4f Sites Research Articles

Doping effects in strontium hexaferrite: Theoretical and experimental analysis

Hexagonal ferrites based on SrFe12O19 (SFO) are vital materials in the field of permanent magnets, valued for their cost-effectiveness and reliable magnetic properties, making them a more affordable alternative to rare-earth permanent magnets. This study focuses on understanding the doping effects of commonly used elements in SFO system. DFT calculation results show that doping La or La-Co can enhance the phase stability of SFO, in which Co has a preference for the 4f1 sites of Fe. The total magnetic moment was observed to slightly decrease with La doping, whereas it exhibited an increasing trend with La-Co and La-Ca-Co co-doping and the largest increase of 6.5 % can be obtained in Sr2LaCaFe44Co4O76 (SLCFCO). The increase in the total magnetic moment is mainly attributed to the smaller antiferromagnetic moment of Co than that of Fe on 4f1 sites, as well as the reduction in the antiferromagnetic Fe moments at 4f1 and 4f2 sites. The total density of states indicates a transition from semiconductor behavior to a metallic character both in the La-Co and La-Ca-Co doped versions. The experimental results validate the theoretical predictions, confirming enhancements in magnetic performance. The saturation magnetization increased from 63.86 Am2/kg in SrFe11.7O19 (SFO) to 67.53 Am2/kg in Sr0.4La0.4Ca0.2Fe10.5Co0.5O19 composition. These findings provide a vital understanding of tuning the magnetic and electronic properties in doped SFO, which will enable the development of advanced applications in magnetic materials.

Study of the magnetic and structural properties of BaFe12 – xCuxO19 ferrites obtained by hydrothermal synthesis

Hexagonal ferrites of M-type (in particular, BaFe12O19) are magnetic materials with functional characteristics affected both by chemical composition and technology of their synthesis. We present the results of studying the magnetic and structural properties of BaFe12 – xCuxO19 hexaferrites (x = 0, 0.1, 0.2, 0.3, 0.4) obtained by hydrothermal synthesis with partial substitution of copper for iron. The composition of the synthesized samples was analyzed using X-ray diffraction, and the magnetic characteristics were measured using a vibration magnetometer. It has been revealed that the coercivity of the ferrite powders depends non-monotonically on the copper concentration and reaches the maximum (5629 Oe) and minimum (4698 Oe) values at x = 0 and x = 0.2. The presence of copper reduces the coercive force, but at the same time the values remain rather high compared to the results of similar studies. The saturation magnetization of the obtained ferrites gradually decreases (from 65.88 to 60.75 A · m2/kg at x = 0 and x = 0.4, respectively) with increasing. The distribution of Cu over ferrite sublattices was studied using Mössbauer spectroscopy. It is shown that in the hexaferrite structure, copper ions preferentially occupy 12k and 4f1 sites. Hence, a decrease in the saturation magnetization with increasing x is most likely attributed to the presence of side non-magnetic phases observed on X-ray diffraction patterns. It is also revealed that during synthesis, copper participates in the formation of low-melting phases on the surface of hexaferrite grains which promotes agglomeration of the particles. Thus, the resulting powders can potentially be sintered at lower temperatures and, therefore, without a significant increase in the size of crystallites. Herewith, the coercivity retains its original high values. The results obtained can be used in developing ferrite permanent magnets with improved characteristics.

Occupation distribution analysis of Co in La-Co substituted M-type hexaferrite and its significant effect on magnetic properties

In recent years, La-Co co-substituted M-type strontium hexaferrite (La-Co SrM) has gained significant attention due to its improved permanent magnet performance and elevated operating temperature. However, the occupation of Co ions within the lattice of La-Co SrM has sparked considerable debate over the past two decades. To address this, neutron powder diffraction and Raman spectroscopy were employed in this study to analyze Co occupancy in La-Co SrM single crystal and polycrystalline samples. Results consistently reveal a preference for most Co ions to occupy the spin-down tetrahedral 4f1 site, which enhances the net magnetic moment and uniaxial magnetic anisotropy. Differences in Co ion distribution between single crystals (octahedral 12k and 2a sites) and polycrystalline powders (bipyramidal 2b sites) are attributed to sample preparation conditions, often overlooked in previous discussions. Therefore, precise regulation of Co occupancy, particularly maximizing the occupation of the 4f1 site, proves pivotal in enhancing magnetic properties.

Influence of Ba content on magnetic properties of the Sr0.4-xBaxLa0.4Ca0.2Fe10.8Co0.2O19 hexaferrite from recycled rare-earth tailings

We synthesized Sr0.4-xBaxLa0.4Ca0.2Fe10.8Co0.2O19 (x = 0, 0.01, 0.02, 0.03, 0.04, 0.05) through a solid-phase reaction. In this study, the strontium-barium mixed eutectic powders derived from rare earth tailings were used as alternative sources of strontium and barium sources to synthesize M-phase hexaferrite. All the samples have a single M-phase hexaferrite phase and magnetoplumbite structure. The magnetic properties and their correlation with the structure were investigated through magnetic measurements and Mössbauer spectroscopy. The sample with x=0.03 has the optimal magnetic properties at room temperature. This enhancement can be attributed to the combined substitution of Sr2+-Ba2+ ions affecting the 4 f1 site, thereby positively influencing the magnetic properties. The coercivity of the permanent magnet component prepared from the magnetic powder with optimal magnetic properties reaches 356 kA/m. This work provides the possible applications of the ferrite derived from rare-earth tailings.

Theoretical Study of the Magnetic Properties of the SmFe12−xMox (x = 1, 2) and SmFe10Mo2H Compounds

We present theoretical investigations examining the electronic and magnetic properties of the SmFe12−xMox (x = 1, 2) and SmFe10Mo2H compounds, including magneto-crystalline anisotropy, magnetic moments, exchange-coupling parameters, and Curie temperatures. The spin-polarized fully relativistic Korringa–Kohn–Rostoker (SPR-KKR) band structure method has been employed, using the coherent potential approximation (CPA) to deal with substitutional disorder. Hubbard-U correction was applied to the local spin density approximation (LSDA+U) in order to account for the significant correlation effects arising from the 4f electronic states of Sm. According to our calculations, the total magnetic moments increases with H addition, in agreement with experimental data. Adding one H atom in the near-neighbor environment of the Fe 8j site reduces the magnetic moments of Fe 8j and enhances the magnetic moment of Fe 8f. For every investigated alloy, the site-resolved spin magnetic moments of Fe on the 8i, 8j, and 8f sites exhibit the same magnitude sequence, with msFe (8i) > msFe (8j) > msFe (8f). While the addition of H has a positive impact on magneto-crystalline anisotropy energy (MAE), the increase in Mo concentration is detrimental to MAE. The computed exchange-coupling parameters reveal the highest values between the closest Fe 8i spins, followed by Fe 8i and Fe 8j spins, for all investigated alloys. The Curie temperature of the alloys under investigation is increased by decreasing the Mo concentration or by H addition, which is qualitatively consistent with experimental findings.

Effect of post-annealing on magnetic anisotropy in La–Co co-substituted M-type strontium ferrite

We investigated the effect of post-annealing on the magnetic anisotropy in La–Co co-substituted magnetoplubite-type strontium ferrite, a matrix phase of commercial high-performance ferrite magnets. Post-annealing was found to be effective in controlling the distribution of substituted Co2+. The Co2+ distribution obtained by lower-temperature annealing shows higher magnetic anisotropy, suggesting that uniaxial magnetic anisotropy is enhanced only when Co2+ occupies the most stable occupied site, i.e., the 4f1 site in the tetrahedral coordination. Furthermore, the slower the cooling rate, the higher the anisotropy. The temperature-dependent Co2+ distribution predicted from the reported density functional theory calculations was compared with the actual anisotropy after annealing, yielding +2.3 meV/ion as the Co2+ anisotropy constant at the tetrahedrally coordinated site and −1.2 meV/ion as that at the octahedrally coordinated sites. Careful selection of heat treatment conditions leads to efficient use of cobalt, an element with supply risks.

Mean-field model for BaFe[formula omitted]Ti[formula omitted]O19 ([formula omitted] 0 and 0.2) hexaferrite: Structural and magnetic properties

An experimental and theoretical investigation of the magnetic properties of Ti-doped barium hexaferrite (BaM) is conducted. Polycrystalline samples of BaFe12−xTixO19 with x≤ 0.2 were synthesized using the solid-state reaction method. X-ray diffraction (XRD) analyses, along with Rietveld refinement, confirmed the presence of BaFe12O19 as the predominant BaM phase and a minor amount of hematite in all samples. Structural analysis revealed a slight increase in lattice parameters a and c for x=0.2. A result mostly related to the preferential occupation of Ti4+ ions at the 4f2 and 12k sites, coupled with the reduction of Fe3＋ ions to Fe2＋. It was also found that the magnetic response of samples is affected by Ti4+ substitution. Saturation magnetization slightly decreased from ∼ 45 (x= 0) to ∼ 44 emu/g (x= 0.2), the anisotropy constant from ∼4.08× 105 to ∼4.06× 105 erg/g2, while coercivity slightly increased from ∼ 2456 to ∼ 2468 Oe, and the anisotropy field from ∼ 18.2 to ∼ 18.6 kOe. The magnetic features of the samples were analyzed in terms of a proposed model based on the mean-field theory. Results indicated that the substitution of Ti4+ ions resulted in a decrease in the magnetic moment of the 2b sublattice. Such behavior is influenced by its closest neighboring sites 12k and 4f2 sublattices, which are the preferred occupancy sites for the Ti4+ cations.

Crystal, electronic structure and hydrogenation properties of the Mg5.57Ni16Ge7.43 cluster phase with a new type of polyhedron.

The ternary germanide Mg5.57Ni16Ge7.43 (cubic, space group Fm-3m, cF116) belongs to the structural family based on the Th6Mn23-type. The Ge1 and Ge2 atoms fully occupy the 4a (m-3m symmetry) and 24d (m.mm) sites, respectively. The Ni1 and Ni2 atoms both fully occupy two 32f sites (.3m symmetry). The Mg/Ge statistical mixture occupies the 24e site with 4m.m symmetry. The structure of the title compound contains a three-core-shell cluster. At (0,0,0), there is a Ge1 atom which is surrounded by eight Ni atoms at the vertices of a cube and consequently six Mg atoms at the vertices of an octahedron. These surrounded eight Ni and six Mg atoms form a [Ge1Ni8(Mg/Ge)6] rhombic dodecahedron with a coordination number of 14. The [GeNi8(Mg/Ge)6] rhombic dodecahedron is encapsulated within the [Ni24] rhombicuboctahedron, which is again encapsulated within an [Ni32(Mg/Ge)24] pentacontatetrahedron; thus, the three-core-shell cluster [GeNi8(Mg/Ge)6@Ni24@Ni32(Mg/Ge)24] results. The pentacontatetrahedron is a new representative of Pavlyuk's polyhedra group based on pentagonal, tetragonal and trigonal faces. The dominance of the metallic type of bonding between atoms in the Mg5.57Ni16Ge7.43 structure is confirmed by the results of the electronic structure calculations. The hydrogen sorption capacity of this intermetallic at 570 K reaches 0.70 wt% H2.

Magnetic properties and coupled spin‒phonon behavior of M‒type Sr1-xSmxFe12O19 (0 ≤ x ≤ 0.15) nanoparticles

We thoroughly investigated the effects of Sm³⁺ doping on the magnetic, structural, hyperfine, and vibrational properties of Sr1-xSmxFe12O19 (0 ≤ x ≤ 0.15) nanoparticles, which were synthesized using the proteic sol-gel process. The Rietveld refined XRD patterns confirmed the presence of M-type SrFe12O19, alongside a minor fraction of α-Fe2O3. As Sm³⁺ doping increased, the unit cell volume fluctuated, decreasing from 691.73 ų at x = 0 to 690.67 ų at x = 0.10 and slightly rising thereafter. The incorporation of Sm³⁺ induced a conversion of Fe3+ to Fe2+, affecting lattice parameters and crystallinity. At x = 0.10, maximum crystallite size (56.48 nm) and X-ray density (4.83 g/cm³) were attained, alongside decreased microstrain, indicating improved crystallinity and reduced defects. However, at x = 0.15, an increase in microstrain suggested lattice instability. FTIR confirmed that bands in the range 400–600 cm⁻1 might be due to the stretching vibration of oxygen and metal (Fe–O) ions, confirming the formation of hexaferrite. Temperature-dependent Raman spectroscopy studies confirmed the presence of spin-phonon coupling in all samples in the ferrimagnetic transition at ∼725-735 K. Mössbauer spectroscopy elucidated that the substitution of Sr2+ by Sm3+ induced the most pronounced effect on the 12k, 4f2, and 2a sites, corroborating the evolution of the lattice constants and the fact that these three sites have Sr sites in their close vicinity. The room-temperature hysteresis loops exhibited narrow features, indicative of soft magnetic behavior. Saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) varied with Sm3+ doping content (x). Ms initially decreased from 62.62 emu/g and 22.03 emu/g at x = 0.00 to 54.96 emu/g and 19.63 emu/g at x = 0.10, respectively, and then increase to 59.77 emu/g and 21.86 emu/g at x = 0.15. Conversely, Mr and Hc showed a non-monotonic trend with x, suggesting complex interactions between dopant concentration and magnetic properties.

Site-selective cobalt substitution in La–Co co-substituted magnetoplumbite-type ferrites: 59Co-NMR and DFT calculation study

The La–Co co-substituted magnetoplumbite-type (M-type) ferrites AFe12O19 (A = Ca, Sr and Ba, ion sizes Ca2+ < Sr2+ < Ba2+) with Co compositions around 0.2 have been subjected to 59Co-NMR. The results show that Co occupies the 4f1, 2a and 12k sites, and that the smaller the A ion, the more Co tends to occupy the 4f1 minority spin site, which is effective in enhancing both uniaxial anisotropy and magnetisation. First-principles total energy calculations based on density functional theory (DFT) of undoped AFe12O19 and a supercell ( 2×2×1 of the unit cell) in which 1/96 of Fe3+ is replaced by Co2+ were performed to predict the stable structure and Co occupancy sites. The results show that regardless of A, Co is most stable when it occupies the 4f1 site, followed by the 2a and 12k sites with energy differences on the order of 100 meV, and Co practically does not occupy the 2b and 4f2 sites. As the A ion becomes smaller, the energy difference when Co occupies each Fe site tends to increase, and the Co occupancy of the 4f1 site also increases. The site selectivity of Co can be roughly explained as a result of the difference in uniaxial strain along the c-axis associated with the difference in A. However, the influence of the A ion differs between the R and S blocks and the local strain also has a secondary effect on the Co distribution. Based on these results, the guidelines for improving the performance (anisotropy and magnetisation) of La–Co co-substituted M-type ferrite magnets with a limited amount of Co can be summarised as follows: It is effective to select as small A ions as possible and to post-anneal at low temperature or cool slowly to concentrate Co at the 4f1 site in tetrahedral coordination.

Investigating the preferential sites of Co2+ substitution in SrCoxFe12−xO19 using Mössbauer spectroscopy

In this paper, a series of M-type hexagonal SrCoxFe[Formula: see text]O[Formula: see text] ([Formula: see text], 0.25, 0.5) samples have been prepared by sol–gel method. X-ray diffraction (XRD) showed that the samples produced were all p63/mmc space groups of single hexagonal crystals, and the lattice parameters a, b and c did not show the expected trend with increasing x. This was related to the valence changes and lattice distortion caused by the substitution of Co[Formula: see text] at the 2a and 4f2 sites. Room temperature Mössbauer spectra studies showed that when [Formula: see text], Fe[Formula: see text] ions were not uniformly distributed, preferring the 4f1, 4f2 and 2a sites; when [Formula: see text], Co[Formula: see text] ions preferentially displaced the 2a and 4f2 sites and the presence of Fe[Formula: see text] was detected; when [Formula: see text], Co[Formula: see text] ions preferentially displaced the 4f1 and 2b sites and the percentage of Fe[Formula: see text] occupancy was reduced, with a greater occupancy ([Formula: see text] occurring at the 12k site, indicating a preference of Fe[Formula: see text] of the doped sample for this site. This work will provide an important reference for the variation of properties induced by different ion doping substitutions of the Fe sites in the hexagonal ferrite family.

Microstructure parameter-dependent non-collinear magnetic structures in scandium-doped M-type hexaferrite nanocrystals.

The quest for materials with non-collinear magnetic structures has been driven by their unique properties and potential applications in advanced spintronics and data storage technologies. In this study, we investigate the induction of a non-collinear conical state in BaFe12O19 (M-type) nanocrystal fibers through the substitution of Fe3+ ions with diamagnetic Sc3+ ions. This substitution introduces an additional parameter for tuning the magnetic structure and allows precise control over the substitution amount. We demonstrate that the non-collinear conical state remains stable within a temperature range of 125 K to 325 K and can be finely adjusted by varying the Sc3+ substitution amount. The selective occupancy of Sc3+ ions at the 2a, 4f2, and 2b sites within the M-type ferrite lattice weakens the super-exchange interaction between Fe1, Fe2, and Fe5 ions. This weakening disrupts interactions between different blocks S/R (R*/S*) and stabilizes the conical state. These findings highlight a significant approach to modulating non-collinear magnetic structures in hexagonal ferrites, with implications for both fundamental research and practical applications in the development of novel magnetic materials.

Synthesis of a new potassium-substituted lead fluorapatite and its structural characterization.

Prismatic crystals of partially potassium substituted lead fluorapatite Pb5.09Ca3.78K1.13(PO4)6F0.87 were grown through a solid-state reaction. The structural study conducted by single-crystal X-ray diffraction revealed that the compound crystallizes in the hexagonal P63/m space group, with unit cell parameters a = b = 9.7190(5) Å, c = 7.1700(6) Å and V = 587.37(7) Å3(Z = 1), as well as final values amounting to R and wR of 0.0309 and 0.0546, respectively. The structural refinement demonstrated that Pb occupies both the (6h) and (4f) structural sites of hexagonal fluorapatite, K occupies the (6h) site, and Ca is placed on the (4f) site. Powder X-ray diffraction study indicated the absence of additional phases or impurities. Chemical analysis using atomic absorption spectrometry and energy-dispersive X-ray spectroscopy confirmed the expected chemical formula. The electrical conductivity measured over a wide temperature range was found to be governed by the ion mobility mechanism in the tunnel along the c axis (probably attributed to the fluorine ion located there). We, therefore, could infer from the analysis of the complex impedance spectra that the electrical conductivity of our apatite depends essentially on the temperature and frequency, which produces a relaxation phenomenon and semiconductor-like behavior. Moreover, the strong absorption in the UV-Visible region was substantiated through studies of the optical properties of the developed sample. Fluorescence spectra exhibited emissions in the orange regions when excited at 375 nm. The findings of the phenomena resulting from the emission and conduction of the apatite in question suggest its potential for application in various technological fields such as photovoltaic cells, optoelectronics, photonics, LED applications, catalysis and batteries.

Machine Learning Approach for the Material Search of Fluoride-Ion Conductors

Fluoride-ion conductors are key materials for realizing all-solid-state fluoride batteries, which theoretically exhibit high energy densities of 4,400 Wh L–1 using Cu cathode and LaF3 anode1-2. To date, a high fluoride-ion conductivity has been reported for limited compounds, such as fluorite-type MSnF4 (M = Pb or Ba)3, tysonite-type Ln 1−x AEx F3−x (Ln = La–Sm, AE = Ca, Sr, or Ba)4, and ASn2F5 (A = Na, K, Rb, or NH4)5. However, the conductivities of these structures can be improved only to a limited extent, necessitating the search for novel phases in multielement systems6. Material search in a multielement system is cost- and effort-intensive due to the extremely large compositional space. In this study, we used regression learning of the ionic conductivities based on compositional descriptors aiming to accelerate the material search for fluoride-ion conductors. Training data on ionic conductivity at room temperature for 317 crystalline or amorphous fluorides were collected from the literature. Descriptors were prepared using only compositional properties (e.g., polarizability, ionic radius, and electronegativity). Regression learning using lasso, ridge, elastic net, support vector machine, and random forest were performed. Fig. 1 shows the correlation between the measured and predicted conductivities from random forest regression. Random forest yields the highest R 2 values of 0.96 and 0.85 for the training and test data, respectively. The highest permutation importance was given for the descriptor of the cation polarizability. Highly polarizable cations such as Bi3+, Sb3+, Pb2+, Sn2+, Tl+, Ag+, Cs+, and Rb+ are essential for achieving a high fluoride-ion conductivity. Moreover, the mean of melting point of metal fluorides and mean of M–F binding energy exhibit high importance scores, suggesting that a weak interaction between the metal cation and fluoride ion is preferred for fluoride-ion conduction. The predictive performance was experimentally verified, and the correlation between the measured and predicted ionic conductivities of the BaF2–SnF2–LaF3 (or KF) pseudo-ternary system was demonstrated. The ionic conductivities of the experimentally obtained samples, as illustrated as green circles in Fig.1, suitably match the predicted values. During experimental verification, new tysonite-type Ba0.2Sn0.8F2 was identified in the SnF2-rich composition region, where a high ionic conductivity was predicted. Rietveld analysis was performed using synchrotron X-ray diffraction (SXRD) data. Refinement was conducted with space group of P63/mmc. Barium and tin simultaneously occupy the 4e site with occupancies of 0.2 and 0.8, respectively, and form a hexagonal close-packed structure. Fluoride ions occupy two distinct sites, 2b and 4f, which adopt three- and four-fold coordination by M 2+ ions (M = Ba0.2Sn0.8), respectively. The occupancies of the fluoride ions at the 2b and 4f sites are 0.626 and 0.687, respectively, suggesting that numerous vacancies are randomly distributed at both sites, which is a unique feature of Ba0.2Sn0.8F2. The composition-based model for the prediction of the ionic conductivity enables to ascertain the compositional regions with high predicted ionic conductivities; consequently, it reduces the cost of material search and accelerates the discovery of fast fluoride-ion-conducting materials.Acknowledgement: This presentation is based on results obtained from a project, JPNP21006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).Reference: (1) M. A. Reddy, et al., J. Mater. Chem., 21, 17059 (2011). (2) F. Gschwind, et al., J. Fluorine Chem., 182, 76 (2016). (3) G. Denes, et al., Solid State Ionics, 13 , 213 (1984). (4) C. Rongeat, et al., ACS Appl. Mater. Inter. 6 , 2103 (2014). (5) J. P. Battut, et al., Solid State Ionics, 22 , 247 (1987). (6) K. Suzuki, et al., J. Mater. Chem. A, 8 , 11582 (2020).Figure 1. Predicted versus measured logarithm of the ionic conductivity. Red, blue, and green circles correspond to the training, test, and experimental data of the newly synthesized compounds. The black line represents the ideal line, i.e., where the predicted and measured conductivities are equal. Figure 1

Prussian Blue Analogues as Positive Electrodes for Mg Batteries: Insights into Mg2+ Intercalation.

Potassium manganese hexacianoferrate has been prepared by co-precipitation from manganese (II) chloride and potassium citrate, with chemical analysis yielding the formula K1.72 Mn[Fe(CN)6 ]0.92 □0.08 ⋅ 1.1H2 O (KMnHCF). Its X-ray diffraction pattern is consistent with a monoclinic structure (space group P 21 /n, no. 14) with cell parameters a=10.1202(6)Å, b=7.2890(5)Å, c=7.0193(4)Å, and β=89.90(1)°. Its redox behavior has been studied in magnesium containing electrolytes. Both K+ ions deintercalated from the structure upon oxidation and contamination with Na+ ions coming from the separator were found to interfere in the electrochemical response. In the absence of alkaline ions, pre-oxidized manganese hexacianoferrate showed reversible magnesium intercalation, and the process has been studied by operando synchrotron X-ray diffraction. The location of Mg2+ ions in the crystal structure was not possible with the available experimental data. Still, density functional theory simulations indicated that the most favorable position for Mg2+ intercalation is at 32f sites (considering a pseudo cubic F m-3m phase), which are located between 8c and Mn sites.

The influence of Ho doping on the morphology and magnetic properties of M-type strontium ferrite with different Fe/Sr

Ho-doped strontium ferrite powder samples with different Fe/Sr ratio were successfully prepared by sol-gel auto-combustion method. Different Fe/Sr ratios (Sr1-xHoxFe12O19, Sr1-x/2HoxFe12-x/2O19, SrHoxFe12-xO19, Sr1+xHoxFe12-xO19, Sr1+xHoxFe12-2xO19) were designed to study the influence of Ho doping on the morphology and magnetic properties of M-type strontium ferrite. XRD analysis revealed a relative change in the content of the secondary phases Fe2O3, HoFeO3, and SrFeO3-x in Ho-doped samples as the Fe/Sr ratio varies. The excess of Sr and Fe in the internal environment resulted in such changes in the phase. SEM analysis confirmed that Ho doping led to an increase in grain size. Different doping methods could lead to significant difference in the size of crystal grain. XPS results indicated that the secondary phases caused by Ho doping led to a change in the oxidation state of the iron element. It was inferred from FTIR and Raman spectra that Ho doping into the lattice might occupy both 2b and 4f1 sites. The VSM showed that under the SrHoxFe12-xO19 doping mode (x = 0.05–0.2), the obtained samples showed a decrease in coercivity (Hc) with increasing Ho doping level (from 5.26 to 4.78 kOe), and the saturation magnetization (Ms) also decreased (from 76.73 to 73.28 emu/g). As the Fe/Sr ratio gradually increases, the saturation magnetization of the sample shows a trend of first increasing and then decreasing, while the coercivity shows a trend of first decreasing and then increasing. The intermediate ionic radius of Ho3+ (0.89 Å) among the rare earth elements results in its unique occupancy behavior, which also influences the formation of different phases, resulting in corresponding changes in coercivity and saturation magnetization.

Correlated microstructure and magnetic properties of Ce-substituted sintered Nd-Fe-B magnets: Role of various phases at triple junctions and grain boundaries

We report the investigation of the microstructure and magnetic properties of a sintered (Nd,Pr)30.3-xCexFebalM1.4B0.92 (in wt.%, xCe = 0) magnet with varying Ce substitutions (xCe = 2, 4, 6, 8). Structure analysis revealed a lattice contraction with Ce addition of up to xCe < 6, and the further increase in the Ce content to 6 wt.% led to a suppression in lattice contraction. The magnetic properties showed a notable deviation from the linear trend in coercivity and remanent magnetization for the magnets with Ce content of xCe > 4. The detailed microstructure analysis revealed that the magnets with xCe > 4 exhibit the REFe2 phase at the expense of a reduced mass fraction of a REOx phase, suggesting that the generation of the REFe2 phase suppressed the formation of the REOx phase. The site occupancy and valence state of Ce, analyzed using atomic-resolution energy dispersive spectroscopy and electron energy loss spectrometry, established that the suppressed lattice contraction is accompanied by a preferential site occupation of Ce from a random case to a 4f-centric site, along with a valence change from a mixed state to a trivalent state, inferring that Ce3+ ions are rejected from the 4g site of the RE2Fe14B phase to occupy the 4f site with a subsequently reduced number of Ce4+ ions due to the magneto-volume and chemical bonding effects. These findings display a new phase formation pathway of the REFe2 phase from a valence state perspective, i.e., the part of Ce4+ ions is considered to be rejected from the RE2Fe14B phase to nucleate the REFe2 phase, which is later found to separate chemically into RE-rich and Fe-rich regions. Further, a bi-layer interfacial structure, composed of Fe-rich and RE-rich layers, is formed in between the RE2Fe14B and the REFe2 phases due to the weak wettability of the REFe2 phase and a slightly positive mixing enthalpy between Ce and Fe. The deviation in magnetic properties is discussed based on the structural and microstructural results. These findings provide structural insight for developing low-cost and high-performance Ce-substituted Nd-Fe-B magnets.

Phase boundary mapping for high thermoelectric performance β-Zn4Sb3 in Zn–In–Sb ternary system

Zn4Sb3 with complex structure shows excellent thermoelectric performance in the low temperature region. Herein, three batches of In doped Zn4Sb3 were synthesized and the influence of In dopant and Zn content on the thermoelectric properties of Zn4Sb3 was systematically investigated by using a phase boundary method. The isothermal ternary phase diagram of Zn–In–Sb is explored at 623 K. The solubility limit of In in the single phase region with β-Zn4Sb3 structure is strongly related to the ratio between Zn + In and Sb. In atoms preferentially occupy C interstitial of 48f site in the crystallographic Wyckoff position and Zn deficiency increases the concentration of Zn vacancies on this site, enhancing the solubility limit of In in the system. The modification of structure via doping with In hinders the movement and diffusion of Zn interstitial in Zn4Sb3, accompanying with the disappearance of the low temperature phase transitions from α′ to α phase and from α to β phase. The regulation of Zn and In contents in single phase region with β-Zn4Sb3 structure effectively optimizes the thermoelectric performance. A maximum ZT value of 1.43 is attained at 673 K for (Zn0·96In0.04)4.15Sb3 compound, which demonstrates a 47% improvement in comparison with that of Zn4·15Sb3 compound. This work provides a new avenue to further improve thermoelectric properties of Zn4Sb3 compounds.

Tb2-xNdxZn17-yNiy (x= 0.5, y= 4.83): a new intermetallic with a maximum disordered structure and its hydrogen storage properties.

The ternary Tb2-xNdxZn17-yNiy (x= 0.5, y= 4.83) disordered phase belongs to the structural family based on the rhombohedral Th2Zn17 structure type. The structure is maximally disordered since all the sites are occupied by statistical mixtures of atoms. The Tb/Nd mixture of atoms occupies the 6c site (site symmetry 3m). The statistical mixtures Ni/Zn consisting of more Ni atoms are located in the 6c and 9d (symmetry .2/m) sites. In the following 18f (site symmetry .2) and 18h (site symmetry .m) sites are located Zn/Ni statistical mixtures which consist of more Zn atoms. Zn/Ni atoms form three-dimensional networks with hexagonal channels that fill statistical mixtures of Tb/Nd and Ni/Zn. The Tb2-xNdxZn17-yNiy compound belongs to the family of intermetallic phases capable of absorbing hydrogen. In the structure, there are three types of voids, namely, 9e (site symmetry .2/m), 3b (site symmetry \overline{3}m) and 36i (site symmetry 1), in which hydrogen can be inserted, and the maximum total absorption capacity can reach 1.21 wt% H2. Electrochemical hydrogenation shows that the phase absorbs 1.03% of H2, which indicates partial filling of the voids with H atoms.

First principles investigation of the substitutional doping of rare-earth elements and Co in La4MgNi19 phase

RE-Mg-Ni alloys, including AB3, A2B7, A5B19 and AB4 types, have received extensive attention due to their excellent hydrogen storage properties. La4MgNi19 is the one of the outstanding candidate for hydrogen storage. In this work, the structure, phase stability and electronic structure of the different La-site partial substituted by Pr, Sm, Gd, Nd and NI-site substituted by Co have been investigated by means of the density functional theory.The calculation results show that La4MgNi19 alloy shows the negative enthalpy of formation, indicating the more stable in the thermodynamic. When the substitution of La occurs, among the two sites La(2c) and La(4f), Pr, Nd, Sm and Gd preferentially occupy the La(4f) site. And the addition of the four doping elements will reduce the stability of the phase. Among them, Pr substituted La4MgNi19 has the highest structural stability. When Co substituting Ni, a single Co atom occupies the Ni(12k) preferentially among the seven different Ni positions. During this process, the crystal structure will be destabilized. The DOS results show that the system still puts up the metallic character after substitution. Sm(La(4f)) has the maximum valence band width. The stability of four doped alloys from high to low is: Pr, Nd, Sm, Gd, which is consistent with the enthalpy of formation results. The enthalpy of formation of hydrides shows that, the bounding hydrogen capacity of the system can be obtained as Nb > Sm > Pr > Gd.