Nanocrystalline La0 Research Articles

Magnetic exchange coupled composite behavior in the doped manganite nanoparticles: A proposed phenomenological model

In this paper, we comprehensively investigate the isothermal magnetization behavior of doped perovskite manganite nanoparticles. The focus is on understanding the impact of variation of particle sizes on the soft and hard magnetic phases with respect to the changes in the coercive field and remanent magnetization, both theoretically and experimentally. The study seeks to correlate experimental findings with the proposed phenomenological model to gain deeper insights into the underlying mechanisms governing exchange coupling and anisotropy effects in the nanocrystalline composites. The proposed phenomenological model beautifully demonstrates how the values of saturation magnetization and coercive field changes with changing the particle size in the nanocrystalline La0.48Ca0.52MnO3 (LCMO48) and La0.46Ca0.54MnO3 (LCMO46) compounds. In addition, the model provide an insights into the limitations of critical radius, size and shape of the nanocrystalline particle. This investigation looks into how the size of particles affects their magnetic properties, specifically coercive field and remanent magnetization.

Photocatalytic activity (dye degradation) of pristine and doped LaFeO3 (dopant Y at a site and Ni at B site) by impedance spectroscopy

Lanthanum iron oxide (LaFeO3) nanoparticles, doped with yttrium (Y) at the A-site and nickel (Ni) at the B-site, were synthesized using a sol-gel method at 600 °C. This study focuses on the synthesis of nanocrystalline La1−xYxFeO3 (Y doped LaFeO3), LaFe1−xNixO3 (Ni doped LaFeO3) and La1−xYxFe1−xNixO3 (Y/Ni doped LaFeO3) materials, where x = 0.01. The Synchrotron-based XRD analysis confirmed the single-phase formation of orthorhombic LaFeO3 (Pbnm space group) in each sample. The characterization through scanning electron microscopy and EDX revealed grains of irregular shape with an average particle size of around 200 nm. Optical properties were explored using UV–Visible spectroscopy, indicating a band gap of 1.7 eV for the pure sample. Additionally, the photocatalytic properties of the synthesized nanoparticles were evaluated for dye degradation. Electrical properties were systematically examined using impedance spectroscopy across a frequency range of 1 Hz–10 MHz at room temperature. The effect of doping on crystal structure and possible implications on dielectric and photocatalytic properties were studied. The complex impedance diagram displayed a depressed semicircle, suggesting non-Debye relaxation processes. A correlation was established between impedance and UV–Vis spectroscopy. However, the current literature lacks a comprehensive investigation for isovalent i.e. +3 substitution at A and B site simultaneously for dye degradation properties of LaFeO3-based materials. This study addresses this research gap by systematically exploring the influence of separate as well as simultaneous Y+3 and Ni+3 doping on the structural, optical, electrical, and photocatalytic characteristics of LaFeO3 nanoparticles, thereby providing valuable insights for the development of advanced functional materials for environmental applications. It was found that Y+3 dopants had a notable impact on the dye degradation properties for Mythlene blue dye at room temperature.

Synthesis of nano-crystalline K2NiF4-type phases La1+xSr1-xCoO4 (x = 0.0, 0.2, and 0.5): Effect of mixed valence state of Co on structural, magnetic and photocatalytic properties

A lot of work has already been reported on bulk K2NiF4-type layered perovskite oxides but hardly is known about their nano-crystalline state as they are generally prepared at high temperatures. In this connection, we have synthesized nanocrystalline La1+xSr1-xCoO4 layered perovskites oxides via sol-gel Pechini method. Efforts were also done to synthesize phases with x > 0.5 but remained unsuccessful. The Rietveld refinement investigation has confirmed the successful formation of single-phase compounds that crystallize in the I4/mmm space group with tetragonal symmetry. The X-ray photoelectron spectroscopy (XPS) analysis revealed that Co in La1+xSr1-xCoO4 has mixed valence states of +2 and + 3. Dominant anti-ferromagnetic (AFM) super-exchange Co2+―O―Co2+ and Co3+―O―Co3+ interactions were observed in all the synthesized nanophases and the existence of weak ferromagnetic (FM) interactions due to the generation of double-exchange Co2+―O―Co3+ interaction because of mixed valent state of Co. The increase in Néel temperature (TN) values with La doping has been attributed by the increased prevalence of anti-ferromagnetic (AFM) super-exchange Co2+―O―Co2+ and Co3+―O―Co3+ interactions. The band gap energy values (Eg) for all the prepared nano samples vary from 1.57 to 1.64 eV, rendering them highly competent catalysts for degradation of dyes. Additionally, the photocatalytic performance of the prepared nano photocatalysts was evaluated using cationic (MB & RhB) and anionic (MO) dye contaminants as well as their ternary mixture. LaSrCoO4 photocatalyst has been found to efficiently and swiftly degrade all three dye pollutants separately as well as in mixtures, with outstanding recyclability and durability. It has been observed that the x = 0.0 photocatalyst showed best degradation efficacy for RhB dye, i.e., ∼99 % in 100 min, compared to ∼91 % and ∼84 % for the x = 0.2 and 0.5 photocatalysts in the same time interval, respectively. These results suggest that LaSrCoO4 nano powder can be considered a promising photocatalyst in the dyes degradation.

Fully dense nanocrystalline (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 high-entropy ceramics fabricated under ultra-high pressure

In this work, nanocrystalline high-entropy ceramics (HECs) were prepared by the combination of combustion synthesis and ultra-high pressure sintering. The prepared high-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 ceramic powder had the average grain size of 11 nm and displayed the disordered defective fluorite structure. The HECs sintered under ultra-high pressure showed the defective fluorite structure, whereas the control samples fabricated by pressureless sintering showed the ordered pyrochlore structure. The HECs sintered under pressure of 10 GPa possessed much smaller grain size than that obtained by pressureless sintering. In particular, the grain size of ceramics sintered under 10 GPa at 600 °C was not significantly larger than that of raw powder and its Vickers hardness was 11.9 GPa. Ultra-high pressure sintering could remarkably increase the density of ceramics and restrain the growth of grains. The plastic deformation under ultra-high pressure was believed as the main densification mechanism for the grain refinement and performance improvement.

Microstructure evolution, dielectric response, and conduction mechanism of La1–xYxFeO3, (0 < x < 0.3) annealed perovskites synthesized via a sol–gel combustion technique

The structural and dielectric characteristics of nano-crystalline La1−xYxFeO3, (0 < x < 0.3) perovskite samples synthesized through the citrate auto-combustion route were investigated. The findings of X-ray diffraction (XRD) analysis revealed that the synthesized perovskites exhibited a polycrystalline orthorhombic structure with a space group Pnma. Furthermore, the tolerance factor was indicative of the phase stability of the orthoferrite series. Based on the refined XRD data, the size of the crystallites was estimated to be between 11.9 and 18.5 nm using size-strain plots (SSP). To gain more insight into the crystal structure, high-resolution transmission electron microscopy (HRTEM) was employed. The estimated size based on HRTEM calculations agreed with the XRD findings. The influence of annealing temperature on the frequency dependence of the dielectric response was thoroughly investigated for nano-crystalline La1−xYxFeO3 perovskites. The investigation took place at room temperature as well as different annealing temperatures, with a comprehensive frequency range of 50 Hz to ~ 2 MHz. The incorporation of Y3+ ions into LaFeO3 (LFO) has resulted in a significant enhancement of the dielectric constant values. This improvement is attributed to the successful integration of Y3+ ions, which have effectively modified the electrical properties of LFO. Furthermore, the annealing has been shown to enhance both the values of the dielectric constant, ε1(ω) and ε2(ω), which are critical parameters for electronic devices. Based on a power law response, the AC conductivity displays an increase with dopant concentration and frequency. Additionally, it has been noted that the activation energy value for LFOY30 is slightly reduced by 0.01 eV in comparison to pure LFO. The AC conductivity of LFOY30 was observed to increase with thermal annealing over the temperature range of 333 K to 463 K. This result confirms the occurrence of thermally activated conduction in the material. Based on this finding, LFOY30 is deemed appropriate for use in applications that require effective heat dissipation. The frequency exponent, s, decreases with increasing temperature from 0.87 to 0.56 for LFO and from 0.488 to 0.42 for LFOY30, respectively, which is associated with the correlated barrier hopping (CBH) model. The maximum barrier height (WM) was around 0.26 eV. The loss tangent (tanδ) variation was estimated as a function of frequency at different dopant concentrations and temperatures. Doping and annealing have been identified as appropriate measures for enhancing the electrical properties of the La1−xYxFeO3 system, thereby making it a useful material for electronic devices.

Effect of bismuth doping on structural, morphological, Griffiths-like phase, and magnetocaloric properties in La0.9−xBixBa0.1MnO3 (x = 0, 0.05, and 0.1)

In this recent work, we have carried out the structural, morphological, Griffiths-like phase, and magneto-caloric studies of sol–gel-synthesized, nanocrystalline manganite La0.9−xBixBa0.1MnO3 (x = 0, 0.05, and 0.1). The Rietveld analysis of X-ray diffraction patterns of nanocrystalline La0.9−xBixBa0.1MnO3 (x = 0, 0.05, and 0.1) confirm the single-phase nature and orthorhombic crystal structure with Pnma space group for all samples. The images acquired with a transmission electron microscope reveal the average particle sizes ranges from 32 nm to 65 nm. The temperature-dependent magnetization measurements, for 0.05 T external applied magnetic field, have revealed paramagnetic (PM) to ferromagnetic (FM) phase transition with transition temperatures (TC) ∼ 228 K, 219 K, and 233 K for x = 0, 0.05, and 0.1, respectively. The downward divergence of temperature dependence inverse susceptibility from the Curie-Weiss fit, indicates a pronounced Griffiths singularity in the samples. The positive slopes of Arrott plots for nanocrystalline La0.9−xBixBa0.1MnO3 (x = 0.05, 0.1) confirm the second-order magnetic phase transition. We have estimated the magnetic entropy change at different values of applied magnetic field for nanocrystalline La0.9−xBixBa0.1MnO3 (x = 0.05, 0.1) samples. Further, using this magnetic entropy data, we have also estimated the magnetic cooling capacity.

A-site K-doped lanthanum manganite nanocrystalline La0.67Ba0.33MnO3 for room-temperature micro-scale magnetic cooling

Bulk nanocrystalline La0.67Ba0.33−xKxMnO3 (with x = 0, 0.05, 0.1, and 0.2) manganites have been prepared by the modified sol-gel method (Pechini). The single-phase rhombohedral crystal structure with the R-3c (no. 167) space group was verified by X-ray diffraction (XRD) and sustained by Rietveld refinement. As follows from the results of XRD structural analyses, the increase in K-doping triggers an increase in the distortion of the MnO6 octahedra, which eventually causes the narrowing of the eg bandwidth. Mn is in a mixed valence state of Mn4+/Mn3+ as inferred by X-ray photoelectron spectroscopy. Magnetic measurements confirm that the Curie temperature decreases from 348 K for La0.67Ba0.33MnO3 to 316 K for La0.67Ba0.13K0.20MnO3. The increasing of the Mn4+ ion concentration at the B-site sublattice and A-site ionic disorder (σ2) breaks up the double exchange interaction between the Mn3+ and Mn4+ ions. The ferromagnetic to paramagnetic second-order magnetic phase transition at TC is also confirmed by electron paramagnetic resonance. According to magnetic field-dependent magnetization isotherms at different temperatures, La0.67Ba0.13K0.20MnO3 shows a relatively large magnetocaloric effect (1400 mJ cm−3 K−1 at 316 K under 5 T applied magnetic field), which raises the possibility of using this material for room-temperature micro-scale magnetic cooling.

Tailoring Multiferroic Characteristics in LaFeO3 Nanocrystals via Rare-Earth Pr3+ Doping

Multiferroic materials have sparked significant interest in the realm of materials science because of their potential impact on various device applications. This study focuses on the synthesis of nanocrystalline La1−xPrxFeO3 (LPFO) materials, where x can be either 0 or 0.5, using a solid-state technique. The aim is to gain insights into their structural, optical, dielectric, and magnetic properties. To confirm the chemical phase of the synthesized materials, X-ray diffractometer and Raman spectroscopy were employed. The outcome of the Rietveld analysis reveals that the LPFO crystallites exhibit orthorhombic symmetry with a Pbnm space group. The functional groups that were present in the LPFO samples were identified using FT-IR spectroscopic analysis. The morphological studies using scanning electron microscope and transmission electron microscope indicate that the synthesized samples exhibit excellent homogeneity with uniformly distributed grains. In order to investigate the dielectric constant (εʹ) and dielectric loss (tan δ) were examined as functions of temperature and frequency. Pr3+ dopants had a notable impact on the dielectric characteristics, particularly within the frequency span of 10 kHz–1 MHz and over the temperature variation of 40–160°C. The ambient temperature magnetic properties of the LPFO ferrite materials displayed antiferromagnetic behavior. Ultimately, this research reveals insightful information on the structural, optical, dielectric, and magnetic properties of the synthesized nanocrystalline LPFO materials, shedding light on their potential applications in the multifunctional devices.

Impact of Crystallite Size on Structural, Optical and Magnetic Characteristics of La0.7Sr0.15Ca0.15MnO3 Nanocrystalline

Nanocrystalline La0.7Sr0.15Ca0.15MnO3 (LSCMO) manganites were prepared by the combustion process and heated to various annealing temperatures (TA) to get various sized crystallites. The X-ray diffraction (XRD) patterns provided evidence that a Rhombohedral structure with space group was formed. Additionally, an increase in the size of the crystallites was observed, from 15.64 to 36.78nm, as the temperature (TA) increased from 700℃ to 1300℃. The FESEM micrographs revealed that homogeneous with porosity. The FTIR spectra showed five absorption peaks. The Optical energy gap of LSCMO nanocrystalline is decreased from 3.51 to 3.28 eV as annealed temperature raised, reveals that the LSCMO nanoparticles are semiconductor in nature. Room temperature Raman spectra of LSCMO nanoparticles demonstrate a notable reliance on annealing temperature. When the Raman modes were analysed with respect to TA, it was observed that the Raman vibrational phonon mode below 200cm-1 (A1g) and four modes (Eg) in the range 200-800cm-1 displayed significant displacements and widening, which were associated with oxygen sublattice distortion. Considerable changes were observed in both the intensity and full width half maximum (FWHM) of the five Raman modes as the annealing temperature increased. Magnetic behaviour using M-H loop at room temperature were measured by the Vibrating sample magnetometer revealed that gradation of saturation magnetization as the function of annealing temperature. Hence there is a remarkable crystallite size effect on optical and magnetic properties of LSCMO nanocrystallites.

Investigation of magnetic properties and colossal magnetoresistance in nanocrystalline doped manganite

Here in, we report structural, magnetic, and magneto-transport properties of nanocrystalline La0.6Ag0.2Bi0.2MnO3 prepared using citrate sol–gel method. By using scanning and transmission electron microscopy measurements, the morphology and particle size of the sample have been confirmed. The Mn2p x-ray photoelectron spectroscopy spectra revealed the nanoparticles contained the coexistence of Mn3+ and Mn4+ ions with Mn3+/Mn4+ ratio of 2:1. Field-cooled and zero field-cooled magnetization protocols with temperature span of 5 K–300 K, confirm the paramagnetic (PM) to ferromagnetic (FM) phase transition at critical temperature, ∼ 146 K. The complete investigation of isothermal magnetization (), Arrott plots, and magnetocaloric effect as well as quantitative analysis of second-order phase transition has been studied. The criticality at the PM-FM transition was examined for the sample, and the obtained critical exponents were verified for their reliability through the utilization of the scaling hypothesis and Kouvel-Fisher plot. We observed a large magnetic entropy change (∼7 J-Kg−1K−1) at upon 5 T magnetic field strength. The renormalized magnetic entropy change plots are found to collapse onto a single curve, thus verifying the universality of the sample. Above the metal–insulator transitions the electrical resistivity shows a small polaron hopping conduction mechanism, however, at low temperatures scattering mechanism dominates and the whole range was explained by the universal percolation model. The colossal value of negative MR is found to be 88% at 168 K under an applied field strength of 2 T. As a result of our experimental data, we can grasp the intuitive understanding of magnetic as well as transport properties in Bi-doped manganite systems potential for magnetic sensors and spintronics applications.

Grain-refining fabrication of nanocrystalline (La0.2Nd0.2Sm0.2Gd0.2Eu0.2)2Zr2O7 high-entropy ceramics by ultra-high pressure sintering

As an important A2B2O7-type ceramic, (La0.2Nd0.2Sm0.2Gd0.2Eu0.2)2Zr2O7 high-entropy pyrochlore possesses promising properties such as high melting point, high chemical durability, and low thermal conductivity. However, the low sintering ability limits its application in thermal barrier coating and radioactive waste immobilization. It usually needs long-term high-temperature soaking to achieve full density, but with inevitable grain growth. In this work, dense and grain-refined nanocrystalline (La0.2Nd0.2Sm0.2Gd0.2Eu0.2)2Zr2O7 ceramics were prepared with ultra-high pressure sintering (UHPS) method under 10 GPa at a low temperature of 800 °C. The densification behavior, microstructure evolution, and properties of the UHPS-ed samples were then investigated. The grain size of as-prepared (La0.2Nd0.2Sm0.2Gd0.2Eu0.2)2Zr2O7 ceramic was only 151 nm, which is 40% smaller than that of raw powder. In addition, it exhibited advantageous properties including both high hardness and aqueous durability. Plastic deformation under ultra-high pressure was believed as the dominant densification mechanism responsible for grain refinement and property improvement.

A comparative study of structural, magnetic and catalytic properties of half doped nanocrystalline La0.5A0.5Mn0.5Fe0.5O3 (A = Ca, Sr) perovskite oxides: Highly efficient and versatile candidates for enhanced oxidative degradation of hazardous organic dyes at ambient conditions

This work presents a systematic study of half doped nanocrystalline La0.5A0.5Mn0.5Fe0.5O3 (A = Ca, Sr) perovskite oxides to examine the effect of A-site cation radius on structural, magnetic and catalytic properties via different characterization techniques. Rietveld refinements of powder X-ray diffraction data confirmed the orthorhombic symmetry for both the samples with Pbnm space group. The results show an increase in lattice parameters and unit cell volume with doping of larger A cation. The BET surface area for Ca and Sr-doped samples has been found to be 32.3 m2/g and 33.8 m2/g respectively. The magnetic properties of nano structured samples show magnetic inhomogeneity due to coexistence of ferromagnetic and antiferromagnetic interactions. The magnetization has enhanced in Sr-doped sample which may be associated with increase in double exchange interactions between mixed valence manganese ions through oxygen. Moreover, the synthesized nano samples have been employed to catalytic degradation of rhodamine B dye as model pollutant under normal atmospheric conditions and it has been found that Sr-doped sample has shown better catalytic dye degradation than Ca-doped (99% degradation in 25 min) due to the presence of greater number of Mn3+/Mn4+ redox couples as determined by iodometric titration. Additionally, methylene blue and methyl orange dyes were also tested for catalytic degradation by more active Sr-doped catalyst in order to check its versability.

Controlling the Magnetic Properties of La0.9A0.1Mn0.9Cr0.1O3 (A: Li, K, Na) Powders and Ceramics by Alkali Ions Doping

Nanocrystalline La0.9A0.1Mn0.9Cr0.1O3 (A: Li, K, Na) powders have been synthesized by combustion method. The powders were used to prepare ceramics by high-pressure low-temperature sintering technique. For all samples the structure, elemental composition and morphology were studied using X-ray diffraction (XRD), Raman spectroscopy, Energy-Dispersive X-ray Spectroscopy (EDS) and Scanning electron microscopy (SEM). Magnetic properties were studied using magnetometry methods and the valency changes of the cations after alkali ions doping were studied using X-ray photoelectron spectroscopy (XPS). The influence of the sintering pressure on the structural and magnetic properties of the manganites doped with different alkali ions and chromium was also investigated. Magnetization properties were studied as a function of sintering pressure and type of the dopant. Chemical doping with alkali ions as well as external pressure significantly changed the magnetic properties of the compounds. It was found that the magnetic properties of the manganites could be predictably modified through the use of a suitable dopant element.

Solution combustion synthesis of nano crystalline La0.67 (Sr0.25Ca0.75)0.33MnO3 powder and its characterizations

Solution combustion synthesis of nano crystalline La0.67 (Sr0.25Ca0.75)0.33MnO3 powder and its characterizations

Observation of Griffith like phase and large magnetocaloric effect in nanocrystalline La0.7Ag0.2Bi0.1MnO3

In this paper, we present structural, magnetic, magnetocaloric, and critical study of perovskite La0.7Ag0.2Bi0.1MnO3 (LABMO) nanocrystalline compound synthesized by the sol–gel method. Temperature dependent magnetization measurements reveal the significant suppression of ferromagnetism in the LABMO sample upon Bi-doping on a La-site. The downturn in inverse magnetic susceptibility (χ−1) observed just above TC (236 K) in the paramagnetic regime corroborates the presence of short-range ferromagnetic correlations, which is the characteristic of the Griffith like phase below 270 K. The deviation from linear paramagnetic behavior in χ−1 implies the strong Griffith singularity. Furthermore, we have employed an integrated Maxwell's thermodynamic relation numerically and used isothermal magnetization data to determine the change in magnetic entropy at various magnetic fields. For a magnetic field change of 5 T, the value of maximum magnetic entropy change is found to be ∼6 J kg−1 K−1. We have also explored the critical behavior of the LABMO sample at transition temperatures using different theoretical models. The value of exponents β, γ, and δ does not fall into any known universality class. Despite this, the scaling relations show that interactions are renormalized around the Curie temperature (TC). The exponent n ≤ 2 extracted from field dependency on the magnetic entropy change confirms the second-order phase transition in LABMO.

Influence of phase boundary on magnetic and magnetocaloric properties of nanocrystalline La0.17Ca0.83MnO[formula omitted] sample

In this present study we report the Magnetic properties and magnetocaloric effect of two nanocrystalline La0.17Ca0.83MnO3 samples with average particle size ∼40 nm and ∼70 nm. According to the reported phase diagram, the bulk compound lies near the phase boundary between charge ordered antiferromagnetic and canted antiferromagnetic state. In contrast to the bulk counterpart, the significant effect of the phase boundary was observed on the physical properties of nanocrystalline compounds. Moreover, the signature of canted magnetic phase in nanocrystalline sample was probed through magnetocaloric effect study.

Effect of sintering on formation, structure, magnetic and dielectric properties of nanocrystalline La0.1Ca0.9MnO3 perovskite

Nanocrystalline La0.1Ca0.9MnO3 perovskite was synthesized employing a co-precipitation technique using sodium hydroxide to co-precipitate nanocrystalline perovskite. The sample sintered in temperature ranging from 473 to 1273 K. The effect of sintering on the formation of the perovskite material has been studied by X-Ray Diffraction (XRD) and Fourier-transform Infrared spectra (FTIR) to explore the structural features of the materials. The morphology and microstructure properties of perovskite materials were studied using Transmission Electron microscopy (TEM). The chemical formula prediction of the obtained perovskite and EDX data were analyzed. Both space charge zones at grain boundaries and the surface of the particle are responsible for decreasing the electrical conductivity at high frequency. All samples obey the paramagnetic regime and Ms enhanced with increasing Ts. Increasing the ratio of Ca to 90% reduces the Ms and Hc of such materials.

Correction to: Study of Critical Magnetic Behaviour in Nanocrystalline La0.65Ce0.05Sr0.3Mn1−xCuxO3 (x = 0, x = 0.05 and x = 0.15) Prepared by Pechini Method

Correction to: Study of Critical Magnetic Behaviour in Nanocrystalline La0.65Ce0.05Sr0.3Mn1−xCuxO3 (x = 0, x = 0.05 and x = 0.15) Prepared by Pechini Method

Mechanochemical Synthesis of Soft Ferromagnetic Perovskite Lanthanum Strontium Manganite (La0.6Sr0.4MnO3)

Nanocrystalline La0.6Sr0.4MnO3 sample with perovskite structure was successfully prepared by mechanochemical synthesis. Mechanical treatment for 10 h followed by annealing at 1100 °C results in the formation of single perovskite lanthanum strontium manganite (LSMO) phase with rhombohedral crystal structure and symmetry. The heat-treated sample crystallite size, calculated from X-ray diffraction pattern, was about 84 nm. The scanning electron microscope images indicated a high degree of fine structure. The formation of single perovskite phase was confirmed by FTIR measurements. The magnetic measurements showed the soft ferromagnetic behavior of the obtained sample. The saturation magnetization was found to be lower than that expected from theoretical calculation for perfect ferromagnetic ordering and lower than the saturation magnetization published for bulk sample. This was explained in the light of the core/shell model of manganite nanoparticles. The obtained sample was found to have a negative coefficient of magnetoresistance at room temperature of about (−3.5%) with the application of 25 kOe magnetic field.

Nanocrystalline high-entropy hexaboride ceramics enable remarkable performance as thermionic emission cathodes

The development of high-entropy borides with combined structural and functional performance holds untold scientific and technological potential, yet relevant studies have been rarely reported. In this work, we report nanocrystalline (La0.25Ce0.25Nd0.25Eu0.25)B6 high-entropy rare-earth hexaboride (HEReB6-1) ceramics fabricated through the high-pressure sintering of self-synthesized nanopowders for the first time. The as-fabricated samples exhibited a highly dense (96.3%) nanocrystalline (94 nm) microstructure with major (001) fiber textures and good grain boundaries without any impurities, resulting in a remarkable mechanical, electrical, and thermionic emission performance. The results showed that the samples possessed outstanding comprehensive mechanical properties and a high electrical resistivity from room temperature to high temperatures; these were greater than the average values of corresponding binary rare-earth hexaborides, such as a Vickers hardness of 23.4 ± 0.6 GPa and a fracture toughness of 3.0 ± 0.4 MPa•m1/2 at room temperature. More importantly, they showed high emission current densities at elevated temperatures, which were higher than the average values of the corresponding binary rare-earth hexaborides. For instance, the maximum emission current density reached 48.3 A•cm−2 at 1873 K. Such superior performance makes the nanocrystalline HEReB6-1 ceramics highly suitable for potential applications in thermionic emission cathodes.