12k Sites Research Articles

Determination of the solubility range and crystal structure of the Mg-rich ternary compound in the Ca–Mg–Zn system

The homogeneity range and crystal structure of the Mg-rich ternary solid solution has been determined using SEM, EPMA/WDS, TEM and X-ray diffraction. This compound has the Ca 3Mg xZn 15−x (4.6 ≤ x ≤ 12) formula at 335 °C. The refinement of the XRD patterns was carried out by Rietveld analysis, XRD data has shown that this solid solution crystallizes in hexagonal structure having P63/mmc (194) space group and Sc 3Ni 11Si 4 prototype. The lattice parameters decrease linearly with decreasing Mg content obeying Vegard’s law. The fractional atomic occupancy of 6h, 4f, 2b and 12k sites of this compound are function of Mg concentration. Focused Ion Beam has been used to lift TEM specimen of the ternary compound and the hexagonal structure has been confirmed by means of selected area electron diffraction data. Based on the atomic occupancy results and the crystallographic details, a three sublattice (Ca)(Zn)(Mg, Zn) 4 model is proposed for this compound.

Mössbauer spectroscopy and magnetic susceptibility studies of Cr–Al substituted strontium ferrite particles

Substituted strontium ferrite SrFe12−xCrx/2Alx/2O19 (x=0–2.5) was prepared by sol-gel method. Ac susceptometer, and F57e Mössbauer spectroscopy were used to analyze the structure, static and dynamic magnetic properties of the prepared samples. Mössbauer spectra show that Al–Cr ions preferentially occupy the 12k sites. Magnetic dynamics of the samples was studied by the measurement of ac magnetic susceptibility versus temperature at different frequencies and dc magnetic fields. A frequency-dependent peak was observed in ac magnetic susceptibility versus temperature, which is well described by Vogel–Fulcher model. It was observed that with increasing substitutions, blocking temperature is decreased.

Effects of Substitution in Barium Hexaferrites BaFe12-xXxO19 (X=Co,Ti; Sc)

The short and long-range atomic and magnetic order in BaFe12-xCoxTixO19 (x=0.4, 0.7, 0.8) and BaFe10.4Sc1.6O19 prepared by soft chemistry routes were studied and compared with parent BaFe12O19 prepared by solid-state reaction. For BaFe10.3Co0.85Ti0.85O19 neutron diffraction and magnetic measurements revealed that (Co2+, Ti4+) substitution causes significant distortions in the local oxygen surrounding of ferric cation sites, while the grain-size effect on the structural parameters is considerably smaller. The thermal expansion coefficient exhibits a strong anisotropy. The refined magnetic moments for x=0.45 and 0.7 based on the five-cation sublattice block-type collinear ferrimagnetic structure of uniaxial type known as Gorter type for BaFe12O19, are considerably lower than the theoretical spin only moments, especially for the 4e and 12k sites, indicating for x = 0.7 a local noncollinearity with short-range ordering. For x = 0.85, at 10 K a block-type conical magnetic structure sets in. For BaFe10.4Sc1.6O19, combined neutron diffraction, field-dependent 57Fe Mössbauer studies and magnetic measurements show that the collinear block-type structure remains effective at 300 K and below it down to about 190 K. In the 190 – 1.6 K range we observe a temperature dependent incommensurate complex block-type conical structure.

Crystal structure and iron topochemistry of erionite-K from Rome, Oregon, U.S.A.

A complete crystal-chemical characterization of erionite-K from Rome, Oregon, was obtained by combining field emission scanning electron microscopy, laboratory parallel-beam transmission powder diffraction, and 57 Fe Mossbauer spectroscopy. Rietveld refinement results evidenced that the most striking difference in comparison with the structure of erionite-Ca is significant K at a K2 site (½, 0, 0), which is empty in erionite-Ca. In addition, site Ca1 shows low occupancy and Ca3 is vacant. The oxidation and coordination state of Fe, whose occurrence was revealed by chemical analysis, have been clarified by exploiting room- and low-temperature 57 Fe Mossbauer spectroscopy. The majority of Fe (95%) was attributed to Fe 3+ -bearing, superparamagnetic, oxide-like nanoparticles with dimensions between 1 and 9 nm, and the remaining 5% was attributed to hematite particles with size ≥10 nm, both located on the crystal surface.

Barium in twilight zone suspended matter as a potential proxy for particulate organic carbon remineralization: Results for the North Pacific

Abstract This study focuses on the fate of exported organic carbon in the twilight zone at two contrasting environments in the North Pacific: the oligotrophic ALOHA site (22°45′N, 158°W; Hawaii; studied during June–July 2004) and the mesotrophic Subarctic Pacific K2 site (47°N, 161°W; studied during July–August 2005). Earlier work has shown that non-lithogenic, excess particulate Ba (Baxs) in the mesopelagic water column is a potential proxy of organic carbon remineralization. In general, Baxs contents were significantly larger at K2 than at ALOHA. At ALOHA, the Baxs profiles from repeated sampling (five casts) showed remarkable consistency over a period of three weeks, suggesting that the system was close to being at steady state. In contrast, more variability was observed at K2 (six casts sampled), reflecting the more dynamic physical and biological conditions prevailing in this environment. While for both sites Baxs concentrations increased with depth, at K2 a clear maximum was present between the base of the mixed layer at around 50 and 500 m, reflecting production and release of Baxs. Larger mesopelagic Baxs contents and larger bacterial production in the twilight zone at the K2 site indicate that more material was exported from the upper mixed layer for bacterial degradation deeper, compared to the ALOHA site. Furthermore, application of a published transfer function [Dehairs, F., Shopova, D., Ober, S., Veth, C., Goeyens, L., 1997. Particulate barium stocks and oxygen consumption in the Southern Ocean mesopelagic water column during spring and early summer: relationship with export production. Deep-Sea Research II 44(1–2), 497–516] relating oxygen consumption to the observed Baxs data indicated that the latter were in good agreement with bacterial respiration, calculated from bacterial production. These results corroborate earlier findings highlighting the potential of Baxs as a proxy for organic carbon remineralization. The range of POC remineralization rates calculated from twilight zone excess particulate Ba contents also compared well with the depth dependent POC flux decrease as recorded by neutrally buoyant sediment traps, except in one case (out of four). This discrepancy could indicate that differences in sinking velocities cause an uncoupling of the processes occurring in the fine suspended particle pool from those affecting the larger particle pool which sustains the vertical flux, thus rendering comparison between both approaches risky.

Lattice vibration characterization and magnetic properties of M-type barium hexaferrite with excessive iron

Nonstoichiometric M-type barium hexaferrite compounds, with nominal compositions BaFe12+xO19+1.5x (0⩽x⩽0.4), have been synthesized by spark plasma sintering technique. The electron probe microanalysis shows that the excessive trivalent iron (Fe3+) ions are in the lattices of BaFe12+xO19+1.5x compounds. Both Raman and FTIR spectra reveal that the excessive Fe3+ ions spin upward. They occupy the octahedron interstices similar to the FeO6 octahedron at the 12k site and the bipyramid interstices similar to the FeO5 bipyramid at the 2b site, in the x range of 0–0.3 and x=0.4, respectively. This explains well significant enhancement in the specific saturation magnetizations for BaFe12+xO19+1.5x with increasing the x in the range of 0–0.4. The gradual increase in the coercivities has been interpreted with the augment of Fe3+ concentration in unit cell.

Magnetic properties of Lu 2Co 17−xSi x single crystals

The Co-sublattice anisotropy in Lu 2Co 17 consists of four competitive contributions from Co atoms at crystallographically different sites in the Th 2Ni 17-type of crystal structure, which result in the appearance of a spontaneous spin-reorientation transition (SRT) from the easy plane to the easy axis at elevated temperatures. In order to investigate this SRT in detail and to study the influence of Si substitution for Co on the magnetic anisotropy, magnetization measurements were performed on single crystals of Lu 2Co 17− x Si x ( x=0−3.4) grown by the Czochralski method. The SRT in Lu 2Co 17 was found to consist of two second-order spin reorientations, “easy-plane”–“easy-cone” at T SR1≈680 K and “easy-cone”–“easy-axis” at T SR2≈730 K. Upon Si substitution for Co, both SRTs shift toward the lower temperatures in Lu 2Co 16Si ( T SR1≈75 K and T SR2≈130 K) with the further onset of the uniaxial type of magnetic anisotropy in the whole range of magnetic ordering for Lu 2Co 17− x Si x compounds with x>1 due to a weakening of the easy-plane contribution from the Co atoms at the 6g and 12k sites to the total anisotropy.

Formation of submicron-sized SrFe 12−xAl xO 19 with very high coercivity

Submicron-sized SrFe 12− x Al x O 19 ( x=1.3) was formed in glass-ceramic matrix using controlled thermocrystallization of the SrO–Fe 2O 3–Al 2O 3–B 2O 3 glass and the hexaferrite powder was obtained by removing the matrix phases. The samples were characterized by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray (EDX) analysis and magnetization measurements. The glass-ceramic material exhibits very high coercivity value up to 10.18 kOe which approaches a theoretically estimated maximum value for the compound. The hexaferrite powder consists of well faceted single crystals, which adopt the shape of a truncated hexagonal bipyramid. The powder saturation magnetization value is close to the theoretically estimated one for bulk material. Crystal structure of the powder was refined by Rietveld method and distribution of Al atoms on Fe sites was determined. Al atoms occupy 41% of 2a sites, 14% of 12k sites and 5% of 4e(1/2) sites, while 4f sites are not affected.

Cr‐site preference of BaFe12–xCrxO19 hexaferrite ceramics monitored by Mössbauer spectroscopy

AbstractSite preference of Cr in the M‐type barium ferrite ceramics was monitored by powder X‐ray diffraction (XRD), magnetization measurements and Mössbauer spectroscopy. The calcined BaFe12–x Crx O19 powders (x = 0.0–0.8) obtained by the sol gel method were die‐pressed into pellets and then sintered at 1300 °C in a oxygen atmosphere for 12 hrs. All the phases of the ceramics were of the pure magnetoplumbite structure. The saturation magnetization (Ms) is found to be a function of Cr ion content. The MS of the ceramics decreases due to the dilution of the magnetic interaction with increasing Cr concentration. The coercivity (Hc) of the ceramics increases due to finer grain observed by scanning electron microscopy (SEM). Fitted Mössbauer spectra show that Cr ions preferentially occupy into the 2a, 12k and 4f2 Fe crystallographic sites. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural and magnetic studies on mechanosynthesized BaFe 12−xMn xO 19

Barium hexaferrite powders with manganese substitution were prepared by mechanosynthesis. The structural and magnetic properties were characterized by X-ray diffractometer and vibration sample magnetometer, respectively. XRD patterns were refined by Rietveld method. Preferential site occupation of manganese ion was investigated by room temperature (RT) Mössbauer measurements. XRD results showed a single-phase barium hexaferrite with some residual hematite. Crystallite size was observed to decrease with substitution amount. Lower saturation magnetization and increased coercivity is observed in substituted samples. RT Mössbauer measurements showed that manganese ions preferentially occupy 12k, 4f 2, and 2a sites.

Effect of aluminum substitution on microwave absorption properties of barium hexaferrite

Aluminum substituted barium hexaferrites were prepared by the self-propagating combustion method and subsequent calcination at 850 °C. The crystalline structure, complex permittivity, complex permeability, and hyperfine parameters of BaFe12−xAlxO19 (x varies from 1.5 to 2.3 in steps of 0.2) were measured with x-ray diffraction (XRD), vector network analyzer and Mössbauer spectroscopy. The XRD results show that all Al3+ ions enter into the lattice of hexagonal barium ferrite. The substitution of Al3+ ions can greatly affect the complex permittivity and permeability of barium ferrite. With increasing substitution, the real part of complex permittivity increases gradually, and the peaks of the imaginary part of complex permeability shift into higher frequency band. When the substitution amount x is 1.9, the largest movement of the peaks is 1.95 GHz, which indicates that the ferromagnetic resonant frequency of barium ferrite increases by 1.95 GHz. The Al3+ ions preferentially occupy the 4f2, 2a, 4f1, and 12k sites in the subcrystalline structure up to x=1.9, and then the Al3+ ions mainly occupy 12k sites. This change also results in 2b sites with a large quadrupole splitting. These occupations lead to a variable magnetocrystalline anisotropy field.

Properties of Cr-substituted M-type barium ferrites prepared by nitrate–citrate gel-autocombustion process

The Cr-substituted M-type barium hexaferrites, BaFe 12− x Cr x O 19, with x = 0.0 – 0.8 have been successfully prepared by nitrate–citrate auto-combustion process using citric acid as a fuel/reductant and nitrates as oxidants. The resulting precursors were calcined at 1100 °C for 1 h and followed by sintering at 1200 °C for 12 h in oxygen atmosphere. The ferrites were systematically investigated by using powder X-ray diffractometer (XRD), magnetic hysteresis recorder, Mössbauer spectrometer, and scanning electron microscope (SEM). The XRD data show the formation of pure magnetoplumbite phase without any other impurity phases. Both a and c lattice parameters calculated by the Rietveld method systematically decrease with increasing Cr content. The effects of Cr 3+ ions on the barium ferrites were reported and discussed in detail. The site preference of Cr 3+ and magnetic properties of the ferrites have been studied using Mössbauer spectra and hystereses. The results show that the magnetic properties are closely related to the distributions of Cr 3+ ions on the five crystallographic sites. The saturation magnetization systematically decreases, however, the coercivity increases with Cr concentration. The magnetization and Mössbauer results indicate that the Cr 3+ ions preferentially occupy the 2a, 12k, and 4f VI sites. The average size of hexagonal platelets obtained by SEM photographs tends to decrease with respect to Cr content.

Ce2Ru3Al15, an intermetallic compound of a new structure type

The title compound, dicerium triruthenium pentadecaaluminium, is a new intermetallic compound from the Al-rich region of the Ce–Ru–Al phase diagram. Two Ce atoms (6g and 2a sites of space group P63/mcm) are coordinated by 18 atoms, one Ru atom (12i site) and five Al atoms (12k, 12i, 12j, 12j and 12k sites) have 12 nearest neighbours each.

Site occupancy and anisotropy distribution of Al substituted Ba‐ferrite with high coercivity

AbstractThe site occupancy and anisotropy distribution of Al substituted BaFe12−xAlxO19 (0.0 ≤ x ≤ 4.0) have been studied with Mössbauer spectroscopy, X‐ray diffraction, and vibrating sample magnetometry. The results suggest that the coercivity, magnetization, and magnetocrystalline anisotropies are closely related to the distributions of Al3+ ions on the five iron sites. Mössbauer spectra indicated that Al3+ ions have a strong preference for the 4f1, 2a and 12k sites. The substitution of Al3+ by Fe3+ in the system causes a drastic reduction of the saturation magnetization while the coercivity first increases to 8.6 kOe, for x = 2.0, and then decreases to 6.9 kOe, for x = 4.0. The origin of the changes in the magnetic properties caused by Al3+ substitution can be attributed to the site preferences. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Mössbauer investigation of Sr 1− xLa xFe 12− yCo yO 19 ferrites

Sr 1− x La x Fe 12− y Co y O 19 powders with y/ x=0.75 and x=0.1, 0.2, 0.3, 0.4 were prepared according to a ceramic process. X-ray diffraction analysis shows that all the powders are of single, hexagonal M-type, phase. The Mössbauer investigation confirms that Co 2+ substitutes for Fe 3+ in both 4f 2 (mainly) and 2a sites. The most important hyperfine parameters changes concern the 12k, 4f 2 and 2b sites and are related to both Co 2+/Fe 3+ and La 3+/Sr 2+ substitution effects.

First-principles calculations ofC14-type Laves phase Ti-Mn hydrides

We have performed first-principles calculations of hexagonal $C14\ensuremath{-}\mathrm{type}$ Laves phase Ti-Mn metals and their hydrides, namely, ${\mathrm{Ti}}_{1.0}{\mathrm{Mn}}_{2.0}{\mathrm{H}}_{3},$ ${\mathrm{Ti}}_{1.25}{\mathrm{Mn}}_{1.75}{\mathrm{H}}_{3},$ and ${\mathrm{Ti}}_{1.5}{\mathrm{Mn}}_{1.5}{\mathrm{H}}_{3},$ to understand their hydrogen storage properties, particularly focusing on materials trend of the heat of formation. We employed the ultrasoft pseudopotential method within the generalized gradient approximation to density functional theory; in calculations, we neglected effects of magnetic ordering and assumed that all hydrogen atoms are located on the 12k sites of the space group ${P6}_{3}/mmc.$ We have found that the calculated heat of formation reproduces the experimental trend, and it involves two dominant contributions: (i) from an energy cost by volume expansion, and (ii) from an energy gain by hydrogen insertion. We thus emphasize that an equilibrium volume and a bulk modulus of the host metal are the keys to describe the heat of formation for these systems.

Mossbauer investigation of Sr-Fe-O hexaferrites with La-Co addition

M-type Sr/sub 1-x/La/sub x/Fe/sub 12-x/Co/sub x/O/sub 19/ ferrite powders with x=0 to 0.4 were investigated using Mossbauer spectrometry in the 77 K-550 K temperature range. The Mossbauer spectra were fitted by assuming that Co/sup 2+/ is present in both 4f/sub 2/ and 2a octahedral sites, and that some Fe/sup 2+/ is present in the 2a site. On increasing x, the main changes concern the Mossbauer contributions of the 12k, 4f/sub 2/ and 2b sites. These changes are interpreted in relation with the Co/sup 2+/-Fe/sup 3+/ substitution at the 4f/sub 2/ site, the Fe/sup 3+/-Fe/sup 2+/ valence change at the 2a site and the La/sup 3+/-Sr/sup 2+/ substitution. The temperature dependence of the hyperfine field at the Fe sites of the Sr/sub 0.6/La/sub 0.4/Fe/sub 11.6/Co/sub 0.4/O/sub 19/ compound is similar to that of the unsubstituted compound. However, the curvature of the temperature dependence of the hyperfine field at the 12k site slightly increases with x. This reflects the increase of the curvature of the temperature dependence of the magnetization of the substituted compounds.

Nanosize effects on the microstructure of BaFe10.3Co0.85Ti0.85O19 hexaferrite

Nanosized and microcrystalline BaFe10.3Co0.85Ti0.85O19 samples were investigated by neutron diffraction. A strong anisotropy of the thermal expansion coefficient and its dependence on the grain size was established. The cation distributions and the magnetic moments were determined. The refined magnetic moments proved to be much lower than the theoretical spin only moments, especially for the 4e and 12k sites, indicating a local noncollinearity with short-range ordering.

A Mössbauer spectral study of the magnetic properties of Ho2Fe17 and Ho2Fe17D3.8

The Mössbauer spectra of Ho2Fe17 and Ho2Fe17D3.8 have been measured between 4.2 and 295 K and analyzed with a model which takes into account both the disordered nonstoichiometric hexagonal Th2Ni17-like structure and the basal orientation of the iron magnetic moments. The isomer shifts of the five crystallographically inequivalent iron sites in both Ho2Fe17 and Ho2Fe17D3.8 follow the sequence of Wigner–Seitz cell volumes and their temperature dependence follows the typical second-order Doppler shift. An increase in the weighted average isomer shift upon deuterium insertion results from the lattice expansion. The sequence of the site-averaged hyperfine fields is in agreement with the increasing number of iron near neighbors. The five quadrupole interactions agree with those observed in the paramagnetic spectra of other R2Fe17 compounds. For the dumbbell 4e and 4f sites, the asymmetry parameter and the θ angle are zero and 90° in agreement with the site point symmetry and the basal orientation of the iron magnetic moment. For the 6g site, the asymmetry parameter is zero in agreement with the site point symmetry. In both compounds the 12j site asymmetry parameter is 0.9 and for the 12k site this parameter increases from 0.0 in Ho2Fe17 to 0.8 in Ho2Fe17D3.8 as a result of the insertion of an additional deuterium near neighbor. The θ angles for the magnetically inequivalent 12j and 12k sites differ by 60° as expected in the hexagonal symmetry. The variations with rare-earth of the weighted average isomer shifts and hyperfine fields in R2Fe17, R2Fe17Hx, and R2Fe17N3, are discussed in terms of the lanthanide contraction and lattice expansion upon insertion of hydrogen, or deuterium, or nitrogen and the axial and basal magnetic anisotropy.

Influence of Co–Ti substitution on coercivity in Ba ferrites

Polycrystalline BaCo x Ti x Fe 12−2 x O 19 samples with 0⩽ x⩽1.2 ions per formula units were prepared by modified citrate precursor method with the initial ratio of Ba:Fe equal to 1:10.8. The cationic site preferences of Co 2+ and Ti 4+ in Co–Ti substituted Ba ferrite were investigated by magnetic measurements and Mössbauer spectroscopy. The coercivity H c, specific magnetic polarisation J s-m( J s-r) and Curie temperature T c is decreasing with increasing Co–Ti substitution. The variation of these parameters can be explained by the replacement of the Fe 3+ ions in the 2b and 4f 2 sites, and by splitting the 12k site on the two magnetically non-equivalent sublattices (12k and 12k′), which reduces the effective uniaxial anisotropy of Ba ferrite.