M2Y8(SiO4)6O2: Tb (M = Ca, Sr) phosphors: Sol-gel synthesis from N-2-aminoethylic-3-aminopropyldiethoxysilane and different photoluminescence behavior

M₂Y₈(SiO₄)₆O₂: Tb³⁺ (M = Ca, Sr) phosphors have been synthesized with a new silicon source silane crosslinking reagent (N-2-aminoethylic-3-aminopropyldiethoxysilane [NH₂(CH₂)₂NH(CH₂)₃SiCH₃(OCH₃)₂], abbreviated as AEAPMMS) through the sol-gel process, both of which present the characteristic emission ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) of Tb³⁺ ions. It is interesting to be found that the high energy level blue emission (⁵D₃ → ⁷F_J (J = 6, 5, 4, 3) transition) still can be found in the emission spectrum of Ca₂Y₈(SiO₄)₆O₂: Tb³⁺ while it disappears in the emission spectrum of Sr₂Y₈(SiO₄)₆O₂: Tb³⁺ for the cross-relaxation-induced quenching.     

Template synthesis of magnetic one-dimensional nanostructured spinel MFe2O4 (M = Ni, Mg, Co)

The iron nitrate and suitable salt of nickel, cobalt, or magnesium with molar ratio of 2:1 were introduced into the pores of mesoporous silica SBA-15 via two-solvent method. The thermal decomposition of the precursors and the formation of one-dimensional nanostructured spinel ferrite in SBA-15 pores were monitored by XRD, TG-DTA, N₂ adsorption-desorption, and TEM. The pure spinel MFe₂O₄ nanowires obtained through complete removal of the silica template with aqueous NaOH solution were confirmed by TEM and HRTEM. The spinel MFe₂O₄ nanowires showed increasing normal configuration and exhibited superparamagnetism in comparison with the bulk ones.     

Transformation of Dion-Jacobson phase to Aurivillius phase: synthesis of (PbBiO2)MNb2O7 (M = La, Bi)

We describe transformations of the Dion-Jacobson (D-J) phases, KLaNb₂O₇ and RbBiNb₂O₇, to the Aurivillius (A) phases, (PbBiO₂)LaNb₂O₇ (1) and (PbBiO₂)BiNb₂O₇ (2), in a metathesis reaction with PbBiO₂Cl. Oxide 1 adopts centrosymmetric tetragonal structure (a = 3.905(1) Å, c = 25.66(1) Å), whereas oxide 2 crystallizes in a noncentrosymmetric orthorhombic (A2₁am) (a = 5.489(1) Å, b = 5.496(2) Å, c = 25.53(1) Å) structure. Oxide 2 shows a distinct SHG response towards 1064 nm laser radiation. The role of La³⁺ versus Bi³⁺ in the perovskite slabs for the occurrence of noncentrosymmetric structure/ferroic property in these materials is pointed out.     

Magnetic and transport properties of La0.3Ca0.7Mn0.9T0.1O3 (T = Cr, Fe, and W)

We have carried out structural, magnetic and magneto transport measurements of the electron-doped manganite La_0.3Ca_0.7MnO₃ substituted with 10% of Cr, Fe and W on the Mn site. The substitution by Cr, Fe and W suppresses the charge order transition present at 260 K in the parent compound. All the samples show a semiconducting behavior. Whereas the parent compound does not show any magneto resistance (MR) even in a field of 14 T, a maximum MR of 6% in 5 T at 25 K is observed for the Cr substituted sample that is attributed to a spin-cluster glass like states induced by Cr. The Fe and W substituted samples showed a MR of 1.5 and 3%, respectively which may be attributed to a smaller number of FM domains/spin-clusters and to an increase in anti-ferromagnetic interaction.     

Synthesis and photoluminescence of new phosphors M2(Mg, Zn)Si2O7:Mn (M = Ca, Sr, Ba)

New phosphors M₂(Mg, Zn)Si₂O₇:Mn²⁺ (M = Ca, Sr, Ba) were prepared by sol-gel process, and their luminescent properties in ultraviolet and vacuum ultraviolet region were investigated. The results showed that the (Ca, Sr, Ba)₂MgSi₂O₇:Mn²⁺ samples did not emit any visible light; the Sr₂ZnSi₂O₇:Mn²⁺ and Ca₂ZnSi₂O₇:Mn²⁺ samples showed green light. The Ba₂ZnSi₂O₇:Mn²⁺ sample mainly showed green light under 254 nm excitation and red light under 147 nm excitation. The different emission was due to the Mn²⁺ ions occupied different sites, which were excited selectively. Among the three phosphors Sr₂ZnSi₂O₇:Mn²⁺ showed the highest green emission intensity, and its decay time was shorter than that of Zn₂SiO₄:Mn²⁺ under 147 nm excitation.     

Synthesis of SrO(SrTiO3)n (n = 1, 2, ∞) compounds and electronic structure analysis

SrO(SrTiO₃)_n compounds were prepared by a modified sol-gel self-propagating combustion, which is a low-temperature combustion synthesis procedure using microwave-assisted sol-gel as precursors. The thermal treating conditions were determined by DTA/TG analysis of the powders. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for the sample characterization. The results confirm that the high temperature and long reaction time which occur in classical solid-state reaction method are avoided. The band structure, total density of states (DOS), and partial density of states (PDOS) of SrO(SrTiO₃)_n were calculated in order to study the electronic structures of SrO(SrTiO₃)_n.     

Intercalation of polyethylene oxide PEO in layered MPS3 (M = Ni, Fe) materials

The intercalation compounds Li_0.96(H₂O)_0.77(PEO)_0.63Ni_0.48PS₃ and Li_0.94(H₂O)_0.92(PEO)_0.94Fe_0.48PS₃ obtained by the insertion of PEO in MPS₃ form lithium-polyethylene oxide complexes containing Li⁺ exchangeable cation in the interlayer space. Polyethylene oxide (PEO) is able to associate interlayer cation increasing the ionic conductivity of NiPS₃ and FePS_3. These compounds constitute a new family of intercalates MPS₃ (M = Ni, Fe) host-layer materials.The new materials were characterized by powder X-Ray diffraction (XRD), Fourier-transformed infrared (FT-IR), differential thermal and thermogravimetric analyses (DTA/TG), energy dispersive X-Ray (EDX), inductively coupled plasma (ICP) and electrochemical impedance measurements. The intercalation compound Li_0.96(H₂O)_0.77(PEO)_0.63Ni_0.48PS₃ shows an ionic conductivity of 0.13 μS/cm, and dc electronic conductivity of ca. 0.1 μS/cm which is twice that of NiPS₃.     

Synthesis and characterization of K2NiF4-type CaLnCoO4 (Ln = Sm and Gd)

K₂NiF₄-type CaLnCoO₄ (Ln = Sm and Gd) has been synthesized at 1173 or 1223 K in air using citric acid (CA) and ethylene glycol (EG). CaLnCoO₄ (Ln = Sm and Gd) has an orthorhombic structure with the space group Bmab. The average particle sizes are approximately 300 nm for CaSmCoO₄ and approximately 170 nm for CaGdCoO₄, respectively. The global instability index (GII) indicates that the crystal structure of CaGdCoO₄ is more stable than that of CaSmCoO₄. CaLnCoO₄ (Ln = Sm and Gd) is a p-type semiconductor and shows paramagnetic behavior above 5 K. The 1/χ-T curve of CaSmCoO₄ deviates from the Curie-Weiss law, whereas the 1/χ-T curve of CaGdCoO₄ follows the Curie-Weiss law in the temperature range of 5 ≤ T ≤ 300 K. From the values of the observed effective magnetic moment (μ_eff) of CaLnCoO₄ (Ln = Sm and Gd), it is considered that the spin state of the Co³⁺ ion is low.     

Generalized large-scale synthesis of MTiO3 (M = Ba, Sr, Pb) nanocrystals

A general hydrothermal synthesis technique has been developed for the large-scale preparation of perovskite oxide nanocrystals of BaTiO₃ (BT), SrTiO₃ (ST) and PbTiO₃ (PT). X-ray diffraction pattern (XRD) and Raman spectrum revealed that tetragonal BT was successfully synthesized. The obtained ST and PT were in cubic and tetragonal phase, respectively. Scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) images showed that all of the products were on nanometer scale. These nanocrystals should provide an ideal candidate for fundamental studies of nanoscale ferroelectricity, piezoelectricity, and paraelectricity. Meanwhile, the synthetic strategy may be easily extended to prepare other nearly monodispersed nanocrystals of perovskite oxides or solid solutions. The relative studies are in progress and will be reported later.     

Electrical and magnetic properties of new rhodium perovskites: La2MRhO6, M = Cr, Fe, Cu

The compositions La₂FeRhO₆, La₂CrRhO₆, La₂CuRhO₆ were prepared for the first time, and their electrical and magnetic properties were determined. Known La₂MnRhO₆, La₂CoRhO₆, and La₂NiRhO₆ were also prepared and their properties determined for comparison. All six phases have the orthorhombic perovskite structure. Electrical resistivity measurements as a function of temperature indicate that all phases are semiconducting. Seebeck measurements show that all are p-type conductors. Magnetic measurements indicate general antiferromagnetic interactions for all compositions, although evidence for weak ferromagnetism is found in the case of La₂CrRhO₆ and La₂FeRhO₆. The oxidation state of M cation appears to be close to 3+ for Fe and Cr, and a mixture of 2+/3+ for Cu. A favorable combination of electrical conductivity and Seebeck coefficient results in a relatively high thermoelectric power (1.11 W/m-K² at ∼575 K) factor for La₂CuRhO₆.     

Combustion synthesis and photoluminescence of Ce-activated MHfO3 (M = Ba, Sr, or Ca)

A simple sol-combustion route involving EDTA was employed to prepare nanosized MHfO₃:Ce (M = Ba, Sr, or Ca) which were characterized by XRD, TEM and luminescent measurements. The as-prepared powders consist of particles with cube-like morphologies for Ce-doped BaHfO₃ and SrHfO₃ and uniform polyhedron-like morphology for Ce-doped CaHfO₃. The average sizes of crystallites of BaHfO₃:Ce, SrHfO₃:Ce and CaHfO₃:Ce are 48, 34 and 30.4 nm, respectively. Broad emissions for nanocrystals of MHfO₃:Ce (M = Ba, Sr, or Ca) resulting from Ce³⁺ 5d → 4f transitions were observed under UV excitation, and the Stokes shift for nanocrystals of Ce-doped BaHfO₃, SrHfO₃ and CaHfO₃ are around 9000, 7400 and 7020 cm⁻¹, respectively.     

Pechini sol-gel deposition and luminescent properties of SrLa1−xRExGa3O7 (RE = Eu, Tb) phosphor films

SrLa_1−xRE_xGa₃O₇ (RE = Eu³⁺, Tb³⁺) phosphor films were deposited on quartz glass substrates by a simple Pechini sol-gel method. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy, field-emission scanning electron microscopy, photoluminescence spectra, and lifetimes were used to characterize the resulting films. The results of XRD indicated that the films began to crystallize at 700 °C and crystallized fully at 900 °C. The results of FT-IR spectra were in agreement with those of XRD. Uniform and crack-free films annealed at 900 °C were obtained with average grain size of 80 nm, root mean square roughness of 46 nm and thickness of 130 nm. The RE ions showed their characteristic emission in crystalline SrLa_1−xRE_xGa₃O₇ films, i.e., Eu³⁺⁵D₀-⁷F_J (J = 0, 1, 2, 3, 4), Tb³⁺⁵D₄-⁷F_J (J = 6, 5, 4, 3) emissions, respectively. The optimum concentrations (x) of Eu³⁺ and Tb³⁺ were determined to be 50, and 80 mol% in SrLa_1−xRE_xGa₃O₇ films, respectively.     

Electrical properties of MgAl2-2xZrxMxO4 (M = Co, Ni and x = 0.00-0.20) synthesized by coprecipitation technique using urea

This paper presents the results of a study concerning the structural and electrical properties of MgAl_2-2xZr_xM_xO₄ (x = 0.00-0.20 and M = Co²⁺ and Ni²⁺) prepared by a coprecipitation technique using urea as a precipitating agent. The X-ray diffraction data for the pure and its doped samples are consistent with the single-phase spinel and their crystallite sizes are in the range 7-20 ± 4 nm. The DC resistivity increases from 3.09 × 10⁹ Ω cm to 6.73 × 10⁹ and 8.06 × 10⁹ Ω cm whereas dielectric constant decreases from 5.80 to 5.11 and 4.95 on doping with Zr-Co and Zr-Ni, respectively. The electrical resistivity variations with increase in the dopant contents indicate two types of conduction mechanisms in operation. Several parameters such as, hopping energy (W), metal-semiconductor transition temperature (T_MS) and Debye temperature (θ_D) have also been determined. The increase in DC resistivity and decrease in dielectric constant suggest that the synthesized materials can be considered for application as an insulating and structural material in fusion reactors.     

Crystal growth and structure of R2Ir2O7 (R = Pr, Eu) using molten KF

Single crystals of R₂Ir₂O₇ (R = Pr, Eu) have been synthesized using molten KF at 1373 K. The pyrochlore compounds crystallize in a cubic space group (No. 227, origin choice 2), with Z = 8. At room temperature, the lattice parameters are a = 10.3940(4) Å, V = 1122.92(7) Å³ and a = 10.274(3) Å, V = 1084.5(6) Å³ for Pr₂Ir₂O₇ and Eu₂Ir₂O₇, respectively. In this paper, we report the crystal growth of R₂Ir₂O₇ (R = Pr, Eu) and their structure determinations from single crystal X-ray diffraction experiments at temperatures of 110, 115, and 298 K.     

New cathode compositions based on La0.84Sr0.16Mn1−xMxO3, where M = Al, Ga for solid oxide fuel cell

In order to identify new cathode compositions for the high temperature solid oxide fuel cell, we have investigated the effect of the trivalent cations Al and Ga at the Mn site of the well-studied cathode composition La_0.84Sr_0.16MnO₃. All the compositions have been prepared by the low temperature citrate-nitrate auto-ignition process and sintered within the temperature range of 1150-1350 °C for 4 h. In order to understand the compatibility of the prepared samples as alternative cathode materials, we compared their electrical conductivity and thermal expansion coefficient with those of La_0.84Sr_0.16MnO₃ and yttria-stabilized zirconia. A 10 mol% Al doped La_0.84Sr_0.16MnO₃ composition exhibited a conductivity of around 122 S cm⁻¹ at 950 °C and a thermal expansion coefficient of 11.04 × 10⁻⁶ K with a minimum reactivity towards yttria-stabilized zirconia. Though the conductivity of the new composition is lower than that of La_0.84Sr_0.16MnO₃ (169 S cm⁻¹ at 950 °C), it is still high enough for use as a cathode material.     

Influence of the B-site ordering on the magnetic properties of the new La3Co2MO9 double perovskites with M = Nb or Ta

Double perovskites La₃Co₂NbO₉ and La₃Co₂TaO₉ have been prepared by both solid state and sol-gel synthesis. The crystal structures have been studied from X-ray and neutron powder diffraction data. Rietveld refinements show that the crystal structure is monoclinic (P2₁/n), with different degrees of ordering of B′ and B″ cations, with octahedra tilted according to the Glazer notation a⁻b⁻c⁺. Occupancy refinements show that the solid state materials are more B-site ordered than the sol-gel ones. Magnetization measurements show that these perovskites show two magnetic contributions, one with spontaneous magnetization and other with linear behaviour with the magnetic field associated to antiferromagnetic correlations. In the samples synthesized by solid state the spontaneous magnetization is more important than those synthesized by the sol-gel and present T_C of 62 K for Nb and 72 K for Ta. On the other hand, materials prepared by sol-gel have T_C 20 K for Nb and 40 K for Ta, respectively and major presence of the antiferromagnetic contribution. The competition between these magnetic behaviours is interpreted, by a microscopic point of view, as to be due to the different degrees of Co²⁺ ions disorder on the B site of the double perovskite structure. This disorder affects the ratio between the antiferromagnetic Co²⁺-O-Co²⁺ and the ferromagnetic Co²⁺-O-M⁵⁺-O-Co²⁺ couplings proposed for the system.     

Synthesis of xLi2MnO3·(1 − x)LiMO2 (M = Cr, Mn, Co, Ni) nanocomposites and their electrochemical properties

A series of 0.4Li₂MnO₃·0.6LiMO₂ (M = Ni_1/3Co_1/3Mn_1/3 and Ni_1/3Cr_1/3Mn_1/3) cathode materials are prepared by a co-precipitation method with subsequent quenching. Crystal structures of samples are investigated by X-ray diffraction and electron diffraction, which show a co-existence of rhombohedral and monoclinic structures indicating nanocomposite characteristics of the sample of 0.4Li₂MnO₃·0.6Li Ni_1/3Cr_1/3Mn_1/3O₂. The average particle size distributions of the powders are analyzed to be an order 400 and 100 nm. The 0.4Li₂MnO₃·0.6LiMO₂ (M = Ni_1/3Co_1/3Mn_1/3 and Ni_1/3Cr_1/3Mn_1/3) electrodes, which consist of a well balanced partial phases of rhombohedral and monoclinic can deliver a high reversible capacity of 220-230 mAh/g during an extended cycling.     

Fabrication of β-BaB2O4 layers on (0 0 1) Sr-doped α-BaB2O4 by vapor transport equilibration (VTE)

Crystalline β-BBO layers have been successfully prepared on (0 0 1)-oriented Sr²⁺-doped α-BBO substrates using vapor transport equilibration technique. The layers were characterized by X-ray diffraction, X-ray rocking curve and transmission spectra. The present results manifest that the VTE treatment time and powder ratio are important factors on the preparation of β-BBO layers. β-BBO layers with a highly (0 0 l) preferred orientation were obtained according to XRD profiles. The full width at half-maximum of the rocking curve for the layer is as low as about 1000 in., which shows the high crystallinity of the layer. These results reveal the possibility of fabricating β-BBO (0 0 1) layers on (0 0 1)-oriented Sr²⁺-doped α-BBO substrates by VTE.     

Mixed cation effect in xR2O-(40 − x)Na2O-50B2O3-10Bi2O3 (R = Li, K) glasses

Mass density, glass transition temperature and ionic conductivity are measured in xLi₂O-(40 − x)Na₂O-50B₂O₃-10Bi₂O₃ and xK₂O-(40 − x)Na₂O-50B₂O₃-10Bi₂O₃ glass systems with 0 ≤ x ≤ 40 mol%. The strength of the mixed alkali effect in T_g, dc electrical conductivity and activation energy has been determined in each glass system. The magnitudes of the mixed alkali effect in T_g for the mixed Li/Na glass system are much smaller than those in the mixed K/Na glasses. The impact of mixed alkali effect on dc electrical conductivity in mixed Li/Na glass system is more pronounced than in the K/Na glass system. The results are explained based on dynamic structure model.     

Single crystal growth and structure refinements of CsMxTe2−xO6 (M = Al, Ga, Ge, In) pyrochlores

Single crystals of CsM_xTe_2−xO₆ pyrochlores with M = Al, Ga, Ge, and In have been grown from a TeO₂ flux. Structure refinements from single crystal X-ray diffraction data are reported. These results are used to discuss deviations from ideal stoichiometry that result in electronic conductivity presumably related to mixed valency of tellurium.