Magnetic and phonon transitions in B-site Co doped BiFeO3 ceramics

Magnetic susceptibility and phonons have been characterized in multiferroic Bi(Fe_1−xCo_x)O_3−δ ceramics for x=0.0, 0.05, and 0.10 (BFO100xCo) as functions of temperature. A preferred (100) crystallographic orientation and increasing average oxygen vacancies were observed in BFO5Co and BFO10Co. The Fe and Co K-edge synchrotron X-ray absorptions revealed mixed valences of Fe³⁺, Fe⁴⁺, Co²⁺, and Co³⁺ ions in BFO5Co and BFO10Co, which exhibit a ferromagnetic (or ferrimagnetic) phase below room temperature due to appearance of ferromagnetic B–O–B (B=Fe and Co) superexchange interactions. Field–cooled (FC) and zero–field–cooled (ZFC) magnetic susceptibilities exhibit a significant spin-glass splitting below room temperature in BFO5Co and BFO10Co. Two Raman-active phonon anomalies at ~170 K (or 200 K) and ~260 K were attributed to the Fe³⁺–O–Co³⁺ and Co³⁺–O–Co³⁺ magnetic orderings, respectively. This work suggests that the low-spin Co²⁺–O–Co²⁺, Fe³⁺–O–Fe³⁺ (or Fe⁴⁺), and high-spin Co²⁺–O–Co²⁺ superexchange interactions are responsible for phonon anomalies at ~290 (or ~300 K), ~400, and ~470 K (or ~520 K) in BFO5Co and BFO10Co.     

(La,Sr)(Mn,Me)O3 manganites doped with d metals: Study of charge compensation mechanisms by crystallographic and magnetic characterizations

A comparison between theoretically calculated unit cell volume and interatomic distances in the system La_0.7Sr_0.3Mn_1−xMe_xO_3+δ (where Me = Cu, Fe, Cr, Ti) and the experimental data obtained by the full-profile Rietveld X-ray analysis as well as an analysis of magnetic properties allowed us to suggest possible mechanisms of charge compensation occurring when d metals substitute for manganese. It has been shown that in the case when copper, iron, chromium and titanium ions substitute for manganese ions in the system La_0.7Sr_0.3Mn_1−xMe_xO₃ charge compensation is described by the model 2Mn³⁺ → Mn⁴⁺ + Cu²⁺, Mn³⁺ → Fe³⁺, Mn³⁺ → Cr³⁺ and Mn⁴⁺ → Ti⁴⁺, respectively. In the latter case, a decrease in oxygen nonstoichiometry occurs with increasing x.     

Magnetic properties of Cu1+xMn2−xO4 and Ni1+xMn2−xO4 solid solutions

Magnetic properties of two spinel oxides solid solutions, Cu_1+xMn_2−xO₄ and Ni_1+xMn_2−xO₄, are reported. These series are characterized by two magnetic transitions: the upper one, of ferrimagnetic type, occurs at about 85 K (for copper-based) and at 105–110 K (for nickel-based spinels), independently of the x-content; the lower transition may be related to a Néel-type collinear ordering and takes place at 30 and 45 K, respectively. Application of moderate fields (H > 250 Oe) make both transitions to merge into one broad maximum in the magnetization, which takes place at lower temperature when applying larger fields. Magnetization cycles with temperature (ZFC/FC) or field (loops) allowed us to well characterize the ordered state. The effective moment follows the expected behavior when manganese ions are being substituted by ions of lower magnetic moment (Ni²⁺ and Cu²⁺).     

Novel 18-membered hexanuclear metallamacrocycle: Synthesis,crystal structure,and magnetic properties

The novel 18-metallacrown-6 metallamacrocycle, with the formula of [Mn₆(amshz)₆(CH₃OH)₆]·6H₂O (amshz = N-acetyl-3-methyl-salicylhydrazide), has been prepared and characterized. Six Mn(III) ions and six deprotonated N-acetyl-3-methyl-salicylhydrazide (amshz^3?) ligands construct a planar 18-membered ring based on Mn–N–N–Mn linkage. Due to the coordination, the ligand enforces the stereochemistry of the Mn³⁺ ions as a propeller shape with alternating …ΔΛΔΛ… configurations. The magnetic properties of the metallacrown molecules are characterized by a weak antiferromagnetic exchange interaction, with μ_eff = 5.12 μ_B at 300 K between the Mn³⁺ ion spins with S = 2 in the cyclic system.     

Blue-violet ceramic pigments based on Co and Mg Co2−xMgxP2O7 diphosphates

Solid solutions of Co and Mg diphosphates with compositions Co_2?xMg_xP₂O₇ (x = 0, 0.1, 0.2, 0.3, 0.5, 0.7, 1.0, 1.5 and 1.8) have been prepared and characterized for the first time as alternative low-toxicity blue ceramic pigments. The compositions were prepared through the conventional coprecipitation route and calcined up to 1000 °C/2 h. Samples were characterized by thermal analysis, XRD, SEM/EDX, UV–vis-NIR spectroscopy and colour measurements (CIE-L^*a^*b^*). Isostructural Co_2?xMg_xP₂O₇ diphosphate solid solutions (monoclinic system and P21/c spatial group) formed successfully within the studied range of compositions, accompanied only by a minor quantity of residual Co or Mg orthophosphates (M₃(PO₄)₂). Interestingly, the obtained solid solutions developed nice blue-violet colourations even with high Mg doping after enamelling within double-firing (x = 1.5–1.8) and single-firing (x = 1.0–1.5) ceramic glasses. These optimal compositions containing a minimized Co amount (measured values around 7–16 wt%) could be therefore less toxic alternatives to the conventional Co₃(PO₄)₂ blue ceramic pigment.     

Microstructure and electrical transport properties of Ca3Co4−xMnxO9+δ ceramics fabricated under two-step external magnetic field

A series of Ca₃Co_4−xMn_xO_9+δ (x=0, 0.25 and 0.5) samples were prepared under two-step external magnetic field during both processes of sol–gel and pre-pressing. The effects of Mn doping and external magnetic field on the microstructure and electrical transport properties of the samples have been studied systematically. The investigated results show that Mn-doping can improve the electrical transport properties slightly. After using external magnetic field, the textured structures of all the samples are enhanced dramatically, which results in the obvious decrease of the electrical resistivity and increase of Seebeck coefficient. The difference of ρ between the x=0.5 samples without and with external magnetic field is about 24.73 mΩ cm at room temperature. All the samples with magnetic field show higher power factors (PF), for example, the PF at x=0.5 with magnetic field can reach the maximum value of 0.29 mW/m K² at 1073 K.     

Dynamical behavior of step-like transition of La0.5Sr0.5Mn1−xTixO3 in a widened field sweep rate

The magnetization behavior of Ti⁴⁺ doped perovskite manganites La_0.5Sr_0.5Mn_1−xTi_xO₃ (x=0.15, 0.175 and 0.2) was investigated. Experimental results show that Ti⁴⁺ dopant suppresses the antiferromagnetic charge ordering and leads to a step-like magnetization behavior below 3 K. The step-like transitions and the critical magnetic fields are strongly dependent on the Ti doping level, magnetic field sweep rate and temperature. Above 3 K, the step-like transitions transform to broad ones. In addition, under pulsed high magnetic fields with an ultrafast field sweep rate of ~10³ T/s, the sharp step-like transitions observed in the static magnetization measurements become smooth metamagnetic transition at low temperatures. This feature is correlated with the collapse of the balance between the magnetic energy and elastic energy in the phase separation system within a martensitic-like scenario.     

Remarkable effect of Co and Mn on the activity of Fe3 − xMxO4 promoted oxidation of organic contaminants in aqueous medium with H2O2

In this work substituted magnetites Fe_3 − xMn_xO₄ (x=0.21, 0.26 and 0.53), Fe_3 − xCo_xO₄ (x=0;0.19;0.38;0.75) and Fe_3 − xNi_xO₄ (x=0;0.10;0.28;0.54) have been used to promote two different reactions involving H₂O₂: (i) the oxidation of organic molecules namely phenol, hydroquinone and methylene blue dye in aqueous medium and (ii) the peroxide decomposition to O₂. The presence of Co or Mn in the magnetite structure strongly increased the rate of H₂O₂ decomposition and the oxidation of the organic molecules whereas the presence of Ni inhibited both reactions. Kinetic data and CEMS Mössbauer spectroscopy suggest that the H₂O₂ decomposition and the organic oxidation are competitive reactions involving oxidizing species generated by surface M²⁺ (M=Fe, Co or Mn).     

Role of spin-glass phase for magnetoresistance enhancement in nickel substituted lanthanum calcium manganite

Effect of Ni substitution in lanthanum calcium manganite (LCMO) has been investigated for change in magnetoresistance (MR). Scanning electron microscopy images revealed decrease in grain size from 3.72 µm to 0.55 µm by Ni substitution. Maximum increase in MR has been found 28% at low temperature (100 K) for x=0.10, Ni substitution at Mn site. Metal insulator transition temperature has been decreased from 253.2 K for x=0.0–90 K for x=0.10. Above x=0.10, Ni substitution no metal-insulator transition temperature appeared due to the presence of porosity in the samples. Ni substitution lowered the magnetic transition temperature from 255 K for x=0.0–125 K for x=0.25. Lowering of irreversible temperature (T_irr) from 250 K for x=0.0–135.4 K for x=0.20 has been obtained by zero field cooled (ZFC) and field cooled (FC) measurements confirm reduction of ferromagnetic clusters and spin-glass phase like behavior due to Ni presence. The spin-glass phase presence allows spin-polarized tunneling even at low magnetic field, which ultimately results in enhancement of MR at low temperature. Core level X-ray photoelectron spectroscopy measurements confirm Ni²⁺ charge state of Ni ions and increase in Mn⁴⁺/Mn³⁺ ratio with increasing Ni content. Increase in resistivity and weakening of ferromagnetism with Ni substitution at Mn site has been observed due to the reduction in grain size and dilution of double exchange interaction.     

Spark plasma sintering of MgAl2O4–YCr0.5Mn0.5O3 composite NTC ceramics

New high temperature negative temperature coefficient (NTC) thermistor ceramics based on a xMgAl₂O₄–(1 − x)YCr_0.5Mn_0.5O₃ (x = 0.1, 0.4, 0.6) composite system have been successfully fabricated through spark plasma sintering (SPS) with a low sintering temperature and a short sintering period. The X-ray diffraction analysis indicates that the SPS-sintered composite ceramics consist of a cubic spinel MgAl₂O₄ phase and an orthorhombic perovskite YCr_0.5Mn_0.5O₃ phase isomorphic to YCrO₃. The SPS-sintered composite ceramics have high relative density ranging from 94.1 to 97.4% of the theoretical density. X-ray photoelectron spectroscopy analysis corroborates the presence of Cr³⁺, Cr⁴⁺, Mn³⁺, and Mn⁴⁺ ions on lattice sites, which may result in the hopping conduction. The obtained ρ₂₅, B_25–150, and B_700–1000 of the SPS-sintered composite NTC thermistors are in the range of 1.53 × 10⁶–9.92 × 10⁹ Ω cm, 3380–5172 K, and 7239–9543 K, respectively. These values can be tuned by adjusting the MgAl₂O₄ concentration.     

Preparation and characterization of Co2+ substituted Li–Dy ferrite ceramics

Stoichiometric compositions of ferrites with the chemical formula Li_0.5?0.5xCo_xFe_2.4?0.5xDy_0.1O₄ with x=0, 0.25, 0.5, 0.75, 1.0 were prepared by the standard double sintering ceramic method. X-ray diffraction analysis confirmed the cubic spinel structure of the prepared samples. The structural, morphological and magnetic properties were studied by X-ray diffraction, infra-red spectroscopy (IR), scanning electron microscopy (SEM), vibrating sample magnetometry (VSM) and ac susceptibility measurements. Lattice constant, grain size and density increase whereas porosity decreases with the increase in Co²⁺ substitution. IR measurements show the characteristic ferrite bands. Spectral absorption bands were observed in IR spectroscopic analysis at ν₁=564?601 cm^?1, ν₂=486?519 cm^?1 and ν₃=551?578 cm^?1. The cation distribution estimated by the X-ray diffraction is supported by magnetization and susceptibility studies. The saturation magnetization decreases from 44.25 to 17.14 emu/g whereas coercivity remarkably increases from 240.69 to 812.14 emu/g with increasing Co²⁺ substitution. The mechanisms involved are discussed.     

Dependence of dielectric properties on structural characteristics of (Zn1/3A2/3)0.5 (Ti1−xBx)0.5O2 (A = Nb5+, Ta5+, B = Ge4+, Sn4+) ceramics at microwave frequencies

The effects of structural characteristics on the dielectric properties of (Zn_1/3A_2/3)_0.5(Ti_1?xB_x)_0.5O₂ (A = Nb⁵⁺, Ta⁵⁺, B = Ge⁴⁺, Sn⁴⁺) (0.1 ≤ x ≤ 0.3) ceramics were investigated at microwave frequency. The sintered specimens showed solid solutions with a tetragonal rutile structure within the solid solution range of compositions. With an increase of BO₂, the temperature coefficient of resonant frequency (TCF) and dielectric constant (K) decreased with a decrease of oxygen octahedral distortion and dielectric polarizabilites, respectively. However, the quality factor (Qf) of the sintered specimens was increased with BO₂ due to the reduction of Ti⁴⁺ ions. The Qf value of the specimens with A = Ta was higher than that of the specimens with A = Nb.     

Hydrothermal synthesis,characterization and magnetic properties of a new 2D arsenic–vanadate layers supported cobalt coordination complex: [Co(en)2(H2O)]{[Co(en)2]2As8V14O42(SO4)} · 3H2O

The title compound [Co(en)₂(H₂O)]{[Co(en)₂]₂As₈V₁₄O₄₂(SO₄)} · 3H₂O (1) (en = ethylenediamine) has been hydrothermally synthesized and characterized by single crystal X-ray diffraction, IR spectrum and thermogravimetric analysis. The single crystal diffraction analysis shows that compound 1 exhibits {[Co(en)₂]₂As₈V₁₄O₄₂(SO₄)}n²ⁿ⁻ layers act as ligand of coordination cations [Co(en)₂]²⁺, and water molecules are inserted in the layers. The magnetic measurement indicates a weak antiferromagnetic exchange coupling within the molecular unit, which is deemed to attribute to the contribution of V^iv and Co²⁺.     

Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method

Mn²⁺-doped Sn_1−xMn_xP₂O₇ (x = 0–0.2) are synthesized by a new co-precipitation method using tin(II)oxalate as tin(IV) precursor, which gives pure tin pyrophosphate at 300 °C, as all the reaction by-products are vaporizable at <150 °C. The dopant Mn²⁺ acts as a sintering aid and leads to dense Sn_1−xMn_xP₂O₇ samples on sintering at 1100 °C. Though conductivity of Sn_1−xMn_xP₂O₇ samples in the ambient atmosphere is 10⁻⁹–10⁻⁶ S cm⁻¹ in 300–550 °C range, it increases significantly in humidified (water vapor pressure, pH₂O = 0.12 atm) atmosphere and reaches >10⁻³ S cm⁻¹ in 100–200 °C range. The maximum conductivity is shown by Sn_0.88Mn_0.12P₂O₇ with 9.79 × 10⁻⁶ S cm⁻¹ at 550 °C in ambient air and 2.29 × 10⁻³ S cm⁻¹ at 190 °C in humidified air. It is observed that the humidification of Sn_1−xMn_xP₂O₇ samples is a slow process and its rate increases at higher temperature. The stability of Sn_1−xMn_xP₂O₇ samples is analyzed.     

A novel three-dimensional coordination polymer containing tetranuclear [Co4(μ3-OH)2] building blocks: Synthesis,crystal structure and magnetic behavior of {[Co2(btc)(μ3-OH)(H2O)2] · H2O}n

A cobalt coordination polymer {[Co₂(btc)(μ₃-OH)(H₂O)₂] · H₂O}_n 1 (btc = 1,2,4-benzenetricarboxylate) was synthesized hydrothermally and structurally characterized by single-crystal X-ray diffraction method. In 1, the six-coordinated Co(II) ions are held together by two bridging hydroxides to form a tetranuclear [Co₄(μ₃-OH)₂] unit, which are connected further through btc³⁻ ligands in a new μ₇-bridging mode to give a porous three dimensional framework. The study of magnetic properties indicated dominant antiferromagnetic coupling interactions between the adjacent Co(II) sites.     

Electronic conductivity,oxygen permeability and thermal expansion of Sr0.7Ce0.3Mn1−xAlxO3−δ

The maximum solubility of aluminum cations in the perovskite lattice of Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ is approximately 15%. The incorporation of Al³⁺ increases oxygen ionic transport due to increasing oxygen nonstoichiometry, and decreases the tetragonal unit cell volume and thermal expansion at temperatures above 600 °C. The total conductivity of Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ (x = 0–0.2), predominantly electronic, decreases with aluminum additions and has an activation energy of 10.2–10.9 kJ/mol at 350–850 °C. Analysis of the electronic conduction and Seebeck coefficient of Sr_0.7Ce_0.3Mn_0.9Al_0.1O_3−δ, measured in the oxygen partial pressure range from 10⁻¹⁸ to 0.5 atm at 700–950 °C, revealed trends characteristic of broad-band semiconductors, such as temperature-independent mobility. The temperature dependence of the charge carrier concentration is weak, but exhibits a tendency to thermal excitation, whilst oxygen losses from the lattice have an opposite effect. The role of the latter factor becomes significant at temperatures above 800 °C and on reducing p(O₂) below 10⁻⁴ to 10⁻² atm. The oxygen permeability of dense Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ (x = 0–0.2) membranes, limited by both bulk ionic conduction and surface exchange, is substantially higher than that of (La, Sr)MnO₃-based materials used for solid oxide fuel cell cathodes. The average thermal expansion coefficients of Sr_0.7Ce_0.3Mn_1−xAl_xO_3−δ ceramics in air are (10.8–11.8) × 10⁻⁶ K⁻¹.     

Dielectric and magnetoelectric properties of BaTiO3–Co0.6Zn0.4Fe1.7Mn0.3O4 composite

In this paper we studied the structural, dielectric, magnetic and magnetoelectric properties of (x)BaTiO₃–(1 ? x)Co_0.6Zn_0.4Fe_1.7Mn_0.3O₄ particulate composite series where x = 0.50, 0.60 and 0.70. BaTiO₃–Co_0.6Zn_0.4Fe_1.7Mn_0.3O₄ composite has the advantage of being non-toxic and environmental friendly from the point of view of device fabrication. High ME voltage coefficients were obtained in the whole series with the highest value of α_E ～ 73 mV/cm Oe achieved in sample x = 0.50 containing equal mole fractions of both the component phases. This value of α_E is an order of magnitude higher than that of particulate sintered BaTiO₃–CoFe₂O₄ composites (～2–4 mV/cm Oe). Dielectric characteristics for these samples indicated two anomalies: (i) one at low temperature close to ferroelectric to paraelectric transition temperature of pure BaTiO₃ and (ii) another at higher temperature related to the magnetic transition in ferrite, a characteristic dielectric feature of composite sample.     

Nanocrystalline/nanosized manganese substituted nickel ferrites – Ni1−xMnxFe2O4 obtained by ceramic-mechanical milling route

Manganese substituted nickel ferrites, Ni_1−xMn_xFe₂O₄ (x=0, 0.3, 0.5 and 0.7) have been obtained by a combined method, heat treatment and subsequent mechanical milling. The samples were characterised by X-ray diffraction, differential scanning calorimetry and magnetic measurements. The increase of the Mn²⁺ cations amount into the spinel structure leads to a significant expansion of the cubic spinel structure lattice parameter. The crystallite size decreases with increasing milling time up to 120 min, more rapidly for the nickel–manganese ferrites with a large amount of Mn²⁺ cations (x=0.7). After only 15 min of milling the mean crystallites size is less than 25 nm for all synthesised ferrites. The Néel temperature decreases by increasing Mn²⁺ cation amount from 585 °C for x=0 up to 380 °C for x=0.7. The magnetisation of the ferrite increases by introducing more manganese cations into the spinel structure. The magnetisation of the milled samples decreases by increasing milling time for each ratio among Ni and Mn cations and tends to be difficult to saturate, a behaviour assigned to the spin canted effect.     

The effect of Co doping on the magnetic and magnetocaloric properties of Pr0.7Ca0.3Mn1−xCoxO3 manganites

In this work, we report the effect of low amount of cobalt doping at the Mn-site on the magnetic and magnetocaloric properties of Pr_0.7Ca_0.3Mn_1−xCo_xO₃ (0≤x≤0.1) powder samples. Our samples, elaborated using the solid–solid reaction method at high temperature, are single phase and crystallize in the orthorhombic system with Pnma space group. While the parent compound Pr_0.7Ca_0.3MnO₃ exhibits a charge order state at low temperature, the substituted samples with low amount of cobalt exhibit a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature T_C increases with Co content from 105 K for x=0 to 116 K for x=0.1. The maximum values of the magnetic entropy change $\left|\mathrm{\Delta }{S}_{\mathrm{M}}^{\max }\right|$ are found to be 0.8 J/kg K, 2.2 J/kg K, 3.1 J/kg K and 3.2 J/kg K in a magnetic field change of 5 T for x=0, 0.02, 0.05 and 0.1 respectively. The maximum value of the relative cooling power RCP is found to be 378.2 J/kg in the Pr_0.7Ca_0.3Mn_0.95Co_0.05O₃ at 5 T. This value of RCP is about 92% of that obtained in gadolinium metal, known as one of the most important materials for magnetic refrigeration, at the same magnetic field change of 5 T.     

First bistable monocobalt complex with six free nitrogen donors: 3,5-Di-tert-butylbenzoquinonatocobalt complex containing triethanolaminetriisonicotinate

A novel cobalt-benzoquinone complex containing triethanolaminetriisonicotinate as a coligand, [Co(3,5-dbq)₂(tati)₂] (3,5-dbq = 3,5-di-tert-butyl-1,2-benzoquinone; tati = triethanolaminetriisonicotinate) is consisted of a valence tautomeric mixture of [Co^II] and [Co^III]. The tati coligand acts as a monodentate, resulting in the formation of discrete trans-[Co(3,5-dbq)₂(tati)₂] with six free nitrogen donors.