Effects of Sr doping on the structure,magnetic properties and microwave absorption properties of LaFeO3 nanoparticles

Lightweight, broadband microwave absorbing materials, with strong absorption capacities, are an urgent demand for practical applications. The microstructural and microwave absorption properties of LaFeO₃ samples prepared by a sol-gel method using different amounts of Sr are investigated systematically. X-ray diffraction and Rietveld refinement studies showed that Sr²⁺ doping can distort the crystal structure of LaFeO₃, leading to lattice expansion and spin tilt of the Fe-O-Fe bond angle. The improvement of magnetic properties mainly originates from the synergistic effect of the bond angle spin tilt and crystal structure defects. Oxygen vacancies will be generated due to the fluctuations in the valence state of Fe³⁺ resulting from the substitution of La³⁺ by Sr²⁺ as deduced from the X-ray photoelectron spectroscopy analysis. The generation of oxygen vacancies, electronic hopping and polarization loss may be one of the main reasons for changes in the electromagnetic parameters. The minimum reflection loss (RL) of La_1–xSr_xFeO₃ nanoparticles with the Sr doping of 0.2 can reach approximately -39.3 dB at 10 GHz for the thickness of 2.2 mm, and the effective absorption bandwidth (RL ≤ -10 dB) can reach approximately 2.56 GHz. In addition, the La_1–xSr_xFeO₃ nanoparticles also can obtain better microwave absorbing performance in the C-band (4–8 GHz) with the minimum RL of -36.8 dB for the matching thickness of 3.0 mm and Sr content of 0.3. Consequently, La_1–xSr_xFeO₃ nanoparticles are promising materials for use in a high-performance and adjustable electromagnetic wave absorber, particularly in C-band and X-band.     

La1−xSrxFeO3 solid solutions in magnetic field

Magnetic properties of La_1-xSr_xFeO₃ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) have been investigated in a wide magnetic field range from 0 to 35 T using pulsed high magnetic field magnetization and electron spin resonance measurements. Experimental results show that both the non-magnetic Sr²⁺ dopant and high magnetic fields affect the stability of the robust antiferromagnetic coupling between Fe ions and induce new phase transitions. Especially the phase transition temperatures and hysteresis behavior show a high sensitivity on the Sr dopant and its doping level. On the other hand, a nonlinear Sr doping level dependence of the transition temperatures and hysteresis behavior were observed, indicating that the charge disproportionation is not the sole influence. Thus, in the series Sr doping La_1-xSr_xFeO₃ samples, the dopant induced FeO₆ octahedron distortion and charge disproportionation are coexistence and competition, which leads to a complicated and Sr doping level nonlinearly dependent magnetization behavior.     

Chemical looping combustion characteristics and kinetic behaviour of Sr-doped perovskite-type CaFeO3 oxygen carriers: Theoretical and experimental investigations

Oxygen carriers (OCs) with typical perovskite structures have attracted attention for use in chemical looping combustion (CLC) owing to their unique tunable structures and excellent performance. Thus, a further improvement in the reactivity and a deep understanding of the kinetic behaviour in CLC are highly desirable for such perovskite OCs. In this study, a series of Sr-doped perovskite-structured CaFeO₃ OCs (denoted as Sr_xCa_1−xFeO₃) were synthesized. The CLC characteristics, kinetic behaviour, and doping mechanism were systematically investigated via experiments and density functional theory (DFT) calculations. The activation energies of Sr_xCa_1−xFeO₃ OCs with various Sr contents were found to be in the range of 36.6–40.1 kJ/mol and lower than that of CaFeO₃ (62.7 kJ/mol), indicating that the Sr doping enhanced the reactivity of CaFeO₃. Among the OCs, Sr_0.4Ca_0.6FeO₃, which had the lowest activation energy and the fastest release of lattice oxygen, was regarded as the optimum OC. DFT calculations indicated that the reaction energy barrier of Sr_xCa_1−xFeO₃ (0.73–1.06 eV) was lower than that of CaFeO₃ (2.18 eV). This suggests that Sr doping and the regulation of the reaction pathways are essential drivers for enhancing the reactivity of Sr_xCa_1−xFeO₃, which affects the release of lattice oxygen and the morphological properties of OC particles.     

Role of A-site (Sr), B-site (Y), and A,B sites (Sr,Y) substitution in lead-free BaTiO3 ceramic compounds: Structural,optical, microstructure,mechanical, and thermal conductivity properties

Strontium and Yttrium-doped and co-doped BaTiO₃ (BT) ceramics with the stoichiometric formulas BaTiO₃, B_1-xSr_xTiO₃, Ba_1-xY_xTiO₃, BaTi_1-xY_xO₃, Ba_1-xY_xTi_1-xY_xO₃, and Ba_1-xSr_xTi_1-xY_xO₃ (x = 0.075) noted as BT, BSrT, BYT, BTY, BYTY, and BSrTY have been synthesized through sol-gel method. X-ray diffraction (XRD) patterns of the prepared ceramics, calcined at a slightly low temperature (950 °C/3h), displayed that BT, BSrT, and BYT ceramics possess tetragonal structures and BTY, BYTY, and BSrTY have a cubic structure. The incorporation of the Ba and/or Ti sites by Sr²⁺ and Y³⁺ ions in the lattice of BaTiO₃ ceramic and the behaviors of the crystalline characteristics in terms of the Y and Sr dopant were described in detail. The scanning electron microscopy (SEM) images demonstrated that the densification and grain size were strongly related to Sr and Y elements. UV–visible spectroscopy was used to study the optical behavior of the as-prepared ceramic samples and revealed that Sr and Y dopants reduce the optical band gap energy to 2.74 eV for the BSrTY compound. The outcomes also demonstrated that the levels of Urbach energy are indicative of the created disorder following the inclusion of Yttrium. The measurements of the thermal conductivity indicated the influence of the doping mechanism on the thermal conductivity results of the synthesized samples. Indeed, the thermal conductivity of BaTiO₃ is decreased with Sr and Y dopants and found to be in the range of 085–2.23 W.m^-1. K^?1 at room temperature and decreases slightly with increasing temperature from 2.02 to 0.73-W.m^-1. K^?1. Moreover, the microstructure and grains distribution of the BT, BSrT, BYT, BTY, BYTY, and BSrTY samples impacted the compressive strength, hence; the compressive strength was minimized as the grain size decreased.     

Effect of Y doping on transport properties of La0.8Sr0.2MnO3 polycrystalline ceramics

In this study, La_0.8-xY_xSr_0.2MnO₃ (LYSMO) polycrystalline ceramics were prepared by means of sol-gel technique using methanol as solvent. X-ray diffraction (XRD) showed all samples to possess standard perovskite structure. Scanning electron microscopy (SEM) revealed samples with high compactness and grain size from 27.80 to 29.73 μm. Resistivity–temperature tests indicated sharp metal-insulator transition behavior of all samples accompanied by rapid transformation from ferromagnetism to paramagnetism (FM-PM). As Y³⁺ doping amounts rose, radius of A-site ions decreased, metal-insulator transition temperature (T_p) of polycrystalline samples shifted to lower temperatures, and resistivity increased. Temperature coefficient of resistance (TCR) and magnetoresistance (MR) were affected by introduction of Y³⁺. At x = 0.06, peak TCR and peak MR reached 4.85% K⁻¹ and 52.34%, respectively. Using double exchange (DE) interaction mechanism, electric transport performances of as-prepared ceramics were explained. These findings look promising for future applications of LYSMO materials in magnetic devices and infrared detectors.     

Structure Stability and Microwave Dielectric Properties of (1 − x) Ba(Mg1/2W1/2) O3 + xBa(Y2/3W1/3)O3 Ceramics

The structure stabilities of double perovskite ceramics‐ (1 ? x) Ba(Mg_1/2W_1/2)O₃ + xBa(Y_2/3W_1/3)O₃ (0.01 ≤ x ≤ 0.4) have been studied by X‐ray powder diffraction (XRD), scanning electron microscopy (SEM), and Raman spectrometry in this study. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 8–11 GHz. The results showed that all the compounds exhibited face‐centered cubic perovskite structure. Part of Y³⁺ and W⁶⁺ cations occupied 4a‐site and the remaining Y³⁺ and Mg²⁺ distributed over 4b‐site, respectively, and kept the B‐site ratio 1:1 ordered. Local ordering of Y³⁺/Mg²⁺ on 4b‐site and Y³⁺/W⁶⁺ cations on 4a‐site within the short‐range scale could be observed with increasing Y‐doping content. The decomposition of the double perovskite compound at high temperature was successfully suppressed by doping with Y on B‐site. However, Ba₂Y_0.667WO₆ impurity phase appeared when x > 0.1. The optimized dielectric permittivity increased with the increase in Y doping. The optimized Q × f value was remarkably improved with small amount of Y doping (x ≤ 0.02) and reached a maximum value of about 160 000 GHz at x = 0.02 composition. Further increasing in Y doping led to the decrease in Q × f value. All compositions exhibited negative τ_f values. The absolute value of τ_f decreased with increasing Y‐doping content. Excellent combined microwave dielectric properties with ε_r = 20, Q × f = 160 000 GHz, and τ_f = ?21 ppm/°C could be obtained for x = 0.02 composition.     

A-site strontium doping effects on structure,magnetic, and photovoltaic properties of (Bi1−xSrx)FeO3−δ multiferroic ceramics

Raman spectroscopy, X-ray diffraction (XRD), magnetization hysteresis loop, synchrotron X-ray absorption spectroscopy, and photovoltaic effects have been measured in (Bi_1−xSr_x)FeO_3−δ (BFO100xSr) ceramics for x=0.0, 0.05, 0.10, and 0.15. Raman spectra and XRD reveal a rhombohedral R3c structure in all compounds. A-site Sr²⁺ doping increases fluctuations in cation-site occupancy and causes broadening in Raman modes. BFO15Sr exhibits a strong ferromagnetic feature due to reduction of FeOFe bond angle evidenced by the extended synchrotron X-ray absorption fine structure. The heterostructure of indium tin oxide (ITO) film/(Bi_1−xSr_x)FeO_3−δ ceramic/Au film exhibit clear photovoltaic (PV) responses under blue illumination of λ=405 nm. The maximal power-conversion efficiency and external quantum efficiency in ITO/BFO5Sr/Au are about 0.004% and 0.2%, respectively. A model based on optically excited charges in the depletion region between ITO and (Bi_1−xSr_x)FeO_3−δ can well describe open-circuit voltage and short-circuit current as a function of illumination intensity.     

Investigation of structural,optical, electrical,and magnetic properties of Fe-doped La0.7Sr0.3MnO3 manganites

In this work, the physical properties of nanocrystalline samples of La_0.7Sr_0.3Mn_1−xFe_xO₃ (0.0 ≤ x ≤ 0.20) perovskite manganites synthesized by the reverse micelle (RM) technique were explored in detail. The phase purity, crystal structure, and crystallite size of the samples were determined using X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. All the samples had rhombohedral crystal structure and crystallite size increased with increase in Fe content in La_0.7Sr_0.3MnO₃. The scanning electron micrographs (SEMs) exhibited smooth surface morphology and nonuniform shape of the particles. The optical properties studied using UV-visible absorption spectroscopy revealed a decrease in the absorbance and optical band gap with an increase in Fe content in La_0.7Sr_0.3MnO₃ compound. The temperature-dependent resistivity measurements revealed semiconducting nature of x = 0 and 0.1 samples up to the studied temperature range, while a metal-to-insulator transition was observed at higher Fe doping. Magnetic studies revealed weak ferromagnetism in all the samples and a reduction in the maximum magnetization with an increase in Fe content. A close correlation between electrical transport and magnetic properties was observed with the doping of Fe ion in La_0.7Sr_0.3MnO₃ at Mn site. These results advocate strong interactions associated with the double exchange mechanism among Fe³⁺ and Mn³⁺ ions.     

Structural,Raman, and Dielectric Studies on Multiferroic Mn‐doped Bi1−xLaxFeO3 Ceramics

Multiferroic Bi_1?xLa_xFeO₃ [BLFO (x)] ceramics with x = 0.10–0.50 and Mn‐doped BLFO (x = 0.30) ceramics with different doping contents (0.1–1.0 mol%) were prepared by solid‐state reaction method. They were crystallized in a perovskite phase with rhombohedral symmetry. In the BLFO (x) system, a composition (x)‐driven structural transformation (R3c→C222) was observed at x = 0.30. The formation of Bi₂Fe₄O₉ impure phase was effectively suppressed with increasing the x value, and the rhombohedral distortion in the BLFO ceramics was decreased, leading to some Raman active modes disappeared. A significant red frequency shift (~13 cm^?1) of the Raman mode of 232 cm^?1 in the BLFO ceramics was observed, which strongly perceived a significant destabilization in the octahedral oxygen chains, and in turn affected the local FeO₆ octahedral environment. In the Mn‐doped BLFO (x = 0.30) ceramics, the intensity of the Raman mode near 628 cm^?1 was increased with increasing the Mn‐doping content, which was resulted from an enhanced local Jahn–Teller distortions of the (Mn,Fe)O₆ octahedra. Electron microscopy images revealed some changes in the ceramic grain sizes and their morphologies in the Mn‐doped samples at different contents. Wedge‐shaped 71° ferroelectric domains with domain walls lying on the {110} planes were observed in the BLFO (x = 0.30) ceramics, whereas in the 1.0 mol% Mn‐doped BLFO (x = 0.30) samples, 71° ferroelectric domains exhibited a parallel band‐shaped morphology with average domain width of 95 nm. Dielectric studies revealed that high dielectric loss of the BLFO (x = 0.30) ceramics was drastically reduced from 0.8 to 0.01 (measured @ 10⁴ Hz) via 1.0 mol% Mn‐doping. The underlying mechanisms can be understood by a charge disproportion between the Mn⁴⁺ and Fe²⁺ in the Mn‐doped samples, where a reaction of Mn⁴⁺ + Fe²⁺→Mn³⁺ + Fe³⁺ is taken place, resulting in the reduction in the oxygen vacancies and a suppression of the electron hopping from Fe³⁺ to Fe²⁺ ions effectively.     

Structural,magnetic and photocatalytic properties of Sr-doped BiFeO3 nanoparticles based on an ultrasonic irradiation assisted self-combustion method

A novel ultrasonic irradiation assisted self-combustion method was developed to prepare single-phase Bi_1−xSr_xFeO_3−δ (BSFO) nanoparticles, which were charactered by XRD, SEM, TEM and UV–vis spectra. The results show that structure, as well as magnetic and photocatalytic properties of BSFO are influenced by the particle size and the Sr²⁺ dopant content. Regarding smaller particles, even if small amount of Sr²⁺ substitution content change can result in the phase transition from the rhombohedral distorted perovskite to the cubic. The doping of heterovalent Sr²⁺ ions in BiFeO₃ (BFO) nanoparticles improves the ferromagnetic property. As ultrasonication can generate particles with larger surface area and more defections, BSFO nanoparticles exhibit efficient photocatalytic activity as a promising photocatalyst.     

Enhanced magnetodielectric properties of Co2Z ferrite ceramics for ultrahigh frequency applications achieved via Cr3+ ion doping

Phase formation, microstructure, magnetic properties, and dielectric properties of Ba_1.5Sr_1.5Co₂Fe₍₂₃−_x)Cr_xO₄₁ (0.0 ≤ x ≤ 1.0) ceramics, in which Fe³⁺ ions were substituted by Cr³⁺ ions, were systematically investigated. X-ray diffraction results reveal that Z-type hexagonal ferrite was formed by sintering at 1250 °C, and Cr³⁺ ions successfully enter lattice without destroying crystal structure. Analysis of the microstructure reveals that Cr³⁺ ion doping has significant effect on crystal micromorphology. Samples with x = 0.4 have the most homogeneous micromorphology and the highest sintering density of 5.12 g/cm³. In addition, under the influence of external magnetic field, all samples exhibit typical soft magnetic character and hysteresis characteristics, with saturation magnetization up to 63.86 emu/g (x = 0.6). Particularly, compared with undoped sample, Cr-doped samples have outstanding magnetic–dielectric properties. Firstly, with increasing Cr³⁺ amount, real part of the permeability (μ′) reaches the maximum value of 10.70 at x = 0.4, while cutoff frequency exceeds 2 GHz, and Snoek constant reaches ∼19.50 GHz. Furthermore, due to more homogeneous microstructure, samples with x = 0.4 have low magnetic loss and can maintain high quality factor (Q) over a broad frequency range. Moreover, real part of the permittivity (ε′) reaches the maximum value of 16.90 at x = 0.6, and dielectric loss remains lower than 0.013 for frequencies below 0.7 GHz. Consequently, magnetic–dielectric materials prepared in this work are expected to have extensive application prospects for ultrahigh-frequency devices.     

Enhanced microwave absorption properties of Zn-substituted SrW-type hexaferrite composites in the Ku-band

Enhanced microwave absorption properties were successfully achievable from SrFe_2-xZn_xFe₁₆O₂₇ (SrFe_2-xZn_xW; x = 0.0, 0.5, 1.0, and 2.0) hexaferrite filler-epoxy resin matrix composites. The composite samples were fabricated with the filler volume fractions (V_f) of 30, 50, 70, and 90%. Compared with fully Zn-substituted SrZn₂W composite (x = 2.0), unsubstituted and partially Zn-substituted SrFe_2-xZn_xW (x = 0.0, 0.5, and 1.0) composites exhibited much higher real and imaginary parts of complex permittivity (ε_r), which is attributable to higher electron hopping between Fe²⁺ and Fe³⁺ ions, and also slightly higher real and imaginary parts of complex permeability (μ_r) due to higher saturation magnetization (M_s). Among all samples, a 2.8 mm-thick SrFe_1·5Zn_0·5W (x = 0.5) composite with the V_f of 90% exhibited the most appropriate for application in the region of 3.4–3.8 GHz, having the minimum reflection loss (RL_min) of ?46 dB at 3.6 GHz with the bandwidth of 0.43 GHz (3.38–3.81 GHz) below ?10 dB, while a 2.15 mm-thick SrFeZnW (x = 1.0) composite with the V_f of 70% showed the most appropriate for application in the region of 5.9–7.1 GHz, possessing the RL_min value of ?23.7 dB at 6.6 GHz with the bandwidth of 1.38 GHz (5.85–7.23 GHz) below ?10 dB. Consequently, partially Zn-substituted SrW-type hexaferrites are very promising microwave absorbers for 5G mobile communications in the Ku band (0.5–18 GHz).     

The magnetization reversal and high temperature dielectric response in Y1−xHoxFe0.5Cr0.5O3

The structural, magnetic, and dielectric properties of the Y_1−xHo_xFe_0.5Cr_0.5O₃ (x=0, 0.05, 0.1, 0.3, and 0.5) compounds have been investigated. Rietveld refinement of the XRD patterns shows that the compounds possess orthorhombic perovskite structure. The dual magnetization reversal is observed in the samples with x=0.05 and 0.1, and it vanishes when x≥0.3. Ferromagnetic-like behavior with large coercive fields is observed in all Ho³⁺ doped YFe_0.5Cr_0.5O₃ samples, indicating a doping induced metamagnetic behavior. This abnormal magnetization behavior can be explained by the antiparallel magnetic coupling between the Ho³⁺ and the canted Cr³⁺/Fe³⁺ moments, as well as the Ho–O–Ho magnetic interaction. The dielectric behavior in the frequency range from 100 Hz to 10 MHz is investigated. The low doped samples (x=0, 0.05, and 0.1) exhibit relaxation-like dielectric behavior and colossal dielectric constant in a wide temperature and frequency range. The dual magnetization reversal under low magnetic field makes these materials attractive candidates for the magnetic dual sensor devices.     

Band gap engineering and microwave dielectric properties evolution of mixed (Sr,La, Ce) TiMgO3 titanate–aluminate system

A mixed perovskite titanate-aluminate [(1-x)(Sr_0.6La_0.2Ce_0.2Ti_0.8Mg_0.2O₃)-xNdAlO₃ for x = 0.1 to 0.4] solid solution was successfully synthesized. X-ray diffraction patterns (XRD) and Rietveld refinement results indicated a stable perovskite phase with a cubic structure, in which Nd³⁺ occupies the A-site randomly while Al³⁺ occupies the B-site. No additional reflection spots (superlattice reflections) were detected in the HRTEM pattern (see SAED), confirming the cubic symmetry. All samples showed small Urbach tails, mainly due to compositional disorder. Microstructural analysis based on atomic force microscopy (AFM) showed no traces of impurity phases. For x = 0.4, excellent microwave dielectric properties (MWD) are obtained with a quality factor (Q × f) of 37,131 GHz at f = 5.2801 GHz, relative permittivity (ε_r) of 43, and temperature coefficient of resonant frequency (τ_f) of +1.3 ppm/°C. Variations in ε_r, Q × f values, and τ_f may be related to changes in relative density (ρ_rel), ion polarizability, optical band gap, and tolerance factor, respectively.     

Optimization of electromagnetic matching of Ba1-xCaxFe11.4Co0.6O19 (0.2 ≤ x ≤ 0.8) ceramics for microwave absorption within 2.6–18 GHz

The Ba_1-xCa_xFe_11.4Co_0.6O₁₉ (x = 0.0, 0.2, 0.4, 0.6, and 0.8) composites with wide-bandwidth and good absorption performance were prepared. The M_S of ceramics increases from about 62.4 to 83.4 emu g⁻¹ as x rises from 0 to 0.6, which demonstrates that the desirable magnetic properties of such ceramics is obtained by adjusting the content of Ca²⁺. A bandwidth of reflection loss (RL) below - 5 dB can be obtained for frequencies from 5.9 to 18 GHz with the Ba_0.4Ca_0.6Fe_11.4Co_0.6O₁₉ ceramic and a thickness of 2.0 mm, and a larger RL value of −34.1 dB is observed at 8.2 GHz for the Ba_0.6Ca_0.4Fe_11.4Co_0.6O₁₉ ceramic. These results suggest the developed ceramics could act as effective and wide-bandwidth microwave absorbing materials to meet commercial and military applications.     

Modification of the band gap of Ruddlesden‒Popper perovskites Srn+1TinO3n+1 (n=1, 2, 3, and ∞) by Fe ion irradiation doping

Ruddlesden?Popper (RP)-type perovskite materials Sr_n+1Ti_nO_3n+1 (n = 1, 2, 3, and ∞) have important application prospects in photocatalysis. However, their wide band gap limits their visible light absorption ability. Doping is an efficient method that can be used to reduce the band gap and improve its light absorption range. In this work, irradiation doping with the advantages of compulsivity and controllability, rather than a chemical doping method, is used to manipulate the band gaps of Sr_n+1Ti_nO_3n+1 to understand the respective contributions of the substitution of the Sr and Ti sites to the band gap reduction as a function of the n value. The solid-state reaction-prepared RP-type perovskites Sr_n+1Ti_nO_3n+1 (n = 1, 2, 3, and ∞) irradiated by a 270 keV Fe ion beam with 0–1 × 10¹⁶ ions/cm² doses were characterized by SEM, XRD and UV–vis diffuse reflectance spectra. Tauc plot analysis showed that the band gaps of Sr_n+1Ti_nO_3n+1 decreased with an increase in Fe doping dose, which was confirmed by DFT calculations. Meanwhile, the effects of the perovskite and rock-salt layers of the RP-type perovskite on the band gap modification were demonstrated, showing that the band gap of Sr_n+1Ti_nO_3n+1 with smaller n values decreases less when the Fe irradiation dose increases. This is explained by more salt SrO layers found in Sr_n+1Ti_nO_3n+1 for smaller n values, which prevents the substitution of Sr and Ti atoms in perovskite layers, significantly influencing the band gap.     

Optimization of room-temperature TCR of polycrystalline La0.9-xSrxK0.1MnO3 ceramics by Sr adjustment

High-density La_0.9-xSr_xK_0.1MnO₃ ceramics (LSKMO, A-site = La, Sr and K, 0 ≤ x ≤ 0.25) are successfully fabricated by using facile sol-gel method. Electrical properties are performed by using combination of phenomenological percolation (PP) model, double exchange (DE) mechanism, and Jahn-Teller (JT) effect. Moreover, X-ray diffraction and scanning electron microscopy are employed to analyze the structure and morphology of LSKMO ceramics. Valence states and ionic stoichiometry are assessed by using X-ray photoemission spectrometry. Results reveal that Sr²⁺ ions, substituting La³⁺ ions, significantly influenced DE mechanism and JT effect. In addition, Sr-doping plays essential role in improving electrical properties of LSKMO ceramics. At optimal doping content of x = 0.09, peak temperature coefficient of resistance (TCR) of the resistivity is found to be 11.56% K^?1 at 297.15 K, which is optimal TCR for A-site K-occupied perovskite manganese oxides. These results confirm that polycrystalline LSKMO ceramics render high room-temperature TCR values due to Sr-doping.     

Role of (La,Gd) co-doping on the enhanced dielectric and magnetic properties of BiFeO3 ceramics

The effect of the La³⁺ and Gd³⁺ co-doping on the structure, electric and magnetic properties of BiFeO₃ (BFO) ceramics are investigated. For the compositions (x=0 and 0≤y≤0.15) in the perovskite structured La_xGd_yBi_1−(x+y)FeO₃ system, a tiny residual phase of Bi₂Fe₄O₉ is noticed. Such a secondary phase is suppressed with the incorporation of ‘La’ content (x). The magnitude of dielectric constant (ε_r) increases progressively by increasing the ‘La’ content from x=0 to 0.15 with a remarkable decrease of dielectric loss. For x=0.15, the system La_xGd_yBi_1−(x+y)FeO₃ exhibits highest remanent magnetization (M_r) of 0.18 emu/g and coercive magnetic field (H_C) of ~1 T in the presence of external magnetic field of 9 T at 300 K. The origin of enhanced dielectric and magnetic properties of La_xGd_yBi_1−(x+y)FeO₃ and the role of doping elements, La³⁺, Gd³⁺ has been discussed.     

Oxygen nonstoichiometry in Pr1−xSrxCo0.2B0.8O3−δ (B = Mn,Fe, x = 0.2, 0.4) perovskite oxides

Thermogravimetric analysis was applied to the study of oxygen nonstoichiometry of perovskite oxides of the compositions Pr_0.8Sr_0.2Co_0.2Mn_0.8O_3−δ and Pr_1−xSr_xCo_0.2Fe_0.8O_3−δ (x = 0.2, 0.4). The measurements were performed in the temperature range from room temperature to 1200 °C in various atmospheres: oxygen, air and argon. The recorded weight loss corresponds to the loss of lattice oxygen. The magnitude of oxygen loss increased and the temperature at which oxygen loss became significant decreased with increasing Sr content. The loss of lattice oxygen became more significant as the oxygen partial pressure decreased. For the same pO₂ and level of Sr doping, the Fe-containing composition became more easily oxygen deficient than the corresponding Mn-containing one, suggesting that Fe is more resistant to oxidation from the trivalent to the tetravalent state than Mn. Electrical conductivity measurements, performed in air, showed that the temperature ranges at which conductivity decrease was observed, correspond fairly well with those ranges where weight loss was detected.     

Doping effect on secondary phases,microstructure and electrical conductivities of LaGaO3 based perovskites

The intermediate temperature electrolytes La_1?xSr_xGa_1?yMg_yO_3?δ (LSGM, where δ = (x + y)/2) with perovskite structure were prepared using a poly(vinyl alcohol) (PVA) solution polymerization method. Three secondary phases were identified by X-ray diffraction, LaSrGaO₄, LaSrGa₃O₇ and La₄Ga₂O₉. The relative amount of these secondary phases depended on the doping compositions. Sr doping produced more Sr rich secondary phases with increasing content, while enhanced solid solubility was observed with Mg addition. Sintered samples showed dense microstructures with well-developed equiaxed grains, and the secondary phases were mainly in the grain boundaries. LaSrGaO₄ could not be detected by SEM for the sintered pellets. The oxygen ionic conductivity was enhanced by doping with Sr and Mg. Mg doping showed the increased conductivity activation energy. La_0.8Sr_0.2Ga_0.9Mg_0.1O_2.85 had the highest ionic conductivity σ = 0.128 S/cm at 800 °C in this work.