Influence of V2O5 ion addition on the conductivity and grain growth of Ni–Zn–Cu ferrites

Polycrystalline nickel–zinc–copper ferrites with chemical formula Ni_0.6+xZn_0.2Cu_0.2V_xFe_2−2xO₄,(0.0 ⩽ x ⩽ 0.25) were prepared by the ceramic route. The X-ray diffraction (XRD) analysis of the samples results confirms single-phase spinel structure. Scanning electron microscopy (SEM) of the prepared ferrites reveal that vanadium addition resulted in a rapid grain growth with large pores trapped inside the grains as the vanadium concentration increases. The ac conductivity σ_ac has been studied as a function of frequency and temperature over the temperature range (300–600 K). The results obtained for these materials reveal a semiconductor – to semimetal transition as V⁵⁺ content increases. All studies composition exhibit a transition with change in the slope of conductivity. The obtained temperature T_c is found to be decrease with the increasing vanadium content. The hopping of electrons between Fe³⁺ and Fe²⁺ as well as the hole hopping between Ni³⁺ and Ni²⁺ are found to responsible for the conduction mechanism. The relation of the universal exponent s with temperature gives evidence for the presence of the correlation barrier hopping (CHB) mechanism in these compounds. The impedance technique has been used to study effect of grain and grain boundary on the electrical properties. The analysis data show only one semi-circle for all samples except for sample with x = 0.05. The results suggested that the conduction mechanism takes place predominantly through the grain in the studied samples.     

Maximum magnetic entropy change modulated toward room temperature in perovskite manganites La0.7−xNdx(Ca,Sr)0.3MnO3

With Nd³⁺ doping and Ca²⁺, Sr²⁺ modulating in the sol–gel technique, a series of polycrystalline perovskite samples La_0.7?xNd_x(Ca,Sr)_0.3MnO₃ (x = 0, 0.05, 0.1, 0.15, 0.20, 0.25) was prepared, their maximum magnetic entropy changes were tuned to room temperature (ΔS_H = ?1.47 J/kg K at 298 k for La_0.45Nd_0.25(Ca,Sr)_0.3MnO₃), an enhancement of the maximum magnetic entropy change (ΔS_H = ?1.89 J/kg K at 315 k) and its refrigerant capacity (about 45.3 J/kg) had also been obtained under 9 kOe magnetic field variation for La_0.55Nd_0.15(Ca,Sr)_0.3MnO₃ contrast to La_0.7(Ca,Sr)_0.3MnO₃.     

Effect of the substitution of Pr for Nd on microstructure and magnetic properties of nanocomposite Nd2Fe14B/α-Fe magnets

Microstructure and magnetic properties of melt-spun nanocomposite magnets with nominal compositions of (Nd_1−xPr_x)₉Fe₈₆B₅ (x=0–1) were investigated. Substitution of Nd by Pr could significantly improve the hard magnetic properties of the nanocomposite magnets; the intrinsic coercivity (_iH_c) and the maximum magnetic energy product ((BH)_max) increase from 414 kA/m and 124 kJ/m³ for x=0 to 493 kA/m and 152 kJ/m³ for x=0.6, respectively. Further substituting Nd by Pr (x>0.6) strongly weakens exchange-coupling interaction between magnetically hard and soft phases.     

Energy levels fitting and crystal-field calculations of Nd3+ doped in GYSGG crystal

The single crystal Nd³⁺-doped in GdY₂Sc₂Ga₃O₁₂ (Nd³⁺:GYSGG) was grown by Czochralski method successfully, and its absorption spectra was analyzed in a wider spectral wavelength range at 7.6 K and 300 K, respectively. The free-ions and crystal-field parameters were fitted to the experimental energy levels at 7.6 K and 300 K with the root mean square deviation of 11.25 and 12.48 cm^?1, respectively. According to the crystal-field calculations, 116 levels of Nd³⁺ at 7.6 K and 114 levels of Nd³⁺ at 300 K were assigned. The fitting results of free-ions and crystal-field parameters were compared with those already reported of Nd³⁺:GSGG and Nd³⁺:YSAG. The results indicated that the free-ions parameters are similar to those of the Nd³⁺ in GYSGG, GSGG and YSAG crystals, and the crystal-field interaction of GSGG and YSAG is stronger than that of GYSGG, which results in the dual-wavelength properties of Nd³⁺:GYSGG crystal.     

BaCO3 mediated modifications in structural and magnetic properties of natural nanoferrites

Preparing M-type barium hexaferrite and improving the magnetic response of natural ferrites by incorporating barium carbonate (BaCO₃) is ever-demanding. Series of barium carbonate doped ferrites with composition (100−x)Fe₃O₄·xBaCO₃ (x=0, 10, 20, 30 wt%) are prepared through solid state reaction method and sintered gradually at temperatures of 800 and 1000 °C. Nanoparticles of natural ferrite and commercial BaCO₃ are used as raw materials. Impacts of BaCO3 on structural and magnetic properties of these synthesized ferrites are inspected. The obtained ferrites are characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) at room temperature. Uniform barium hexaferrite particles in terms of both morphology and size are not achieved. The average crystallite size of BaFe₁₂O₁₉ is observed to be within 30–600 nm. The sintering process results phase transformation from Fe₃O₄ (magnetite) to α-Fe₂O₃ (hematite) and the formation of hexagonal barium ferrite crystals. The occurrence of barium crystal is found to enhance with the increase of BaCO₃ concentrations up to 20 wt% and suddenly drop at 30 wt%. Saturation and remanent magnetization of the doped ferrites are significantly augmented up to 16.37 and 8.92 emu g⁻¹, respectively compared to their pure counterpart. Furthermore, the coercivity field is slightly decreased as BaCO₃ concentrations are increased. BaCO₃ mediated improvements in the magnetic response of natural ferrites are demonstrated.     

NMR and electric impedance study of lithium mobility in fast ion conductors LiTi2 − xZrx(PO4)3 (0 ≤ x ≤ 2)

Materials of the LiTi_2 − xZr_x(PO₄)₃ series (0 ≤ x ≤ 2) were prepared and characterized by powder X-ray (XRD) and neutron diffraction (ND), ⁷Li and ³¹P Nuclear Magnetic Resonance (NMR) and Electric Impedance techniques. In samples with x < 1.8, XRD patterns were indexed with the rhombohedral R3̅c space group, but in samples with x ≥ 1.8, XRD patterns display the presence of rhombohedral and triclinic phases. The Rietveld analysis of the LiTi_1.4Zr_0.6(PO₄)₃ neutron diffraction (ND) pattern provided structural information about intermediate compositions. For low Zr contents, compositions deduced from ³¹P MAS-NMR spectra are similar to nominal ones, indicating that Zr⁴⁺ and Ti⁴⁺ cations are randomly distributed in the NASICON structure. At increasing Zr contents, differences between nominal and deduced compositions become significant, indicating some Zr segregation in the triclinic phase. The substitution of Ti⁴⁺ by Zr⁴⁺ stabilizes the rhombohedral phase; however, electrical performances are not improved in expanded networks of Zr-rich samples. Below 300 K, activation energy of all samples is near 0.36 eV; however, above 300 K, activation energy is near 0.23 eV in Ti-rich samples and close to 0.36 eV in Zr-rich samples. The analysis of electrical data suggests that the amount of charge carriers and entropic terms are higher in Zr-rich samples; however, the increment of both parameters does not compensate lower activation energy terms of these samples. As a consequence of different contributions, the bulk conductivity of Zr-rich samples, measured at room temperature, is one order of magnitude lower than that measured in Ti-rich samples.     

Synthesis and near-infrared luminescence properties of LaOCl:Nd3+/Yb3+

Near-infrared emitting phosphors LaOCl:Nd³⁺/Yb³⁺ were prepared by the solid-state method, and their structures and luminescent properties were investigated by using X-ray diffraction and photoluminescence analysis, respectively. The studies shows that tetragonal LaOCl:Nd³⁺/Yb³⁺ can be synthesized by the solid-state reaction at 600 °C for 3 h. Upon 353 nm UV excitation, LaOCl:Nd³⁺/Yb³⁺ sample shows strong near-infrared emission lines in the region of 1060–1150 nm (corresponding to ⁴F_3/2 → ⁴I_J′ transition of Nd³⁺, J′ = 9/2, 11/2, 13/2, 15/2) and 980–1050 nm (corresponding to ²F_5/2 → ²F_7/2 transition of Yb³⁺). The decreasing emission intensity of Nd³⁺ with increasing doping concentration of Yb³⁺ proved the energy transfer in LaOCl:Nd³⁺/Yb³⁺. The possible near-infrared emission and energy transfer mechanism between Nd³⁺ and Yb³⁺, as well as the energy transfer efficiency of LaOCl:Nd³⁺/Yb³⁺ were discussed.     

Finite size effect and influence of temperature on electrical properties of nanocrystalline Ni–Cd ferrites

Electrical conductivity and dielectric measurements have been investigated for four different average grain sizes ranging from 3 to 7 nm of nanocrystalline Ni_0.2Cd_0.3Fe_2.5−xAl_xO₄ (0.0 ⩽ x ⩽ 0.5) ferrites. The impedance spectroscopy technique has been used to study the effect of grain and grain boundary on the electrical properties of the Al doped Ni–Cd ferrites. The analysis of data shows only one semi-circle corresponding to the grain boundary volume suggesting that the conduction mechanism takes place predominantly through grain boundary volume in the studied samples. The variation of impedance properties with temperature and composition has been studied in the frequency range of 120 Hz–5 MHz between the temperatures 300–473 K. The hopping of electrons between Fe³⁺ and Fe²⁺ as well as hole hopping between Ni³⁺ and Ni²⁺ ions at octahedral sites are found to be responsible for conduction mechanism. The dielectric constant and loss tangent (tan δ) are found to decrease with increasing frequency, whereas they increase with increasing temperature. The dielectric constant shows an anomalous behavior at selected frequencies, while the temperature increases, which is expected due to the generation of more electrons and holes as the temperature increases. The behavior has been explained in the light of Rezlescu model.     

Band overlap via chemical pressure control in double perovskite (Sr2−xCax)FeMoO6 (0 ⩽ x ⩽ 2.0) with TMR effect

The chemical pressure control in (Sr_2−xCa_x)FeMoO₆ (0 ⩽ x ⩽ 2.0) with double perovskite structure has been investigated systematically. We have performed first-principles total energy and electronic structure calculations for x = 0 and x = 2.0. The increasing Ca content in (Sr_2−xCa_x)FeMoO₆ samples increases the magnetic moment close to the theoretical value due to reduction of Fe/Mo anti-site disorder. An increasing Ca content results in increasing (Fe²⁺ + Mo⁶⁺)/(Fe³⁺ + Mo⁵⁺) band overlap rather than bandwidth changes. This is explained from simple ionic size arguments and is supported by X-ray absorption near edge structure (XANES) spectra and band structure calculations.     

Dielectric and piezoelectric properties of (Bi1−x−yNdxNa1−y)0.5BayTiO3 lead-free ceramics

New lead-free (Bi_1−x−yNd_xNa_1−y)_0.5Ba_yTiO₃ ceramics were prepared by a conventional ceramic technique and their dielectric and piezoelectric properties were studied. X-ray diffraction studies reveal that Nd³⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) of rhombohedral and tetragonal phases is formed at 0.04 < y < 0.10. The partial substitutions of Nd³⁺ and Ba²⁺ decrease effectively the coercive field E_c and increase significantly the remanent polarization P_r. Because of lower E_c, larger P_r and the formation of the MPB, the piezoelectric properties of the ceramics are significantly enhanced at x/y = 0.02/0.06: d₃₃ = 150 pC/N and k_p = 30.5%. The ceramics exhibit relaxor characteristic, which is probably resulted from the cation disordering in the 12-fold coordination sites. The depolarization temperature T_d shows a strong compositional dependence and reaches a minimum value at the MPB. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both the polar and non-polar regions near the depolarization temperature T_d, which cause the polarization hysteresis loop become deformed near/above T_d.     

Synthesis and characterisation of Cr3+-containing NASICON-related phases

Chromium-containing NASICON-related phosphates of the type Na_(1+x)Cr_xM_(2−x)P₃O₁₂) (M = Ti, Hf, Zr) have been synthesised by solid state reaction and structurally characterised by Rietveld refinement of the powder X-ray diffraction data. Materials of composition A_(1+x)/2Cr_xZr_(2−x)P₃O₁₂ (A = Cd, Ca, Sr), have also been prepared and characterised. The crystal structure of Na_(1+x)Cr_xM_(2−x)P₃O₁₂ corresponds to R-3c symmetry for x values ranging from 0.15 to 2.00, whereas compounds of composition A_(1+x)/2Cr_xZr_(2−x)P₃O₁₂ corresponding to R-3c are obtained when x ≤ 1.00 for Sr²⁺ and Ca²⁺, and x ≤ 1.50 for Cd²⁺. The polarizing effect of the two different metal ions A and M on the phosphorus atom and the P–O bond was studied by both ³¹P MAS NMR and infrared spectroscopy and shows that the electron density on the phosphorus, and thus the strength of the P–O bonds, are affected by both the interstitial (A) and the structural (M) metal ions.     

Defect interactions in La0.3Sr0.7Fe(M′)O3−δ (M′ = Al,Ga) perovskites: Atomistic simulations and analysis of p(O2)-T-δ diagrams

Atomistic modelling showed that a key factor affecting the p(O₂) dependencies of point defect chemical potentials in perovskite-type La_0.3Sr_0.7Fe_1−xM′_xO_3−δ (M′ = Ga, Al; x = 0–0.4) under oxidizing conditions, relates to the coulombic repulsion between oxygen vacancies and/or electron holes. The configurations of A- and B-site cations with stable oxidation states have no essential influence on energetics of the mobile charge carriers, whereas the electrons formed due to iron disproportionation are expected to form defect pair clusters with oxygen vacancies. These results were used to develop thermodynamic models, adequately describing the p(O₂)-T-δ diagrams of La_0.3Sr_0.7Fe(M′)O_3−δ determined by the coulometric titration technique at 923–1223 K in the oxygen partial pressure range from 1 × 10^− 5 to 0.5 atm. The thermodynamic functions governing the oxygen intercalation process were found independent of the defect concentration. Doping with aluminum and gallium leads to increasing oxygen deficiency and induces substantial changes in the behavior of iron cations, increasing the tendencies to disproportionation and hole localization. Despite similar oxygen nonstoichiometry in the Al- and Ga-substituted ferrites at a given dopant content, the latter tendency is more pronounced in the case of aluminum-containing perovskites.     

Synthesis of TiO2 nanoparticles and spectroscopic upconversion luminescence of Nd3+-doped TiO2–SiO2 composite glass

Nd³⁺-doped TiO₂–SiO₂ composites were prepared by sol–gel method. Optical properties such as radiative life-time (τ), stimulated emission cross-section (σ_p) and branching ratio (β) were calculated using Judd–Ofelt theory. Violet to blue upconversion emissions at 380 nm (⁴D_3/2→⁴I_11/2), 399 nm (²P_3/2→⁴I_11/2), 420 nm (²D_5/2→⁴I_9/2) and 452 nm (²P_3/2→⁴I_13/2) were obtained under 578 nm xenon-lamp excitation. The choice of 578 nm is justified by the absorption spectra of the same samples, which shows a strong absorption peak at 578 nm. This 578 nm excitation pump produces upconversion in Nd³⁺ by a sequential two-photon absorption process.     

Near infrared and charge transfer luminescence of trivalent ytterbium in KLa(PO3)4 powders

The polycrystalline powders of condensed polyphosphates KLa_(1 ? x)Yb_x(PO₃)₄ (x = 5, 10, 15, 20%) with linear chain were prepared by solid-state reaction. These samples were characterized by X-ray diffraction, FTIR and Raman scattering spectroscopies. The obtained powders are formed by single monoclinic phase of type III of condensed polyphosphate KLa(PO₃)₄ (KLP) crystallized with P2₁ space group. Lattice parameters varied as a function of the ytterbium concentration. As the Yb³⁺ concentration increased, the crystal lattice parameters were decreased. For the first time, near infrared (NIR) and UV–Visible spectroscopy of Yb³⁺ in KLa(PO₃)₄ powders, at room temperature, are carried out. In the IR range, a broad band relative to the fundamental ²F_5/2 → ²F_7/2 emission was registered. In the UV–Visible spectra, two bands typical of the Yb³⁺ charge transfer band (CTB) luminescence are observed. The registered decay times of these two emission types showed low sensibility to the Yb³⁺ concentration in KLa(PO₃)₄.     

Spin reorientation and magnetocrystalline anisotropy of (Nd1−xDyx) Fe14B

Spin reorientation and magnetocrytalline anisotropy of (Nd_1−xDy_x)₂Fe₁₄B (x=0.25, 0.5, 0.75) have been studied from mangetization curves of magnetically aligned powders. In (Nd_1−xDy_x)₂Fe₁₄B, the spin reorientation temperature (T_SR) decreases linearly on increasing Dy-substitution from 135 to 56 K with the ratio of ΔT_SR=−1.11 K/Dy at% in the composition range of 0⩽x⩽0.75. The spin reorientation angle at 4.2 K decreases on Dy-substitution from 30.4° at x=0 to 14.7° at x=0.75. From the investigation of the magnetocrystalline anisotropy at 4.2 K, the disappearance of the spin reorientation for compositions x≳0.85 is expected.     

Formation of nickel and nickel,zinc-bearing ferrite in aqueous suspension by air oxidation

Stoichiometric Ni-bearing ferrite was formed by air oxidation of an iron(II) hydroxide suspension at an initial Ni : Fe_tot mol ratio (r_Ni) of 0.20 : 2.80 at pH 10.0 and 65°C. Most of products formed at r_Ni=0.40 : 2.60 and 0.60 : 2.40 were Ni-bearing ferrites, of which vacancies of Fe³⁺ ion on the lattice points may be considered. Only Ni, Zn-bearing ferrites were formed in the suspensions at initial (Ni + Zn)  : Fe_tot mol ratios (r_{Ni + Zn}) of 0.20 : 2.80–0.60 : 2.40 at pH 10.0 and 65°C. At higher r_Ni or r_{Ni + Zn} by-products containing Ni, Fe and O₄²⁻ were formed. The formation of the by-products was depressed in the suspensions containing chloride ions in the place of sulfate ions.     

Micro Raman,Mossbauer and magnetic studies of manganese substituted zinc ferrite nanoparticles: Role of Mn

A series of Mn–Zn Ferrite nanoparticles (<15 nm) with formula Mn_xZn_1−xFe₂O₄ (where x=0.00, 0.35, 0.50, 0.65) were successfully prepared by citrate-gel method at low temperature (400 °C). X-ray diffraction analysis confirmed the formation of single cubic spinel phase in these nanoparticles. The FESEM and TEM micrographs revealed the nanoparticles to be nearly spherical in shape and of fairly uniform size. The fractions of Mn²⁺, Zn²⁺ and Fe³⁺ cations occupying tetrahedral sites along with Fe occupying octahedral sites within the unit cell of different ferrite samples are estimated by room temperature micro-Raman spectroscopy. Low temperature Mossbauer measurement on Mn_0.5Zn_0.5Fe₂O₄ has reconfirmed the mixed spinel phase of these nanoparticles. Room temperature magnetization studies (PPMS) of Mn substituted samples showed superparamagnetic behavior. Manganese substitution for Zn in the ferrite caused the magnetization to increase from 04 to18 emu/g and Lande's g factor (estimated from ferromagnetic resonance measurement) from 2.02 to 2.12 when x was increased up to 0.50. The FMR has shown that higher Mn cationic substitution leads to increase in dipolar interaction and decrease in super exchange interaction. Thermomagnetic (M–T) and magnetization (M–H) measurements have shown that the increase in Mn concentration (up to x=0.50) enhances the spin ordering temperature up to 150 K (blocking temperature). Magnetocrystalline anisotropy in the nanoparticles was established by Mossbauer, ferromagnetic resonance and thermomagnetic measurements. The optimized substitution of manganese for zinc improves the magnetic properties and makes these nanoparticles a potential candidate for their applications in microwave region and biomedical field.     

The studies on structural and thermal properties of delithiated LixNi1/3Co1/3Mn1/3O2 (0 < x ≤ 1) as a cathode material in lithium ion batteries

The structural and thermal properties of the delithiated Li_xNi_1/3Co_1/3Mn_1/3O₂ (0 < x ≤ 1) material have been investigated by using diffraction and thermoanalytical techniques such as XRD and TG-DSC methods. XRD result shows that the delithiated materials maintain the O3-type structure with defined stoichiometric number at the range of 0.24 < x ≤ 1, exhibiting good crystal structural stability. The cobalt and nickel ions in the delithiated materials change their valence state (i.e. Co³⁺ to Co⁴⁺ and Ni³⁺ to Ni⁴⁺) when x < 0.49; the irreversible changes of the transformation may affect the first cycle of charge–discharge efficiency of the materials. A comparison of the results of TG-DSC with TPD-MS shows that the irreversible change of oxygen species during the delithiation process of Li_xNi_1/3Co_1/3Mn_1/3O₂ have great influence on the structural and thermal stability and reversibility of the materials.     

Interfacial reactions of Ba2YCu3O6+z with coated conductor buffer layer,LaMnO3

Chemical interactions between the Ba₂YCu₃O_6+x superconductor and the LaMnO₃ buffer layers employed in coated conductors have been investigated experimentally by determining the phases formed in the Ba₂YCu₃O_6+x–LaMnO₃ system. The Ba₂YCu₃O_6+x–LaMnO₃ join within the BaO–(Y₂O₃–La₂O₃)–MnO₂–CuO_x multi-component system is non-binary. At 810 °C (p_O2 = 100 Pa) and at 950 °C in purified air, four phases are consistently present along the join, namely, Ba_2?x(La_1+x?yY_y)Cu₃O_6+z, Ba(Y_2?xLa_x)CuO₅, (La_1?xY_x)MnO₃, (La,Y)Mn₂O₅. The crystal chemistry and crystallography of Ba(Y_2?xLa_x)CuO₅ and (La_1?xY_x)Mn₂O₅ were studied using the X-ray Rietveld refinement technique. The Y-rich and La-rich solid solution limits for Ba(Y_2?xLa_x)CuO₅ are Ba(Y_1.8La_0.2)CuO₅ and Ba(Y_0.1La_1.9)CuO₅, respectively. The structure of Ba(Y_1.8La_0.2)CuO₅ is Pnma (No. 62), a = 12.2161(5) Å, b = 5.6690(2) Å, c = 7.1468(3) Å, V = 494.94(4) Å³, and D_x = 6.29 g cm^?3. YMn₂O₅ and LaMn₂O₅ do not form solid solution at 810 °C (p_O2 = 100 Pa) or at 950 °C (in air). The structure of YMn₂O₅ was confirmed to be Pbam (No. 55), a = 7.27832(14) Å, b = 8.46707(14) Å, c = 5.66495(10) Å, and V = 349.108(14) Å³. A reference X-ray pattern was prepared for YMn₂O₅.     

Crystal structure,thermal expansion and magnetic properties of Nd2Cu0.8Ge3 compound

A new ternary intermetallic compound, Nd₂Cu_0.8Ge₃, was synthesized and its crystal structure was determined by Rietveld refinement of X-ray powder diffraction data. The Nd₂Cu_0.8Ge₃ compound crystallizes in space group I4₁/amd (No. 141), with a tetragonal a-ThSi₂ structure type, and a=0.41783(2) nm, c=1.43689(9) nm, Z=2 and D_calc=7.466 g/cm³. Using the high temperature powder X-ray diffraction (HTXRD) technique, the lattice thermal expansion behavior of the compound was investigated in the temperature range of 298–648 K, and the result shows that its unit-cell parameters increased anisotropically when temperature increased. The magnetic susceptibility measured in the temperature range of 5–300 K indicated antiferromagnetic order of Nd₂Cu_0.8Ge₃ at low temperatures, and the magnetic susceptibility can be well described over the range of 50–300 K using Curie–Weiss law. The calculated effective magnetic moment (μ_eff) is 3.53 μ_B and dominated by the contribution of the Nd³⁺ ions.