Effect of Bi and Sr doping on morphological and magnetic properties of LaCo<Subscript>0.6</Subscript>Fe<Subscript>0.4</Subscript>O<Subscript>3</Subscript> nanosized perovskites

Nanopowders of La _1?x Bi _x Co_0.6Fe_0.4O₃ (x = 0, 0.1, 0.2) and La _1?2x Bi _x Sr _x Co_0.6Fe_0.4O₃ (x = 0.1) multinary perovskites were synthesized by citrate sol–gel autocombustion method. Crystalline phase and the lattice parameters were obtained from X-ray diffraction pattern. The XRD result shows that all compounds have rhombhohedral crystal structure with \(\bar {\mathbf {R}\mathbf {3}}\)c space group and Bi (x = 0.2) have the presence of secondary peaks. Crystallite size, dislocation density, specific area and strain were calculated from XRD. The elemental composition and micrographs of grain were obtained from EDAX (energy dispersive X-ray analysis) and SEM (scanning electron microscopy), with an average grain size below 400 nm. Surface morphological studies using XPS (X-ray photoelectron spectroscopy) were used to find out the chemical states and surface proportion of oxygen present in samples. Finally, using the vibrating sample magnetometer the room temperature magnetic behaviour of compounds was studied and it was observed that the ferromagnetic behaviour of LaCo_0.6Fe_0.4O₃ was reduced by Bi and Sr doping.     

Effects of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> doping on the microstructure and electrical properties of (Ba,Ca)(Zr,Ti)O<Subscript>3</Subscript> lead-free ceramics

Lead-free ceramics (Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃?x wt.%Cr₂O₃ (BCZT-xCr) were prepared via the conventional solid-state reaction method. The microstructure and electrical properties of BCZT-xCr samples were systematically studied. XRD and Raman results showed that all samples possessed a single phased perovskite structure and were close to the morphotropic phase boundary (MPB). With the increase of the Cr content, the rhombohedral-tetragonal phase transition temperature (T _R-T) increases slightly, and the Curie temperature (T _C) shifts towards the low temperature side. XPS analysis reveals that Cr³⁺ and Cr^{5 +} ions co-existed in Cr-doped BCZT ceramics, indicating the different impact on the electrical properties from Cr ions as “acceptor” or “donor”. For the x = 0.1 sample, relative high piezoelectric constants d ₃₃ (～316 pC/N) as well as high Q _m (～554) and low tanδ (～0.8%) were obtained. In addition, the AC conductivity was also investigated. Hopping charge was considered as the main conduction mechanism at low temperature. As the temperature increases, small polarons and oxygen vacancies conduction played important roles.     

Fluorination of Bi<Subscript>1.8</Subscript>Fe<Subscript>1.2</Subscript>SbO<Subscript>7</Subscript> pyrochlore solid solutions

A technique has been developed for fluorinating the pyrochlore oxide Bi_1.8Fe_0.2FeSbO₇, and a compound with the composition Bi_1.8Fe_1.2SbO_7–x/2F_x has been obtained. The synthesized oxyfluoride also has the pyrochlore structure (sp. gr. Fd3m), with a lattice parameter a = 10.4443(1) Å (R _wp = 5.2). It has been shown that the charge balance upon fluorine substitution for oxygen is maintained not through partial reduction of Fe³⁺ to Fe²⁺ but through the incorporation of fluorine into oxygen vacancies. The magnetic behavior of the fluorinated pyrochlore phase is determined by the persisting frustration of the octahedral sublattice, which is responsible for the development of a spin glass state below T _f = 12 K. The fluorination-induced changes in the anion sublattice led to an increase in the antiferromagnetic exchange interaction between neighboring Fe³⁺ ions and changes in the dynamic properties of the spin glass phase.     

Solid-state reactions in the system Li<Subscript>2</Subscript>CO<Subscript>3</Subscript>-Na<Subscript>2</Subscript>CO<Subscript>3</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>

The formation mechanisms of Li _x Na_{1 ?x} Ta _y Nb_{1 ? y} O₃ perovskite solid solutions in the Li₂CO₃-Na₂CO₃-Nb₂O₅-Ta₂O₅ system have been studied by x-ray diffraction, differential thermal analysis, thermogravimetry, IR spectroscopy, and mass spectrometry at temperatures from 300 to 1100°C. The results indicate that the synthesis of Li _x Na_{1 ? x} Ta _y Nb_{1 ? y} O₃ solid solutions involves a complex sequence of consecutive and parallel solid-state reactions. An optimized synthesis procedure for Li _x Na_{1 ? x} Ta _y Nb_{1 ? y} O₃ solid solutions is proposed.     

Photoluminescence of Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>:Eu nanocrystals

The photoluminescence (PL) of Ga₂S₃-5 mol % Eu₂O₃ nanocrystals prepared by mechanical comminution of the initial compound has been studied in the temperature range from 77 to 300 K. It is established that the PL spectrum of nanocrystals, in comparison to that of a massive sample, extends over a broader wavelength interval (430–620 nm) and has two maxima (at 507 and 556 nm) instead of one. The intensity of emission from nanocrystals is significantly higher than that from the massive crystal. The halfwidth in both cases varies with the temperature in proportion to T ^1/2. The intensity of emission at 556 nm for nanocrystals depends on the temperature as lgI ～ 1/T, this dependence having three linear portions corresponding to an activation energy of 0.04, 0.16, and 0.43 eV. The PL bands with maxima at 507 and 556 nm are assigned to the intracenter 4f ⁶5d4f ⁷ transitions in Eu²⁺ ions.     

Rare-earth-doped PbTiO<Subscript>3</Subscript>-PbZrO<Subscript>3</Subscript> solid solutions

(Pb_{1 ? x} Ln _x )(Zr_0.53Ti_0.47)O₃ and (Pb_{1 ? x} Ln _x )(Zr_0.65Ti_0.35)O₃ (x = 0.02, 0.06; Ln = La, Pr, Gd, Yb) solid solutions have been prepared by modified solid-state synthesis using organic-ligand precursors. The solid solutions have been characterized by thermal analysis, IR spectroscopy, x-ray powder diffraction, and atomic force microscopy. All of them have a rhombohedrally distorted perovskite structure (sp. gr. R3c).     

Toroidal spin ordering in BiFeO<Subscript>3</Subscript>, GaFeO<Subscript>3</Subscript>, and Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> in the Faddeev model with a magnetic field

The Fadeev model is used for describing the recently discovered toroidal spin ordering in piezoelectric and ferrimagnetic GaFeO₃ and piezo- and magnetoelectric Cr₂O₃ and BiFeO₃. A stable toroidal solution of the Faddeev model with the topological charge Q= 1 in an external homogeneous magnetic field was obtained using the trial function method. The energy of a toroid as a function of its radius (R) was determined at various values of the external magnetic field (H). It was shown that the energy minimum is shifted toward smaller R’s with an increase in H. At a critical field value, the torus collapses so that the local spin structure disappears. It is suggested to use magnetic field for controlling the torus size in multiferroics, promising materials of spintronics.     

Synthesis,photoluminescence properties of Sr<Subscript>1.95</Subscript>Ba<Subscript>0.05</Subscript>CeO<Subscript>4</Subscript>:Eu<Superscript>3+</Superscript> for LED applications

The Sr_1.95Ba_0.05 CeO₄:Eu³⁺ phosphors are synthesized by the solid-state reaction method. The samples are characterized using X-ray diffraction (XRD), diffuse reflectance spectroscopy and photoluminescence (PL) spectra. The XRD results reveal that the synthesized phosphors are genuine crystalline and belong to the orthorhombic structure. The intense PL emission is optimized from the PL spectra at various doping concentrations of europium ions. The results indicates that the phosphor can be effectively excited under 264 nm wavelength producing on intense emission spectrum of the synthesis material at 484 nm (blue region). The color purity of the phosphor is confirmed by CIE coordinates (x = 0.217, y = 0.265). The experimental data indicate that the prepared phosphors can be used as blue-emitting material in the field of illuminations and display devices.     

Giant Magnetoresistance in Topological Insulator Bi<Subscript>1.98</Subscript>Nd<Subscript>0.02</Subscript>Se<Subscript>3</Subscript> Single Crystal

Doped topological insulators (TI) Bi_2?x Nd _x Se₃ single crystals were prepared by the self-flux method. The phase structure, magnetic properties, and electrical transport properties of the samples were studied. The X-ray diffraction (XRD) patterns of the sample indicate an incorporation of Nd into the Bi₂Se₃, and the crystal can be easily cleaved with silvery surface. The Bi_2?x Nd _x Se₃ sample shows a giant magnetoresistance (MR) with different magnetic field. The positive magnetoresistance (MR) can reach 190 % at the field of 9 Tesla when the field is perpendicular to ab-plane of the crystal. In addition, at low magnetic fields, the MR exhibits a weak antilocalization (WAL) cusp.     

Study of Magnetic Entropy Change in Nd<Subscript>0.67</Subscript>Ba<Subscript>0.33</Subscript>Mn<Subscript>0.98</Subscript>Fe<Subscript>0.02</Subscript>O<Subscript>3</Subscript> by Means of Theoretical Models

Magnetic entropy change (?ΔS _M ) of Nd_0.67 Ba_0.33Mn_0.98Fe_0.02O₃ perovskite have been analyzed by means of theoretical models. An excellent agreement has been found between the (ΔS_M) values estimated by Landau theory and those obtained using the classical Maxwell relation. In order to estimate the spontaneous magnetization M _{s pont}(T), we used the mean-field theory to analyses the (ΔS_M) vs. M ² data. The obtained M _{s pont}(T) values are in good agreement with those found from the classical extrapolation from the Arrott plots(H/M vs. M ²), confirming that the magnetic entropy is a valid approach to estimate the spontaneous magnetization in our system. At a relatively low magnetic field, a phenomenological model has been used to estimate the values of the magnetic entropy change. The results are in good agreement with those obtained from the experimental data using Maxwell relation.     

Phenomenological Model of Magnetocaloric Effect in La<Subscript>0.7</Subscript>Ca<Subscript>0.2</Subscript>Ba<Subscript>0.1</Subscript>MnO<Subscript>3</Subscript> Manganite Around Room Temperature

In this work, we studied in detail the magnetic and magnetocaloric properties of the La_0.7Ca_0.2Ba_0.1MnO₃ compound according to the phenomenological model. Based on this model, the magnetocaloric parameters such as the maximum of the magnetic entropy change ΔS _M and the relative cooling power (RCP) have been determined from the magnetization data as a function of temperature at several magnetic fields. The theoretical predictions are found to closely agree with the experimental measurements, which make our sample a suitable candidate for refrigeration near room temperature. In addition, field dependences of \({{\Delta } S}_{\mathrm {M}}^{\max }\) and RCP can be expressed by the power laws \({\Delta S}_{\mathrm {M}}^{\max }\approx a\)(μ ₀ H) ⁿ and RCP ≈b(μ ₀ H) ^m , where a and b are coefficients and n and m are the field exponents, respectively. Moreover, phenomenological universal curves of entropy change confirm the second-order phase transition.     

Photoinactivation of bacteria by using Fe-doped TiO<Subscript>2</Subscript>-MWCNTs nanocomposites

In this study, nanocomposites of Fe-doped TiO₂ with multi-walled carbon nanotubes (0.1– 0.5?wt. %) were prepared by using sol–gel method. The structural and morphological analysis were carried out with using X-ray diffraction pattern and transmission electron microscopy, which confirm the presence of pure anatase phase and particle sizes in the range 15–20?nm. X-ray photoelectron spectroscopy was used to determine the surface compositions of the nanocomposites. UV–vis diffuse reflectance spectra confirm redshift in the optical absorption edge of nanocomposites with increasing amount of multi-walled carbon nanotubes. Nanocomposites show photoinactivation against gram-positive Bacillus subtilis as well as gram-negative Pseudomonas aeruginosa. Fe-TiO₂-multi-walled carbon nanotubes (0.5?wt. %) nanocomposites show higher photoinactivation capability as compared with other nanocomposites. The photoluminescence study reveals that the Fe-TiO₂-multi-walled carbon nanotubes nanocomposites are capable to generate higher rate of reactive oxygen species species than that of other nanocomposites. Our experimental results demonstrated that the Fe-TiO₂-multi-walled carbon nanotubes nanocomposites act as efficient antibacterial agents against a wide range of microorganisms to prevent and control the persistence and spreading of bacterial infections.     

Phases and morphotropic regions in the PbNb<Subscript>2/3</Subscript> Mg<Subscript>1/3</Subscript>O<Subscript>3</Subscript>-PbTiO<Subscript>3</Subscript> system

Ceramic samples of solid solutions (1 ? x) PbNb_2/3Mg_1/3O₃ · xPbTiO₃ (0 ≤ x ≤ 1.0, Δx = 0.0025–0.05) are prepared by the columbite method. A detailed x?T phase diagram of the system is constructed (isothermal join at 25°C), and dielectric, piezoelectric, and elastic properties are investigated. It is established that the region of the morphotropic phase transition is positioned in the range 0.28 < x ≤ 0.43 and consists of a series of narrower regions. Inside one phase (cubic, rhombohedral, tetragonal), regions are found in which a qualitative and quantitative difference in structural and electrical parameters is observed. An interpretation of the observed effects in the context of the defect structure of the objects is suggested.     

Investigation of Magnetic,Magnetocaloric, and Critical Properties of La<Subscript>0.5</Subscript>Ba<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> Manganite

We present an extensive study of the magnetic properties of a novel La_0.5Ba_0.5MnO₃ perovskite material prepared by the hydrothermal method. The explored sample was structurally studied by the x-ray diffraction (XRD) method which confirms the formation of a pure cubic phase of a perovskite structure with Pm3m space group. The magnetic properties were probed by employing temperature M (T) and external magnetic field M (μ_oH) dependence of magnetization measurements. A magnetic phase transition from ferromagnetic to paramagnetic phase occurs at 339 K in this sample. The maximum magnetic entropy change (\(\left | {{\Delta } S}_{M}^{\max } \right |\)) took a value of 1.4 J kg^??1 K^??1 at the applied magnetic field of 4.0 T for the explored sample and has also been found to occur at Curie temperature (T_C). This large entropy change might be instigated from the abrupt reduction of magnetization at T_C. The magnetocaloric effect (MCE) is maximum at T_C as represented by M (μ_oH) isotherms. The relative cooling power (RCP) is 243.2 J kg^??1 at μ_oH =?4.0 T. Moreover, the critical properties near T_C have been probed from magnetic data. The critical exponents δ, β, and γ with values 3.82, 0.42, and 1.2 are close to the values predicted by the 3D Ising model. Additionally, the authenticity of the critical exponents has been confirmed by the scaling equation of state and all data fall on two separate branches, one for T < T_C and the other for T > T_C, signifying that the critical exponents obtained in this work are accurate.     

Structural and dielectric properties of PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>-PbZrO<Subscript>3</Subscript> solid solutions synthesized at high pressures and temperatures

(1 ? x)PbMg_1/3Nb_2/3O₃ · xPbZrO₃ (1 ? x)PMN · xPZ) solid solutions have been synthesized at a pressure of 5 GPa and temperatures from 1300 to 1700 K, and their structural and dielectric properties have been studied. The composition dependences of the average unit-cell parameter and dielectric permittivity for the solid solutions indicate that the PMN-PZ system has a morphotropic phase boundary near x = 0.65. The solid solutions have a cubic structure for x < 0.65, a rhombohedral structure in the range 0.65 < x < 0.9, and an orthorhombic structure (similar to that of PbZrO₃) for x > 0.9. The temperature and frequency dependences of dielectric permittivity suggest that the (1 ? x)PMN · xPZ samples with x < 0.65 consist of two ferroelectric phases: a relaxor with antipolar dipole order and a normal ferroelectric with a diffuse phase transition. The effect of annealing temperature on the ferroelectric state of the samples with x < 0.65 is examined. In the composition range 0.65 < x < 0.9, the samples have normal ferroelectric properties, independent of annealing temperature.     

DFT study of the stability of oxygen vacancy in cubic <Emphasis Type="Italic">AB</Emphasis>O<Subscript>3</Subscript> perovskites

Rare earth and alkaline earth metal perovskites with general formula ABO₃ have attracted much attention as electrocatalysts for state-of-the-art fuel cells, and catalysts for hydrogen generation and hydrocarbons oxidation. Tuning the ion conductivity through doping A and B and subsequent formation of oxygen vacancies is essential for the performance of perovskites materials. To provide insights into factors that affect stability of oxygen vacancies and understand the origin of the activity of doped perovskite materials, we investigate the structure and energetics of cubic ABO₃ perovskites (A = La and/or Be, Mg, Ca, Sr, Ba; B = Ti, V, Cr, Mn, Fe, Co, and Ni) using density functional theory calculations. It is found that the lattice constant of ABO₃ generally increases as the ionic radius of A and B; the bulk formation energy of ABO₃ is decomposed into the ionization energy and lattice energy, which depend on the ionic radius and valence. The trend of bulk formation energy corresponds to that of ionization energy at a given ionic valence, while corresponds to that of lattice energy as doping La by alkali earth metals with lower valence. There exists a good linear relationship between the bulk formation energy and oxygen vacancy formation energy. This work provides an understanding toward the origin of the activity of perovskites at the atomic level.     

Electrical Conductivity and Electrochemical Characteristics of Na<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>-Based NASICON-Type Materials

NASICON-type materials with the compositions Na₃V_2–xAl_x(PO₄)₃, Na₃V_{2 - x}Fe_x(PO₄)₃, Na_{3 + x}V_2–xNi_x(PO₄)₃, and Na₃V_{2 - x}Cr_x(PO₄)₃ (x = 0, 0.03, 0.05, and 0.1) have been prepared and characterized by X-ray diffraction analysis, electron microscopy, and impedance spectroscopy. The results demonstrate that the highest electrical conductivity among the samples studied is offered by the material doped with 5% Fe: Na₃V_1.9Fe_0.1(PO₄)₃. The activation energy for low-temperature conduction in the doped materials decreases from 84 ± 2 to 54 ± 1 kJ/mol and that for high-temperature conduction is ~33 kJ/mol. The discharge capacity of Na₃V_1.9Fe_0.1(PO₄)₃/C under typical working conditions of cathodes of sodium ion batteries has been shown to exceed that of Na₃V₂(PO₄)₃/C. The capacity of the more porous material prepared by the Pechini process (Na₃V_1.9Fe_0.1(PO₄)₃/C-{II}) approaches the theoretical one at a low charge–discharge rate and retains its high level as the charge rate is raised (its discharge capacity was 117.6, 108.8, and 82.6 mAh/g at a discharge rate of 0.1C, 2C, and 8C, respectively).     

Temperature effect on the photoluminescence intensity and Eu<Superscript>2+</Superscript> excited state lifetime in EuGa<Subscript>2</Subscript>S<Subscript>4</Subscript> and EuGa<Subscript>2</Subscript>S<Subscript>4</Subscript>:Er<Superscript>3+</Superscript>

The photoluminescence (PL) spectra and Eu²⁺ excited state lifetime of EuGa₂S₄ and EuGa₂S₄:Er³⁺ have been studied in the range 78–500 K. The spectra show a band at 545 nm, due to the 4f ⁶5d → 4f ⁷(⁸ S _7/2) transition. With increasing temperature, the full width at half maximum Γ(T) of the PL band of EuGa₂S₄ and EuGa₂S₄:Er³⁺ crystals increases from 0.15 to 0.22 and from 0.13 to 0.19 eV, respectively. Over the entire temperature range studied, Γ(T) is a linear function of T ^1/2. The 545-nm emission intensity and Eu²⁺ excited state lifetime in EuGa₂S₄ and EuGa₂S₄:Er³⁺ vary exponentially with temperature. The luminescence quenching energies evaluated from the Arrhenius plots of I(10³/T) and τ(10³/T) coincide (0.10 eV) within the error of determination.     

Influence of isovalent and heterovalent substitution for Bi<Superscript>3+</Superscript> and Fe<Superscript>3+</Superscript> on the properties of Bi<Subscript>2</Subscript>Fe<Subscript>4</Subscript>O<Subscript>9</Subscript>-based solid solutions

Bi_2–хLa_хFe₄O₉ and Bi₂Fe_4–2xTi_xCo_xO₉ ferrites have been prepared by solid-state reactions at a temperature of 1073 K. X-ray diffraction data indicate that, in the Bi_2–хLa_хFe₄O₉ system, the limiting degree of La³⁺ substitution for Bi³⁺ ions in Bi₂Fe₄O₉ does not exceed 0.05 and that the limiting degree of substitution in the Bi₂Fe_4–2xTi_xCo_xO₉ system lies in the range 0.05 < x < 0.1. The specific magnetization and specific magnetic susceptibility of the samples have been measured at temperatures from 5 to 300 K in a magnetic field of 0.86 T. The field dependences of magnetization obtained for the Bi_2–хLa_хFe₄O₉ and Bi₂Fe_4–2xTi_xCo_xO₉ ferrites at temperatures of 300 and 5 K demonstrate that partial isovalent substitution of La³⁺ for Bi³⁺ ions in Bi₂Fe₄O₉ and heterovalent substitution of Ti⁴⁺ and Co²⁺ ions for two Fe³⁺ ions leads to partial breakdown of the antiferromagnetic state and nucleation of a ferromagnetic state.     

Magnetic properties of Er<Subscript>2</Subscript>Mn<Subscript>2/3</Subscript>Re<Subscript>4/3</Subscript>O<Subscript>7</Subscript>, a new zirkelite-structure oxide

We have studied the magnetic properties of the new compound Er₂Mn_2/3Re_4/3O₇ prepared by reacting Er₃ReO₈, ReO₂, MnO, and metallic Re at 1020°C in silica tubes sealed off under vacuum. The compound is shown to have the zirkelite structure with hexagonal cell parameters a = 7.3174(6) Å and c = 17.365(1) Å (sp. gr. P3₁21₁, Z = 6). Magnetization data obtained in the range 2–300 K demonstrate that, above ～150 K, its magnetic susceptibility exhibits Curie-Weiss behavior with an effective magnetic moment of 9.50μ_B. Dynamic magnetic susceptibility measurements point to spin-glass behavior of this compound at low temperatures.