(2) mm/s, a value rather small for a Dy compound. Both results imply a strong lattice contribution to the electric field gradient. A crystal-field term A⁰₂ = -2450(50) K/a²_o is inferred. Our data are consistent with a point-charge calculation, provided charges of opposite signs are assumed for Fe and B atoms. Hyperfine parameters show some dispersion, reflecting the quasi incommensurate nature of the R and Fe+B sublattices in the R_1+εFe₄B₄ structure.     

Synthesis, structural and conductivity characterization of alginic acid–Fe3O4 nanocomposite

Alginic acid–Fe₃O₄ nanocomposite is synthesized by the precipitation of Fe₃O₄ in the presence of alginic acid (AA). Structural, surface, morphological, thermal and electrical transport properties of the nanocomposite were performed by XRD, FT-IR, TEM-SEM, TGA and conductivity measurements respectively. FT-IR analysis revealed that Fe₃O₄ NPs are strongly capped with AA and TGA analysis showed that nanocomposite have 80% of Fe₃O₄ content. TEM analysis of Fe₃O₄ NPs show an average particle size of 9.5 nm, and upon nanocomposite formation with AA these particles are observed to form aggregates of ~150 nm. The frequency-dependency of the AC conductivity show electrode polarization effect. Analysis of electrical modulus and dielectric permittivity functions suggest that ionic and polymer segmental motions are strongly coupled. DC electrical conductivity is strongly temperature dependent, and is classified into three regions over a limited temperature range of up to 100 °C.     

Superconductivity in the (La,Ln)2−x(Ba,Sr)xCuO4 system

Abstract

Several (La_1?y Ln_y)_2?x Sr_x(Ba_x)CuO₄ systems show superconductivity in the 15–40K region. Marginal metallicity of the non-superconducting phases and the role of the nominal Cu³+ /Cu ²⁺ ratio are discussed. Possible importance of hole-hole coupling (forming resonating O-O bonds) is indicated.     

Synthesis and characterization of Sr3Al2O6：Eu2＋, Dy3＋ phosphors prepared by sol-gel-combustion processing

A type of red luminescent Sr3Al2O6:Eu2+, Dy3+ phosphor powder is synthesised by sol-gel-combustion processing, with metal nitrates used as the source of metal ions and citric acid as a chelating agent of metal ions. By tracing the formation process of the sol-gel, it is found that it is necessary to reduce the amount of NO3- by dropping ethanol into the solution for forming a stable and homogeneous sol-gel. Thermogravimetric and Differential Scanning Calorimeter Analysis, x-ray diffractionmeter, scanning electron microscopy and photoluminescence spectroscopy are used to investigate the luminescent properties of the as-synthesised Sr3Al2O6:Eu2+, Dy3+. The results reveal that the Sr3Al2O6 crystallises completely when the combustion ash is sintered at 1250 C. The excitation and the emission spectra indicate that the excitation broadband lies mainly in a visible range and the phosphors emit a strong light at 618 nm under the excitation of 472 nm. The afterglow of (Sr0.94Eu0.03Dy0.03)3Al2O6 phosphors sintered at 1250 ℃ lasts for over 1000 s when the excited source is cut off.     

Magnetic and superconducting properties of Tl(Sr1.5R0.5)CaCu2O7−δ compounds

DC magnetic susceptibility and resistivity measurements have been performed on 14 Tl(Sr_1.5R_0.5)CaCu₂O_7−δ compounds with R=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. All samples except the Ce-doped compound show superconducting phase transition temperature around 80–90 K. The rare-earth atoms behave as local moments above the superconducting transition temperature of these compounds. Mechanism of filling of holes in the CuO₂ plane can be employed to interpret the suppression or enhancement of superconductivity by rare-earth ions in Tl(Sr_1.5R_0.5)CaCu₂O_7−δ.     

Ln7O6(BO3)(PO4)2:Eu(Ln=La,Gd,Y)的VUV-UV激发和辐射发光

本文报道了Ln 7O6(BO3)(PO4)2:Eu(Ln=La,Gd,Y)在VUV-UV区的激发光谱及Eu3+在可见区的发射光谱.其激发光谱包括基质在真空紫外区的激发带和激活剂离子在紫外区的Eu3+-O2-电荷迁移带,随La3+,Gd3+,Y3+离子半径逐渐减小,Eu3+-O2-电荷迁移带的重心位置逐渐向高能量方向移动,Gd7O6(BO3)(PO4)2:Eu和Y7O6(BO3)(PO4)2:Eu在真空紫外区的吸收与Eu3+-O2-电荷迁移带位于紫外区的吸收的比值要高于在La7O6(BO3)(PO4)2:Eu中的这个比值.激发能可被基质吸收,传递给激活剂离子,得到Eu3+的红光发射.在Gd7O6(BO3)(PO4)2:Eu中,5D0→7F1的发射强度较强,在Y7O6(BO3)(PO4)2:Eu中,5D0→7F2和5D0→7F3的跃迁较强.     

Synthesis and characterization of magnetic diphase ZnFe(2)O(4) /γ-Fe(2)O(3) electrospun fibers

Magnetic nanofibers of ZnFe₂O₄/γ-Fe₂O₃ composite were synthesized by electrospinning from a sol-gel solution containing a molar ratio (Fe/Zn) of 3. The effects of the calcination temperature on phase composition, particle size and magnetic properties have been investigated. Zinc ferrite fibers were obtained by calcinating the electrospun fibers in air from 300 to 800 °C and characterized by thermogravimetric analyses, Fourier transformed infrared spectroscopy, X-ray photoemission spectroscopy, X-ray diffraction, vibration sample magnetometry and magnetic force microscopy. The resulting fibers, with diameters ranging from 90 to 150 nm, were ferrimagnetic with high saturation magnetization as compared to bulk. An increase in the calcination temperature resulted in an increase in particle size and saturation magnetization. The observed increase in saturation magnetization was most likely due to the formation and growth of ZnFe₂O₄/γ-Fe₂O₃ diphase crystals. The highest saturation magnetization (45 emu/g) was obtained for fibers calcined at 800 °C.     

Synthesis and characterization of (Pr0.75Sr0.25)1 − xCr0.5Mn0.5O3 − δ as anode for SOFCs

The complex perovskite (Pr_0.75Sr_0.25)_1 − xCr_0.5Mn_0.5O_3 − δ (PSCM) has been prepared and studied as possible anode material for high-temperature solid oxide fuel cells (SOFCs). PSCM exhibits GdFeO₃-type structure and is both physically and chemically compatible with the conventional YSZ electrolyte. The reduction of PSCM resulted in structural change from orthorhombic Pbnm to cubic Pm-3m. Selected area electron diffraction (SAED) analysis on the reduced phases indicated the presence of a √2 × √2 × 2 superlattice. The total conductivity values of ∼ 75% dense Pr_0.75Sr_0.25Cr_0.5Mn_0.5O_3 − δ at 900 °C in air and 5% H₂/Ar are 9.6 and 0.14 S cm^− 1 respectively. The conductivity of PSCM drops with decreasing Po₂ and is a p-type conductor at all studied Po₂. The average TEC of Pr_0.75Sr_0.25Cr_0.5Mn_0.5O_3 − δ is 9.3 × 10^− 6 K^− 1, in the temperature range of 100–900 °C and is close to that of YSZ electrolyte. The anode polarization resistance of PSCM in wet 5%H₂ is 1.31 Ω cm² at 910 °C and in wet CH₄ at 930 °C; the polarization resistance is 1.29 Ω cm². PSCM was unstable at 900 °C in unhumidified hydrogen. Cell performance measurements carried out using graded PSCM and La_0.8Sr_0.2MnO₃ as anode and cathode respectively yielded a maximum power density of 0.18 W cm^− 2 in wet 5%H₂/Ar at 910 °C and the corresponding current density was 0.44 A cm^− 2 at 0.4 V. The activation energy for the electrochemical cell operating in wet (3% H₂O) 5%H₂/Ar fuel is 85 kJ mol^− 1.     

Eu3+:Ln2O2S(Ln=Lu,Y,Gd,La)晶体的角重迭参数计算

本文使用角重迭模型(AOM),针对Eu3+:Ln2O2S(Ln=Lu,Y,Gd,La)晶体系列,在考虑硫和氧两种不同格位的情况下,对f电子的全部四个角重迭参数eσ、eπ、eδ和eφ进行了计算,得到了一些基本规律,即中心离子与配位体之间的共价性大小次序为Lu～Y>Gd>La,并且Ln-O键的共价性要强于Ln-S键.