Re-examination of the structural properties of solid solutions SrxCa1−xCO3

We have re-examined the evolution of orthorhombic cell parameters as a function of the substitution parameter x in solid solutions Sr_xCa_1−xCO₃ in order to clarify contradictory results found in the literature. Calcium carbonate has been synthesized in the presence of Sr²⁺ ions (Sr/Ca molar ratio ranging from 10⁻² to 1), using experimental conditions that previously allowed us to obtain monophasic aragonite. The precipitates obtained have been analysed using powder X-ray diffractometry (XRD) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The powder XRD data confirm the occurrence of purely monophasic strontian-aragonites. Moreover, the cell parameters as well as the substitution parameter x have been refined for 0 < x < 0.5 against powder XRD data through Rietveld refinement. On the other hand, x was deduced from chemical analysis by ICP-AES. The agreement between both techniques is very satisfactory. The evolution of the cell parameters as a function of x is found to be linear within the studied range, this feature being confirmed for the overall domain (0 ≤ x ≤ 1) if one takes into account the cell parameters of aragonite CaCO₃ and strontionite SrCO₃. This result, that is consistent with the existence of continuous solid solutions obeying the Vegard's law in the Sr_xCa_1−xCO₃ system, contradicts previously published assertions.     

Grain size-dependent electrical transport properties in La0.75Sr0.25Mn1−xMgxO3±δ ceramics

The role of GB in Mg-substituted lanthanum-strontium manganite ceramics is studied with microstructural details. At higher concentrations of Mg (x>0.05), where the average grain size is ∼1 μm, the M-I transition is shifted from 348 to 110 K. Annealing in lower p_O2 (10⁻⁶ atm) at 1375 K for 1 h obliterates the M-I transition and brings in insulating behaviour throughout the temperature of measurement. Re-annealing in oxygen atmosphere for 10-25 min reintroduces the M-I transition, indicating that the electrical transport properties depend on the chemical inhomogeneity introduced by the in- or out-diffusion of oxygen through the GB regions. Samples with larger grain size (∼35 μm) do not exhibit major modifications in electrical resistivity on annealing in different p_O2. The insulating manganites display non-linear J-E characteristics below the magnetic transition temperature at electric field strengths <50 V/cm. The non-linear behaviour is explained on the basis of the inelastic tunnelling through the multiple localised states in the insulating GB regions. The external magnetic field lowers the voltage at which the non-linearity sets in. The tunnelling therefore may be not only through independent defect centres of oxygen vacancies (V_O), but possibly from defect complexes such as Mn³⁺-V_O or Mn²⁺-V_O, where spin-dependent tunnelling can take place.     

Structural and electrical properties of La2−xBaxMo2O9−x/2 (x = 0-0.18)

La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) have been prepared by solid state reaction method. The lattice parameter of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) determined by XRD data refinement shows a linear dependence on the dopant Ba content x. For the specimen with a La/Ba molar ratio of 0.18-0.2, additional reflection of secondary phase exists in the XRD pattern, so the value of solubility limit for Ba in La₂Mo₂O₉ is defined in range of 0.18 < x < 0.2. As the replacement degree of La³⁺ by Ba²⁺ increases, the bulk conductivity of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) decreases initially and then increases, a minimum value at La_1.9Ba_0.1Mo₂O_8.95 exists. Hebb-Wagner studies in argon atmosphere, which use an oxide-ion blocking electrode, show that La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) are predominantly oxide-ion conducting in the temperature ranging from 773 to 1173 K. The average thermal expansion coefficient of La_1.84Ba_0.16Mo₂O_8.92 determined by high-temperature XRD was deduced as great as 17.5 × 10⁻⁶ K⁻¹ between 298 and 1173 K.     

Crystal structures and properties of BiMn1−xAlxO3 with x = 0.03 and 0.1

Crystal structures of BiMn_0.97Al_0.03O₃ (I) at 300 and 470 K and BiMn_0.9Al_0.1O₃ (II) at 90 and 300 K were studied with synchrotron X-ray powder diffraction. The strong Jahn-Teller distortion, observed at 300 K in I and associated with orbital order, disappeared at 470 K completely for one site and partially for the second site. The Mn/Al-O distances were very close to each other in I at 470 K and in II at 90 and 300 K indicating that orbital order did not appear in II even at 90 K. Magnetic properties of I and II were investigated with specific heat, high-temperature dc magnetic susceptibility, and ac magnetic susceptibility using different driving ac and applied dc magnetic fields and different ac magnetic field frequencies. The anomaly on the specific heat associated with a magnetic transition was strongly suppressed in II compared with that of I and BiMnO₃.     

Dielectric and pyroelectric anisotropy in melt-quenched BaBi2(Nb1 − xVx)2O9 ceramics

The (0 0 l) textured BaBi₂(Nb_1 − xV_x)₂O₉ (where x = 0, 0.03, 0.07, 0.1 and 0.13) ceramics were fabricated via the conventional melt-quenching technique followed by high temperature heat-treatment (800-1000 °C range). The influence of vanadium content and sintering temperature on the texture development and relative density were investigated. The samples corresponding to the composition x = 0.1 sintered at 1000 °C for 10 h exhibited the maximum orientation of about 67%. The Scanning electron microscopic studies revealed the presence of platy grains having the a-b planes perpendicular the pressing axis. The dielectric constant and the pyroelectric co-efficient values in the direction perpendicular to the pressing axis were higher. The anisotropy in the dielectric constant is about 100 (at 100 kHz) at the dielectric maximum temperature and anisotropy in the pyroelectric co-efficient is about 50 μC cm⁻² °C⁻¹ in the vicinity of pyroelectric anomaly for the sample corresponding to the composition x = 0.1 sintered at 1000 °C. Higher values of the dielectric loss and electrical conductivity were observed in the direction perpendicular to the pressing axis which is attributed to the high oxygen ion conduction in the a-b planes.     

Synthesis of non-stoichiometric Bi2O4−x by oxidative precipitation

Bi₂O_4−x, a Bi mixed-valence phase was prepared at 95 °C, by a precipitation process, in a basic medium with a highly oxidizing K₂S₂O₈/Na₂S₂O₈. This phase has a low thermal stability as it decomposes below 400 °C in a multiple step process by some O₂ losses prior to finally transforming into γ-Bi₂O₃. The as-prepared powders are 50-60 nm in size with a narrow size distribution. Optical spectra of Bi₂O_4−x exhibit a broad absorption band with a band gap of ∼1.4 eV as compared to 2.61 eV for Bi₂O₃. The composition of this non-stoichiometric phase, which crystallizes in cubic fluorite related structure with a cell parameter of 5.538(3) Å, is Bi₂O_{3.65 ± 0.10}.     

The mechanism of long phosphorescence of SrAl2−xBxO4 (0<x<0.2) and Sr4Al14−xBxO25 (0.1<x<0.4) co-doped with Eu and Dy

The role of B₂O₃ in realizing the long phosphorescence of Eu(II)+Dy(III) doped strontium aluminates has been investigated. IR and solid state MAS NMR spectra show the incorporation of boron as BO₄ in the AlO₄ framework of SrAl₂O₄ and Sr₄Al₁₄O₂₅. Phosphor, made free of glassy phases by washing with hot acetic acid+glycerol, did not show any photoconductivity under UV irradiation, indicating that the mechanism involving hole conduction in valence band is untenable for long phosphorescence. EPR studies confirm the presence of both electron and hole trap centers. Dy³⁺ forms substitutional defect complex with borate; [Dy-BO₄-V_Sr]²⁻, and acts as a hole trap center. The electron centers are formed by the oxygen vacancies associated with BO₃³⁻, i.e. [BO₃-V_O]³⁻. Under indigo light or near UV irradiation, the photoinduced electron centers are formed as [BO₃-V_O(e′)]⁴⁻. The holes are released from [Dy-BO₄-V_Sr(h)]¹⁻ under thermal excitation at room temperature. The recombination of electrons with holes releases energy which is expended to excite Eu²⁺ to induce long phosphorescence.     

Magnetic coupling in La0.67Ca0.33MnO3/La0.67Sr0.33CoO3/La0.67Ca0.33MnO3 sandwiches

La_0.67Ca_0.33MnO₃ (LCMO)/La_0.67Sr_0.33CoO₃ (LSCO)/LCMO trilayer films are fabricated on single-crystal substrates NdGaO₃ (110) and the interlayer coupling are investigated. Compared with LCMO single layer, sandwiches showed the enhanced metal-insulator transition temperature of LCMO layers. The magnetoresistance is dependent on spacer thickness and the peak value dramatically decreases when LSCO layer is thick enough because of shorting by the LSCO layer. The magnetic coercivity H_C shows a nonmonotonic behavior with changing spacer layer thickness and the waist-like hysteresis indicates that there is an indirect exchange coupling between the top and bottom LCMO layers across the spacer layer.     

Luminescence properties of Ca1−xSrxSe:Ce phosphors and their potential application for GaN-based LEDs

A series of solid solution phosphors Ca_1−xSr_xSe:0.02Ce³⁺ were prepared using high-temperature solid-state reaction technique. Their diffuse reflectance and luminescent spectra at room temperature were investigated and discussed. The optical band gap E_g energies of CaSe and SrSe were derived. Because of its broad band absorption in the range of 400-500 nm, Ca_1−xSr_xSe:Ce³⁺ can suit the application requirements for GaN-based light-emitting diodes (LEDs). The emission wavelength and the coordinates systematic shift from yellowish green to bluish green with an increase in x of Ca_1−xSr_xSe:Ce³⁺. Using Dorenbos's empirical equation, the values of energies of the lowest f-d transition absorption E, redshift D and Stokes shift ΔS for Eu²⁺ in the same host were predicted. The results were in good agreement with the experimental data.     

Electrochemical properties of (La1−xTix)0.67Mg0.33Ni2.75Co0.25 (x = 0-0.20, at%) hydrogen storage alloys

(La_1−xTi_x)_0.67Mg_0.33Ni_2.75Co_0.25 (x = 0, 0.05, 0.10, 0.15 and 0.20, at%) alloys are synthesized by arc-melting and subsequent heat solid-liquid diffusing techniques, and the crystalline structures and electrochemical properties of the alloys are investigated systematically. The structural analysis results show that all the alloys mainly consist of (La, Mg)Ni₃ phase with the rhombohedral PuNi₃-type structure and LaNi₅ phase with the hexagonal CaCu₅-type structure. However, when the Ti content is higher than 0.10, a little amount of TiNi₃ phase start to form. Electrochemical measurements show that the alloy electrodes could be activated to their maximum discharge capacity within four cycles, the maximum discharge capacity is around 321.9-384.6 mAh g⁻¹, both the cyclic stability and the high-rate discharge ability first increased and then decrease with increasing x. All the results show that a little amount of Ti substitution for La in AB₃-type hydrogen storage alloys is effective to the improvement of the overall electrochemical properties.     

Synthesis and optical properties of (Gd1−x,Eux)2O2SO4 nano-phosphors by a novel co-precipitation method

(Gd_1−x,Eu_x)₂O₂SO₄ nano-phosphors were synthesized by a novel co-precipitation method from commercially available Gd₂O₃, Eu₂O₃, H₂SO₄ and NaOH starting materials. Composition of the precursor is greatly influenced by the molar ratio of NaOH to (Gd_1−x,Eu_x)₂(SO₄)₃ (the m value), and the optimal m value was found to be 4. Fourier transform infrared spectrum (FT-IR) and thermal analysis show that the precursor (m = 4) can be transformed into pure (Gd_1−x,Eu_x)₂O₂SO₄ nano-phosphor by calcining at 900 °C for 2 h in air. Transmission electron microscope (TEM) observation shows that the Gd₂O₂SO₄ phosphor particles (m = 4) are quasi-spherical in shape and well dispersed, with a mean particle size of about 30-50 nm. Photoluminescence (PL) spectroscopy reveals that the strongest emission peak is located at 617 nm under 271 nm light excitation, which corresponds to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions. The quenching concentration of Eu³⁺ ions is 10 mol% and the concentration quenching mechanism is exchange interaction among the Eu³⁺ ions. Decay study reveals that the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions has a single exponential decay behavior.     

Synthesis and electrochemical behavior of nanosized LiNi1−xCaxO2 cathode materials for high voltage secondary lithium-ion cells

A new class of LiNi_1−xCa_xO₂ (x = 0.0, 0.1, 0.2, 0.3 and 0.5) layered oxide materials has been synthesized by a simple low temperature solid-state route with mixed nitrates/urea with glycerol as the starting materials. First we have taken TG/DTA for observing the phase transformations of LiNi_0.9Ca_0.1O₂. The structure of the synthesized oxides was analyzed using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) to identify the crystal structure and cation environment, respectively. The synthesized ceramic oxide battery materials were examined by using transmission electron microscope (TEM), scanning electron microscope (SEM) analysis to determine the particle size, nature and morphological structure. SEM with energy dispersive X-ray spectroscopic analysis (EDAX) analysis was carried out to explore the composition of the prepared materials. The electrochemical performance of LiNi_1−xCa_xO₂ electrodes was analyzed using cyclic voltammetry (CV) and galvanostatic charge-discharge cycling studies in the voltage range 3.0-4.5 V. Electrode made with cathode active material, acetylene black and poly(vinylidene difluoride) yield a discharge capacity of 178.1 mAh g⁻¹ (x = 0.2) with good specific capacity over several charge-discharge cycles. These results have been also supported by cyclic voltammograms.     

Ba3ZnTa2−xNbxO9 and Ba3MgTa2−xNbxO9 (0≤x≤1): synthesis, structure and dielectric properties

Oxides belonging to the families Ba₃ZnTa_2−xNb_xO₉ and Ba₃MgTa_2−xNb_xO₉ were synthesized by the solid state reaction route. Sintering temperatures of 1300°C led to oxides with disordered (cubic) perovskite structure. However, on sintering at 1425°C hexagonally ordered structures were obtained for Ba₃MgTa_2−xNb_xO₉ over the entire range (0≤x≤1) of composition, while for Ba₃ZnTa_2−xNb_xO₉ the ordered structure exists in a limited range (0≤x≤0.5). The dielectric constant is close to 30 for the Ba₃ZnTa_2−xNb_xO₉ family of oxides while the Mg analogues have lower dielectric constant of ∼18 in the range 50 Hz to 500 kHz. At microwave frequencies (5-7 GHz) dielectric constant increases with increase in niobium concentration (22-26) for Ba₃ZnTa_2−xNb_xO₉; for Ba₃MgTa_2−xNb_xO₉ it varies between 12 and 14. The “Zn” compounds have much higher quality factors and lower temperature coefficient of resonant frequency compared to the “Mg” analogues.     

The demonstration of epitaxial and electronic-magnetic properties for La0.67Sr0.33MnO3−δ films with (1 1 1) orientation

La_0.67Sr_0.33MnO_3−δ films, fabricated on (1 1 1) LaAlO₃ single-crystal substrates using a direct current magnetron sputtering technique, are demonstrated by X-ray diffraction patterns and pole figures to be high quality epitaxial films and there is a perfect matching relationship between the films and the substrates. We observed an obvious difference of the electronic-magnetic transportation properties among films sputtered on (1 1 1), (1 0 0) and (1 1 0) LaAlO₃ substrates, respectively. A mechanism for the difference is discussed briefly.     

Structural and magnetic properties of La0.85Ca0.15Mn1 − xCrxO3

Chromium (Cr) is doped at the Mn site of La_0.85Ca_0.15MnO₃ system to explore its complex ferromagnetic insulating (FMI) state. The Rietveld refinement of neutron diffraction data indicates that there is no structural change owing to Cr substitution in La_0.85Ca_0.15Mn_1 − xCr_xO₃ (0 ≤ x ≤ 0.1). Nevertheless, it strengthens the magnetic couplings and the system shifts towards enhanced ferromagnetic (FM) ordering. Doping with Cr is found to stabilise the FMI state at low temperatures. The magnetic moment of the parent compound (for x = 0) obtained from neutron diffraction data recorded at low temperature (17 K) is found to be ~ 3.53(5) μ_B and is close to the theoretically estimated value of 3.85 μ_B. This value is higher than previously reported value of 2.90 μ_B.     

Microstructure and mechanical properties of small amounts of In2O3 reinforced Pb(ZrxTi1−x)O3 ceramics

Piezoelectric Pb(Zr_xTi_1−x)O₃ (PZT) ceramics with small amount (0.5-2.0 wt.%) of In₂O₃ are prepared by conventional sintering method. Based on X-ray diffraction analysis, the tetragonality of PZT matrix decreases with In₂O₃ content, indicating that In₂O₃ diffuses into PZT matrix. The microstructure of PZT matrix is significantly refined by doping small amounts of In₂O₃. The grain size reduction and the matrix grain boundary reinforcement are the probable mechanism responsible for the high strength and hardness in the PZT/In₂O₃ materials. The enhancement in Young’s modulus is attributed to In³⁺ substitution. The decreased tetragonality with In₂O₃ addition results in less crack energy absorption by domain switching and, hence, causes the small reduction in fracture toughness.     

R3Mn1.5CuV0.5O9 (R = Y, Ho, Er, Tm, Yb and Lu) and Lu3Mn3−3xCu2xVxO9: New noncentrosymmetric oxides related to YMnO3

We report formation of new noncentrosymmetric oxides of the formula, R₃Mn_1.5CuV_0.5O₉ for R = Y, Ho, Er, Tm, Yb and Lu, possessing the hexagonal RMnO₃ (space group P6₃cm) structure. These oxides could be regarded as the x = 0.5 members of a general series R₃Mn_3−3xCu_2xV_xO₉. Investigation of the Lu-Mn-Cu-V-O system reveals the existence of isostructural solid solution series, Lu₃Mn_3−3xCu_2xV_xO₉ for 0 < x ≤ 0.75. Magnetic and dielectric properties of the oxides are consistent with a random distribution of Mn³⁺, Cu²⁺ and V⁵⁺ atoms that preserve the noncentrosymmetric RMnO₃ structure.     

Synthesis and photoluminescent properties of mesoporous (MgO)x(ZnO)1−x materials

(MgO)_x(ZnO)_1−x materials have been synthesized using mesoporous carbon as template. By increasing the MgO content in the materials greater than 25%, the (MgO)_x(ZnO)_1−x materials began to form the mesoporous structure. Pore size distribution curves indicated that the BJH pore diameter decreased with increasing MgO content. In photoluminescence spectra, all the samples except pure ZnO showed both the band-edge emission and the deep-level emission (green band). It was interesting to note that the UV emission peak energy (E_UV) had a red-shift of about 48 meV at the low MgO content range of 0-25%, while when the MgO content varied from 25 to 75%, the E_UV displayed a blue-shift of about 36 meV to the higher energy direction. The optical band gap (E_g) of the (MgO)_x(ZnO)_1−x calculated from the absorption spectra was far smaller than that in literature, and this may be related to the formation of mesoporous structure.     

Critical behaviour and magnetocaloric effect of nano crystalline La0.67Ca0.33Mn1−xFexO3 (x = 0.05, 0.2) synthesized by nebulized spray pyrolysis

The structure, critical exponents and magnetocaloric effect (MCE) of Nebulized Spray Pyrolysis (NSP) synthesized nano crystalline La_0.67Ca_0.33Mn_1−xFe_xO₃ (x = 0.05, 0.2) were investigated. The Reitveld refinement of XRD patterns show that the samples adopt an orthorhombic structure with Pnma space group. TEM inspection reveals that the average particle size is about 15 nm and 42 nm for NSP synthesized LCMFe_0.05 and LCMFe_0.2 samples respectively. The temperature and field dependent magnetization studies reveal the superparamagnetic state of La_0.67Ca_0.33Mn_0.95Fe_0.05O₃ and spin-glass-like state of La_0.67Ca_0.33Mn_0.8Fe_0.2O₃. The critical behaviour at the transition region studied using modified Arrott plot provides a second order nature of phase transition for both samples. The magnetocaloric studies show the maximum value of magnetic entropy change (ΔS_max) is in the range 2.3 J kg⁻¹ K⁻¹ at 158 K for LCMFe_0.05 and 0.3 J kg⁻¹ K at 92 K for LCMFe_0.2 respectively at 5 T field. The field dependence of the magnetic entropy changes are also analysed, which show a power law dependence (ΔS_M ∞ Hⁿ, n = 0.72 (2)) at transition temperature, T_C = 162 K for LCMFe_0.05 and n = 1.11(3) at 92 K for LCMFe_0.2.