4f Sites Research Articles

The stability of newly developed (R,Zr)(Fe,Co)12−xTix alloys for permanent magnets

(R,Zr)(Fe,Co)12−xTix (R = Nd and Sm) alloys of ten different compositions were prepared by strip-casting. The elemental distribution of the sample particles after annealing was quite homogeneous. Existing phases in the prepared alloys were determined by X-ray diffraction and the occupation sites of the substituted elements (Ti, Co, and Zr) were determined using spherical aberration corrected scanning transmission electron microscopy. The ThMn12 structure is stable in Ti1.0 compounds for both R = Nd and Sm, but exhibited obvious precipitation of α-(Fe,Co) phases in low Ti content compounds with Ti0.5. Ti substitution into Fe(8i) sites is important for the stabilization of the ThMn12 structure. Furthermore, the stability of low Ti content compounds could be recovered by the substitution of Zr into R(2a) sites. Co was mainly in Fe(8j) and Fe(8f) sites, which contributed to the stability of the 1–12 structure by increasing the configuration entropy. Analysis based on three Fe hexagons and two local sub-structural units suggested that the stabilization of ThMn12 structure by Ti and Zr substitutions mainly originates by resolving local mismatches in interatomic distances in the structure.

Modulation of physico-chemical, magnetic, microwave and electromagnetic properties of nanocrystalline strontium hexaferrite by Co-Zr doping synthesized using citrate precursor sol-gel method

Co-Zr doped SrCoxZrxFe(12−2x)O19 (x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) have been synthesized by citrate precursor sol-gel method. We have studied the impact of coupled substitution of magnetic Co2+ and non-magnetic Zr4+ for Fe3+ on the morphology, magnetic, microwave and electromagnetic properties of SrCoxZrxFe(12-2x)O19. Characterization of these materials has been done by XRD, FT-IR, EDS, TEM, VSM and Mössbauer spectroscopy techniques. Complex permeability (μ′), complex permittivity (ε′) and reflection loss curves have also been analyzed. XRD analysis indicates the formation of single phase hexagonal ferrites at a low temperature of 800°C. Mössbauer spectra indicates that the dopant ions are located on the tetrahedral 4f1 and octahedral 4f2 sites up to x=0.4, octahedral 12k site and bipyramidal 2b site for x=0.6 and 12k site for x=0.8–1.0. The reflection loss (R.L.) analysis shows that all the ferrites (except x=0.2) have wide absorption bandwidth (R.L.<−10dB for 2–3.5GHz) with peaks of 90–99% absorption in 12.4–18GHz band.

Structure and magnetic properties of Zr–Mn substituted strontium hexaferrite Sr(Zr,Mn) x Fe12−2x O19 nanoparticles synthesized by sol–gel auto-combustion method

In this research, nano-sized powders of Zr–Mn substituted strontium hexaferrite (Sr(Zr,Mn) x Fe12−2x O19 (x = 0, 2, 2.5, 3)) were synthesized by sol–gel auto-combustion route using subsequent heat treatment. The samples were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), Mössbauer spectroscopy and vibration sample magnetometer (VSM) techniques. XRD and Mössbauer spectroscopy results revealed formation of Sr(Zr,Mn) x Fe12−2x O19 accompanied with Mn δ Fe2−δ O4 lateral phase in the samples. Also, FTIR and XRD results demonstrated presence of SrO impurity phase. FESEM micrographs show particle size reduction and presence of two distinct powder morphologies with different brightness levels with Zr4+ and Mn2+ substitutions which approves existence of lateral phases in the substituted samples. TEM micrographs show nanometric particles with sizes smaller than 100 nm with high crystallinity. Mössbauer results showed that at low level of substitution, Zr4+ ions prefer to occupy both 4f1 and 2b however, at higher level of substitution, they prefer exclusively 4f1 site. While, Mn2+ ions distributed approximately equally between 12k and 2a sites. The presence of nonmagnetic Zr4+ cation leads to decrease in exchange interaction, especially at 12k and 2a sites. VSM results showed decrement of coercivity force ( i H c ) from 5593.60 to 3282.46 Oe and maximum magnetization from 62.60 to 46.15 emu g−1, respectively, by increment of Zr–Mn substitution values. Variations in maximum magnetization magnitude have been explained on the basis of occupation of the substituted cations at different iron sites.

Temperature effect on the magnetic property and ferroelectricity in hexaferrite SrFe12O19

We studied the temperature effect on magnetic and electrical properties in bulk SrFe12O19 prepared by conventional ceramic technique. The jumping behavior of magnetization has been observed under the zero-field-cooling mode, but disappeared under the field-cooled cooling mode. The spin moment of iron ions reorients below 50 K leading to the magnetic structure changes. Magnetic parameters, saturation magnetization (Ms) and coercivity field (Hc), show opposite tendency with temperature throughout the measuring range, which is mainly ascribed to the Fe3+ ions situated at 4f2 and 2b sites. The curves of electrical polarization P vs temperature T under different external magnetic field indicate the existence of ferroelectricity and magnetoelectric coupling effect at low temperature, and the transition temperature T P is about 120 K.

Solid-solution stability and preferential site-occupancy in (R-R′)2Fe14B compounds

The rare-earth (R) uniaxial anisotropy of R2Fe14B compounds with magnetic R atoms (e.g., Nd or Pr) is at the origin of the exceptional hard magnetic properties achieved in magnets based on these compounds. The uniaxial anisotropy found in Ce2Fe14B is attributed mainly to the magnetism of Fe. Ce is the most abundant R element and there has been much recent effort to fabricate magnets in which Ce is partially substituted for Nd. In the present neutron study of (R1−xCex)2Fe14B (R = La or Nd), Ce is found to enter the R2Fe14B phase over the entire composition range. The crystallographic parameters decrease with increasing Ce content and the Ce atoms preferentially occupy the smaller 4f sites. It is concluded that Ce in these (RR′)2Fe14B compounds essentially maintains the intermediate valence character found in Ce2Fe14B. It is proposed that, in this intermediate valence state, Ce weakly contributes to uniaxial anisotropy, thus making a link with the fact that significant coercivity is preserved in Ce-substituted NdFeB magnets.

Photoluminescent properties and site occupation preference in Bi3+, Eu3+ doped CaY4(SiO4)3O phosphor

Bi3+, Eu3+ doped CaY4(SiO4)3O phosphors were synthesized through high temperature solid state reaction. Their photoluminescent properties were investigated and site occupation preference of Bi3+ in cationic sites was analyzed. The structure of CaY4(SiO4)3O is characterized by three non-equivalent cationic sites with different coordination and cation-oxygen distances. By means of dielectric theory of the chemical bond for complex crystals, several kinds of chemical bond parameters like fractional covalence of CaY4(SiO4)3O were calculated and integrated to yield environmental factor he. According to quantitative equations between the transition energy of Bi3+ and environmental factor he, the excitation bands at 308 and 226nm were assigned to 1S0→3P1 transition of Bi3+ in Y(6h) and Y(4f) site, respectively. Another excitation band centered at 210nm should be the overlap of Bi3+A-band in Ca site and C-band in Y(6h) site. Optical band gap of pure CYSO was calculated using Kubelka–Munk method from diffuse reflectance spectra. Red emission can be realized in CaY4(SiO4)3O:Bi3+, Eu3+ under UV light excitation because of efficient energy transfer from Bi3+ to Eu3+ and decay behaviors of Bi3+ and Eu3+ emission were investigated. Without optimization, the internal quantum efficiency of CYSO:2%Bi3+, 7%Eu3+ at 310 and 393nm excitations were 31.563%, 74.252%, respectively.

Structural and magnetic properties of Co[sbnd]Ti substituted barium hexaferrite thick films

CoTi substituted M-type barium hexaferrite BaFe12-2xCoxTixO19 (0.0 ≤ x ≤ 1.0) thick films were prepared by screen printing method. X-ray diffraction analysis confirmed the formation of single phase BaFe12O19 (BaM). The preferential site occupation of Co2+ and Ti4+ ion for Fe3+ ion at five different crystallographic sites (12k, 4f1, 4f2, 2a, and 2b) was measured by Mössbauer spectroscopy. The analyses showed CoTi ions preferentially occupy 2b and 4f2 sites which were responsible for drastic decrease in magnetocrystalline anisotropy field (Ha). Magnetization (M) value found to be higher for x = 0.5. The coercivity (Hc) dropped dramatically, recording a reduction of about 95% at x = 1.0, attributed to decreased in Ha. SEM micrographs of CoTi substituted films revealed hexagonal grain structure. The magnetic properties of CoTi substituted BaM, suggests a possible candidate for microwave applications.

Magnetic and dielectric properties of Bi3+ substituted SrFe12O19 hexaferrite

In the present study, SrBixFe12−xO19 (0.0≤x≤1.0) nanomaterials were successfully synthesized by using chemical co-precipitation method. Products were characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Vibrating sample magnetometer (VSM), Mössbauer spectroscopy, AC conductivity and dielectric measurements. The crystal structural information studied by X-ray diffraction (XRD) indicated the formation of single phase pure hexagonal structure, while electron-dispersive X-ray spectroscopy (EDX) revealed the stoichiometric ratio among Bi, Sr, Fe elements. The crystallite sizes of the products were in the range of 65–82nm. VSM analysis showed a tendency in saturation magnetization as Bi2O3 concentration raises, which can be ascribed to preferential site occupied by Bi3+ ions. The frequency-dependent ac conductivity plots exhibited similar trends for all samples. A significant temperature-dependent behavior was only observed at low and medium frequencies. The replacement of non-magnetic Bi3+ ions by Fe3+ ones having magnetic moment of 5µB decrease the magnetic moment of 4f1 site. The AC conductivity increases with frequency as hopping of the charge carriers increases between Fe2+and Fe3+. The DC conductivity exhibited an improvement with increasing temperature and Bi content, and the highest conductivity was measured as 2.84×10−9Scm−1 for x=0.8 at 120°C. The variation of dielectric constant, dielectric loss and tangent loss was observed with the frequency and temperature due to change of electrical conductivity as x changes.

Crystal structure and luminescence properties of Bi3+activated Ca2Y8(SiO4)6O2 phosphors under near UV excitation

Oxyapatite Ca2Y8−x(SiO4)6O2:xBi3+phosphor has been prepared via high temperature solid-state reaction. Its crystal structure and PL properties were investigated by X-ray diffraction, photoluminescence excitation and emission spectra. The results indicated that the Ca2Y8(SiO4)6O2 crystallizes as a hexagonal structure with a space group of P63/m and lattice constants of a=b=9.3507Å, c=6.7899Å, α=β=90.00°, γ=120.00°, V=514.14Å3; The phosphor has two prominent emission bands: when excited under 320–360nm, the phosphors emit a broad band centered at 495nm due to the 3P1–1S0 transition of Bi3+ in 4f (C3) sites; when excited under 380nm, the phosphors emit a broad band centered at 411nm due to the 3P1–1S0 transition of Bi3+ in 6h (Cs) sites. The emission color varies from the greenish blue to blue as the excitation wavelength increases from 335 to 380nm. The optimal intensity of emission band was observed when x=0.015 in the Ca2Y8−x(SiO4)6O2:xBi3+ series. The average critical distance Rc among Bi3+ ions is determined to be 20.15Å.

Synthesis, crystal structure, and physical properties of a new boride Ga2Ni21B20 with a modified Zn2Ni21B20-type structure

A ternary boride Ga2Ni21B20, with modified Zn2Ni21B20-type structure (space group I4/mmm, and lattice parameters a = 7.2164(1) Å, c = 14.2715(4) Å), was synthesized from the constituent elements. Single crystal diffraction data reveal Ni at 8f site splitting into 16m position with nearly half occupancy. In this structure, [Ni6B20] cages share ligand boron atoms with [Ga2B4Ni9] hexa-capped square prisms, forming two dimensional layers. Layers are interconnected via Ga−Ni interactions and build up a three-dimensional framework. Quasi-two-dimensional infinite planar nets formed by intercrossed Ni atoms are embedded. Ga2Ni21B20 is a metallic Pauli paramagnet, in agreement with electronic structure calculations, resulting in 8.2 states eV−1 f.u−1 at the Fermi level.

Single-crystalline M-type Sr Hexaferrites studied by 57Fe Mössbauer spectroscopy

The 57Fe Mossbauer spectra of the single crystalline and the finely ground Sr1−xLaxFe12−yCoyO19 (x = 0 : y = 0, x = 0.192 : y = 0.152 and x = 0.456 : y = 0.225) samples have been measured to investigate the La-Co substitution effects. All observed spectra at 150 K were well fitted using the five subspectra which correspond to the five crystallographical nonequivalent Fe sites in the M-type hexaferrite, indicating that the valence changes to Fe2+ ions in the Fe3+ ions were not observed in our Sr1−xLaxFe12−yCoyO19 samples. In SrFe12O19, the relative absorption intensities in the five subspectra show the large anisotropies in the recoilless fractions at the five Fe sites whereas these anisotropies were not observed in Sr0.544La0.456Fe11.775Co0.225O19. These results indicate the chemical compositional dependence on the anisotropies of the recoilless fractions at the five Fe sites. The substitution of a Co2+ ion for the Fe3+ ion changes the center shifts of the Fe3+ ions near the Co2+ ion by the perturbation of the Fe-O-Co hybridizations. Therefore, the Co2+ ions occupy the 4f1 and the 4f2 sites due to the chemical compositional dependences of the refined magnetic hyperfine field and center shifts of the Fe3+ ions.

Computational and experimental study on the cation distribution of La–Cu substituted barium hexaferrites

The total energies and forces of the Ba0.5La0.5Fe11.5Cu0.5O19 structure were calculated using the density functional theory (DFT) and generalized gradient approximation (GGA). The calculations demonstrated that at 0 K, the magnetic ions Cu2+ preferentially occupy Fe3+ at the 4f2(0) and 4f2(1) configurations. For the site occupancy at the higher temperature (0–1500 K, except 0 K), we computed the formation probability of twenty inequivalent configurations using the Maxwell–Boltzmann statistical distribution. Correspondingly, the Cu2+ ions prefer to enter the 4f2 and 2b sites, especially close to lanthanum layer. In order to confirm the computational conclusions, La–Cu substituted barium hexaferrites with the chemical formula of Ba1−xLaxFe12−xCuxO19 (x = 0.0, 0.1, 0.3, and 0.5), prepared by a conventional ceramic method, were investigated by the Raman spectra and Rietveld refinement of X-ray diffraction patterns (XRD). The experiments revealed that the Cu2+ ions are partitioned at the 2b and 4f2 sites in the ratio of around 1:2.

A novel apatite, Lu5(SiO4)3N:(Ce,Tb), phosphor material: synthesis, structure and applications for NUV-LEDs.

The lutetium containing nitride apatite Lu5(SiO4)3N was prepared by a solid state reaction at high temperature for the first time. Rietveld refinement indicated that the Lu5(SiO4)3N compound has a hexagonal space group of P63/m with cell parameters a = b = 9.700 Å and c = 7.238 Å. Additionally, the results revealed that there are two distinct lutetium sites in the Lu5(SiO4)3N host lattice, i.e. a Lu(1) site with nine coordination (Wyckoff site 4f) and a Lu(2) site with seven coordination (Wyckoff site 6h). Furthermore, the ratio of the number of Lu atoms in Lu(1) and Lu(2) sites is 3 : 2. The band gap for Lu5(SiO4)3N was determined to be 4.12 eV based on the density functional theory (DFT). In the Ce(3+) doped Lu5(SiO4)3N:0.03Ce(3+) compound, the emission peak centered at 462 nm was observed with the Commission International de I'Eclairage (CIE) coordinates of (0.148, 0.184), indicating blue-emission. Remarkably, in Ce(3+) and Tb(3+) co-doped Lu4.97-y(SiO4)3N:0.03Ce(3+),yTb(3+) compounds, the color-tunability was observed with increasing Tb(3+) co-doping rate on moving from blue at Tb(3+) = 0.00 to green at Tb = 0.09, due to the energy transfer from Ce(3+) to Tb(3+) ions being matched well with the decay curve results. Under the excitation at 359 nm, the absolute quantum efficiency (QE) for Lu5(SiO4)3N:0.03Ce(3+) was determined to be 42.13%. This phosphor material could be a platform for modeling a new phosphor and application in the solid-state lighting field.

Site preference and magnetic properties of Ga/In-substituted strontium hexaferrite: An ab initio study

The first-principles density functional theory has been used to study Ga/In-substituted strontium hexaferrite (SrFe12O19). Based on the calculation of the substitution energy of Ga and In in SrFe12O19 and the formation probability analysis, we conclude that in SrFe12−xGaxO19 the substituted Ga atoms prefer to occupy the 12k, 2a, and 4f1 sites, while In atoms in SrFe12−xInxO19 occupy the 12k, 4f2, and 4f1 sites. We used the site occupation probabilities to calculate the magnetic properties of the substituted SrFe12O19. It was found that as the fraction of Ga atoms in SrFe12−xGaxO19 increases, the saturation magnetization (Ms) as well as magnetic anisotropy energy (MAE) decrease, while the anisotropy field (Ha) increases. In the case of SrFe12−xInxO19, Ms, MAE, and Ha decrease with an increase of the concentration of In atoms.

Structure and Thermal Expansion of Calcium-Thorium Apatite, [Ca4]F[Ca2Th4]T[(SiO4)6]O2.

Thorium silicate apatite with the formula [Ca3.84Th0.16]F[Ca2.79Th3.21]T(SiO4)6O2 · x(H) was synthesized by solid-state reaction, and its structure refined in P63/m from powder X-ray diffraction (XRD) data using the Rietveld method (a = 9.50172(9) Å, c = 6.98302(8) Å, V = 545.98(1) Å(3); R-Bragg = 2.102%). It was found that thorium partitions strongly to the tunnel (T) 6h position rather than the framework (F) 4f site. Fourier transform infrared spectroscopy revealed only SiO4 tetrahedron, with SiO5 and SiO6 groups, sometimes observed in siliceous apatites absent, at least to the limit of detection of this technique. Thermal expansion of the thorium apatite determined by high-temperature XRD from 298-1173 K found Δa (0.87%) dilation to exceed Δc (0.73%) with increasing temperature consistent with other silicate apatites.

Structural, magnetic and microwave properties of barium hexaferrite thick films with different Fe/Ba mole ratio

Barium hexaferrite (BaFe12O19) thick films (∼60µm) with different BaO·xFe2O3 mole ratio (x=5.0–6.0) were prepared by screen printing method. X-ray diffraction analysis confirmed the formation of single phase BaFe12O19 (BaM). Preferential site occupation of Fe3+ ion at five different crystallographic sites, with varied mole ratio was measured by Mössbauer spectroscopy. Vacancy fraction found to be higher at 4f1, 4f2 and 2b sites for mole ratio 5.5 and 5.0 respectively. Magnetic measurement shows that the magnetization (M) and magnetocrystalline anisotropy field (Ha) depends upon mole ratio. M and Ha are found to be maximum for mole ratio 5.5, while the coercivity (Hc) remains constant. Reflection losses (RL) in the frequency range of 12–18GHz were also studied. Present investigation demonstrates the effect of mole ratio on structural, magnetic and microwave absorption properties of BaM thick films for microwave device applications.

Phase stability, electronic and elastic properties of Fe6−xWxC (x = 0−6) from density functional theory

The calculated formation enthalpies of Fe3W3C (W in the 48f site) and Fe2W4C are negative, and they are of higher stability than other (Fe,W)6C phases. The 16d and 32e sites are comfortable for Fe atoms, while the W atom is preferential in the 48f site during the formation process of (Fe,W)6C carbides. An exchange of electrons takes place between the Fe/W and the C atom, and also appears between the Fe and W atom. In M–M bonds, besides metallic characters, a valence character between W–W bonds exists. Besides the nonmetal C atom with negative charges (obtaining electrons), the partial metal atoms of Fe and W in different M6C-type carbides show an electronegative phenomenon. The carbides W6C, Fe5WC, Fe2W4C and Fe3W3C-II likely have high temperature superconductive characters. The hardness of Fe3W3C (16.5 GPa) is the highest in these phases, and that of Fe2W4C (13.7 GPa) is the second highest. The Debye temperature (θD) of Fe3W3C–I (Fe in the 48f site) is lowest (254 K) in these carbides, and that of Fe3W3C-II suddenly changes to 568 K when W is in the 48f site. The change of the atom in the 48f site intensively influences the properties of M6C (M = Fe/W).

Investigation of structural and magnetic properties of nanocrystalline Mn-doped SrFe12O19 prepared by proteic sol–gel process

Nanoparticles of SrFe12-xMnxO19 (x=0.0 and 0.10) were synthesized by a proteic sol–gel process. Thermogravimetric and differential thermal analyses (TG–DTA) indicated the formation of nanocrystalline strontium ferrite phase at a calcination temperature of 1000°C. Structural and microstructural evolutions of the samples were studied by X-ray powder diffraction (XRD) and the Rietveld method. XRD patterns demonstrated that all samples consisted of single-phase M-type strontium hexaferrite. The crystal lattice constant did not change significantly with manganese substitution, ranging from 0.5877(3)nm (x=0.0) to 0.5876(3)nm (x=1.0). In addition, the average crystallite size, which was determined from the Williamson–Hall formula, was about 46.4–52.6nm. Infrared spectroscopy (FT-IR) showed the presence of three principal absorption bands in the frequency ranges around 435–535cm−1 and around 595cm−1, indicating the formation of the hexaferrite. Scanning electron microscopy (SEM) revealed that particles consisted of irregular platelets with sizes from 68 to 204nm. Room-temperature Mössbauer investigations revealed that manganese ions preferentially occupied the 12k, 4f1, 4f2, and 2a sites. Hysteresis loops (M–H) showed that the saturation magnetization, remanence, and coercivity decreased with manganese doping. This effect is discussed in terms of the distribution of metal cations in the tetrahedral and octahedral sites.

Experimental study of the crystal structure of the Mg15−xZnxSr3 ternary solid solution in the Mg–Zn–Sr system at 300°C

Abstract A new ternary compound, Mg 15 − x Zn x Sr 3 with extensive solid solubility in the Mg–Zn–Sr system was observed and studied using electron probe microanalysis (EPMA), scanning electron microscopy SEM, and X-ray techniques. The solid solubility limits of this compound were found to be Mg 15 − x Zn x Sr 3 (0.24 ≤ x ≤ 10.58, at.%) at 300 °C using a diffusion couple and several equilibrated alloys. Analysis of the X-ray diffraction (XRD) patterns was carried out by Rietveld method. XRD data has shown that this solid solution crystallizes in the hexagonal P6 3 /mmc (194) space group with the Ni 11 Si 4 Sc 3 prototype. The lattice parameters decrease linearly with decreasing Mg content indicating substitutional solid solubility. The fractional atomic occupancy of the 6h, 6g, 4f, 2b and 12k sites of this compound are function of Mg content.

Effect of atomic sites on electronic and mechanical properties of (Fe,Mo)6C carbides

The phase stability of Fe3Mo3C-II (Mo at the 48f site) is higher than other phases in the present work. The formation enthalpies of Fe3Mo3C-II and Fe2Mo4C are −0.095 and −0.053 eV/atom, respectively. The 16d and 32e sites are comfortable for Fe atoms, and the Mo atoms tend to be at the 48f sites. The electronic characteristics of metal atoms are sensitive to the local short-range order in these crystals. Moreover, the nonmetal C atom is with negative charges (obtaining electrons), and this electronegative phenomenon is also present in some metal atoms (Fe or Mo) in M6C. The hardness of Fe3Mo3C (13.6 GPa) is the highest in these phases, and the hardness of Fe2Mo4C is 11.7 GPa. The θD of Fe3Mo3C-I (Fe at the 48f site) is 397 K, and that of Fe3Mo3C-II abruptly changes to 657 K when Mo in the 48f site.