4f Sites Research Articles

Microstructure and magnetic properties of low-temperature sintered CoTi-substituted barium ferrite for LTCC application

In this article, the influences of the BaCu(B 2O 5) (BCB) additive on sintering behavior, structure and magnetic properties of iron deficient M-type barium ferrite Ba(CoTi) x Fe 11.8−2 x O 19 (BaM) have been investigated. It is found that the maximum sintered densities of BaM change from 86% to 94% as the BCB content varies from 1 to 4 wt%. Single-phase BaM can be detected by the XRD analysis in the sample with 3 wt% BCB sintered at 900 °C, and the microstructure is hexagonal platelets with few intragranular pores. This is attributed to the formation of the BCB liquid phase. Meanwhile, the experimental results illuminate that the CoTi ions prefer to occupy the 4f2 and 2b sites and the magnetic properties depend on the amount of CoTi-substitution. In addition, the chemical compatibility between BaM and silver paste is also investigated; it can be seen that BaM is co-fired well with the silver paste and no other second phase is observed. Especially, the 3 wt% BCB-added Ba(CoTi) 0.9Fe 11O 19 sintered at 900 °C has good properties with the sintered density of 4.9 g/cm 3, saturation magnetization of 49.7 emu/g and coercivity of 656.6 Oe. These results indicate that it is cost effective in the production of Low Temperature Co-fired Ceramics (LTCC) multilayer devices.

Factors Affecting the Superconducting Transition Temperatures of β-Pyrochlore Oxides AOs2O6(A=K, Rb and Cs)

The four crystallographically inequivalent atoms A, B, O, and O' in the face centered cubic unit cell occupy the 16d, 16c, 48f, and 8b sites of the Fd3m space group, respectively. The ideal pyro-chlore structure is composed of two types of cation-oxygen sublattices: one is a corner sharing tetrahedral network compos-ed of A atoms with an O' atom in the center of each tetrahedron, and the other is another tetrahedral network of B atoms with each B atom coordinated quasi-octahedrally by six O atoms. This type of tetrahedral network is called the pyrochlore lattice. The compounds where A is a tr ivalent and B a tetravalent ca-tion (A

Preparation and Photoluminescence Properties of Eu2+-Doped Oxyapatite-Type SrxLa10−x (SiO4)6O3−x/2

Eu2+-doped oxyapatite SrxLa10−x(SiO4)6O3−x/2 phosphors are prepared by solid-state reaction at high temperatures under reducing atmosphere. Their crystal structures and photoluminescence are investigated by x-ray diffraction (XRD) and fluorescence spectroscopy, respectively. The XRD results indicate that the samples are pure oxyapatite phase (P63/m space group). The fluorescence spectra show two peaks corresponding to two sites (4f and 6h sites) for Eu2+ in the host lattice. As the Eu2+ content influences the intensity ratio of the two observed emission peaks, the photoluminescence mechanism is discussed.

Anomalous phase composition in the two-phase region of DyFe3−xAlx (x≤1.0)

The structure transitions and phase relationships of DyFe3−xAlx compounds have been investigated by X-ray powder diffraction. Our XRD results show that each of the compounds with x≤0.45 crystallizes in the rhombohedral PuNi3-type structure with space group R3¯m and Z=9; for the 0.8≤x<1.0 compounds, each has a hexagonal structure of the CeNi3 type with space group P63/mmc and Z=6; and each of the samples with 0.45<x<0.8 is a two-phase mixture of the PuNi3- and CeNi3-type structures. The calculated XRD intensities of the DyFe3−xAlx compounds with x=0.2, 0.33, 0.4, and 0.45 indicate that Dy occupies the 3a and 6c sites, Fe and Al distribute randomly on the 18h site, and the 3b and 6c sites are exclusively occupied by Fe, which agrees well with those of our experimental XRD patterns. The XRD intensities of the DyFe3−xAlx compounds with x=0.8 and 1.0 have also been calculated and found to agree with the experimental results with Dy on the 2c and 4f sites, Fe and Al at the 12k site, and Fe at the 2a, 2b, and 2d sites. In the two-phase region with x=0.45–0.8, the values of unit-cell parameters and phase compositions are linearly dependent on the value of x, indicating that the two phases are constituted by the same composition x with different stacking arrangements. This abnormal two-phase equilibrium is further confirmed by the structural analysis of the DyFe2.33Al0.67 (or x=0.67) sample. The samples with x=1.1 and 1.2 were also analyzed, and each found to be a mixture of more than two phases.

Structure and electrical properties of the new pyrochlore-type protonic solid electrolyte K0.88Nb2O7.58H4.28

Single-crystal, synchrotron powder X-ray diffraction and neutron powder diffraction studies of the novel pyrochlore-type compound with the structural formula K(0.88)(OH)(0.54)H(1.66)(H(2)O)(1.04)Nb(2)O(6) suggests that the water molecules are located in 32e sites, and the hydroxide ions and potassium ions are located in 16d sites with a significant amount of 'free' protons in 96g sites. The total weight loss at temperatures up to 773 K is only about 8%, suggesting the oxygen escape from 48f sites can be excluded and 'free' protons must be preserved in the structure. The bulk conductivity in ambient air reaches 10(-2) S cm(-1) at 623 K. Owing to the extended stability range and resistance to water solubility, the compound can be considered as a candidate for intermediate temperature solid-oxide fuel-cell applications.

Neutron scattering study of short-range correlations and ionic diffusion in copper selenide

The paper reports the results of a neutron scattering study of $$ {\hbox{C}}{{\hbox{u}}_{{2 - \delta }}}{\hbox{Se}} $$ superionic compounds. The crystallographic model fitted to the diffraction data shows the occupation of 8c and 32f sites by Cu atoms. Observed diffuse background is related to correlated thermal vibrations of Se and Cu atoms, with Se↔Cu (8c,32f) and Cu (8c)↔Cu (8c) correlations being most important. The quasi-elastic neutron experiments show the decrease of the self-diffusion coefficient with the deviation from the stoichiometry due to the longer residence time of Cu ions between diffusion hops. Combination of neutron diffraction, diffuse scattering and quasi-elastic scattering experimental data suggests that the Cu atoms diffuse between the nearest 8c sites through the 32f sites.

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.

Comparative analysis of crystal field effects and energy level scheme of six-fold coordinated Cr4+ ion in the pyrochlores, Y2B2O7 (B=Ti4+, Sn4+)

The electronic energy levels of the six-fold coordinated Cr4+ ion in the pyrochlores Y2B2O7 (B=Sn4+, Ti4+), have been computed using the exchange charge model of crystal field theory. The calculated Cr4+ energy levels and their trigonal splitting are in good agreement with experimental spectra. Calculations of the crystal field parameters show that the higher crystal field strength in Y2Sn2O7 (in comparison with Y2Ti2O7) arises from increased orbital overlap effects between the Cr4+ ion and the nearest oxygen ions, which are located at the 48f crystallographic position of the pyrochlore lattice. The increased overlap in Y2Sn2O7 occurs despite the fact that the Cr4+–O2– bond distance in Y2Sn2O7 is longer than in Y2Ti2O7. This is attributed to a lack of hybridization (covalent bonding) between the filled 2p orbital of oxygen ion occupying the 48f site of the pyrochlore lattice and the filled Sn4+ 4d10 orbital. As a result, a stronger crystal field is experienced by Cr4+ ions in Y2Sn2O7, even if the Cr4+–O2− distances are greater in this case, when compared to those in Y2Ti2O7.

Spin dependent Compton scattering study of magnetic transitions in Ir doped CeFe2

First and second order magnetic transitions in Ce(Fe0.97Ir0.3)2 are studied using the magnetic Compton scattering technique. The measurements on polycrystalline sample were carried out at SPring8, Japan for different temperatures and magnetizing fields using 175 keV elliptically polarized synchrotron radiation. The temperature dependent magnetic effects (ratio between magnetic scattering intensity and charge scattering intensity) show the magnetic transitions from antiferromagnetic→ferromagnetic→paramagnetic phases which are consistent with the magnetization data. The temperature and field dependent spin-polarized momentum densities have been analyzed mainly in terms of contribution from Fe(3d) and Ce(4f) sites to determine their role in the formation of total spin moments and thereby magnetic transitions.

Comparative ab initio study of electronic, optical and chemical bonding properties of pyrochlores, Y2B2O7 (B=Ti4+, Sn4+)

The nature of chemical bonding that is responsible for the disparity between the optical properties of the impurity ions in the pyrochlores, Y2Sn2O7 and Y2Ti2O7 has been quantified by computing in the density functional theory framework the structural, physical electronic and optical properties of the host lattices. The partial density of states calculations and the estimation of the Mulliken charges indicate the significant different behavior of the oxygen ions in the two different crystallographic positions (48f and 8b) of the pyrochlore lattice. The hybridization between the filled 2p orbital of oxygen ion on the 48f site (O48f2−) and the empty 3d0 orbital of Ti4+ results in an anomalously lower charge on O48f2− in Y2Ti2O7 when compared with that in Y2Sn2O7 with filled Sn4+ 4d10 orbital. This covalent mixing is chiefly responsible for the disparity between the optical properties of the impurity ions in Y2Sn2O7 and Y2Ti2O7. Additional information on the nature of chemical bonding is provided by calculating the influence of external hydrostatic pressure on the interatomic distances and the elastic properties of the pyrochlores.

A Simple Oxidation Route to Prepare Pseudobrookite from Panzhihua Raw Ilmenite

A simple oxidation route to prepare pseudobrookite from Panzhihua raw ilmenite was developed. By oxidizing ilmenite concentrate in air for 3 h at 1473 K, a pure pseudobrookite was achieved. The X‐ray diffraction studies showed a possible sequence of the phase evolution with temperature, FeTiO3→FeTiO3+Fe2O3+TiO2→Fe2O3+TiO2+Fe2Ti3O9→Fe2O3+TiO2+Fe2TiO5→Fe2TiO5. The sluggish formation reaction of the target product was ascribed to the low heat treatment temperature. During the phase transformation, the extra Ti atoms from FeTiO3 accommodate into the 4c and 8f sites to form the solid solution (Fe,Ti)2TiO5. AC susceptibility determinations indicated that the as‐prepared pseudobrookite was an antiferromagnetic compound and performed the typical anisotropic spin glass behaviors. The decreased phase transition temperature of the prepared pseudobrookite from order to disorder meant a high disorder degree. This oxidation route might be useful to the comprehensive utilization of raw ilmenite, especially for Panzhihua ilmenite in Pan‐Xi region of China.

The role of cations distribution on magnetic and reflection loss properties of ferrimagnetic SrFe12−x(Sn0.5Zn0.5)xO19

The site preference and magnetic properties of Zn–Sn substituted strontium ferrite SrFe12−x(Sn0.5Zn0.5)xO19 particles with x=0–5 were studied using both F57e and S119n Mössbauer spectroscopies and magnetic measurements. The results show that the magnetization and magnetocrystalline anisotropy are closely related to the distributions of Zn–Sn ions on the five inequivalent crystal sites provided by the hexagonal structure of the ferrite. Mössbauer spectra show that the Zn–Sn ions preferentially occupy the 2b and 4f2 sites. The preference for these sites is responsible for the anomalous increase in the magnetization at low Zn–Sn substitutions. Based on microwave measurements of reflectivity, SrFe12−x(Sn0.5Zn0.5)xO19 is a good candidate for electromagnetic compatibility and other practical applications at high frequencies.

Ionic conductivity of apatite-type solid electrolyte material, La 10− XBa XSi 6O 27− X/2 ( X = 0–1), and its fuel cell performance

We prepared Ba substituted lanthanum silicate (La 10− X Ba X Si 6O 27− X/2 ) and examined the effect of Ba substitution on the crystal structure and conductivity. The X-ray diffraction (XRD) results for a series of compositions showed that Ba ion can occupy the La site of an apatite structure with the composition La 10− X Ba X Si 6O 27− X/2 ( X = 0–1). Rietveld analysis of the synchrotron XRD profiles revealed Ba occupation in the La 4f site rather than the La 6 h site and a decrease in the occupation factors of the oxide ions of the SiO 4 tetrahedra. The conductivity of La 10− X Ba X Si 6O 27− X/2 exhibited a maximum at X = 0.6 and the value was the same as that of YSZ (8 mol% Y 2O 3 doped ZrO 2) at 750 °C. On the other hand, the activation energy of about 50 kJ mol −1 for La 9.4Ba 0.6Si 6O 26.7 was smaller than that of YSZ. Thus the conductivity of La 9.4Ba 0.6Si 6O 26.7 was a higher than those of YSZ below 750 °C. The conductivity parallel to the c-axis which is attributed to the 2a site oxide ions migration is known to be dominant in La 10Si 6O 27. However, Ba substitution seems to produce oxygen vacancies and create another pathway for oxide ions perpendicular to the c-axis. The increase in the pathways leads to an increase in the conductivity. We also reported the solid oxide fuel cell (SOFC) performance of La 10− X Ba X Si 6O 27− X/2 with a maximum power density of 65 mW cm −2 using La 0.9Sr 0.1CoO 3− δ as a cathode and NiO-SDC (Sm doped CeO 2) as an anode.

Investigation of (R 1− xR′ x) 2(Fe,Co) 17 powders by Mössbauer spectrometry

We investigated (R 1− x R′ x ) 2Fe 17 and (R 1− x R′ x ) 2Fe 16Co powders by 57Fe Mössbauer spectrometry at 295 K where R and R′ are Y, Nd, Sm, Gd. The spectra of the (R 1− x R′ x ) 2Fe 17 compounds have been fitted with seven sextets: 18f 12, 18f 6, 18h 12, 18h 6, 9d 6, 9d 3 and 6c. The isomer shifts of the different Fe sites are in agreement with the Wigner–Seitz cell volumes. They do not depend on the nature of the rare earth. The hyperfine fields are consistent with the evolution of the Curie temperature. To account for substitution effects due to the partial replacement of Fe by Co, the Mössbauer spectra of the (R 1− x R′ x ) 2Fe 16Co samples have been fitted with four sextets corresponding to the 18f, 18h, 9d, and 6c Fe sites, by considering the site occupancy of the Co atoms in the lattice. The evolution of the hyperfine field with the rare-earth concentration is discussed and interpreted in relation with the magnetic coupling between rare-earth and Fe magnetic sublattices, in agreement with previous magnetic measurements.

Local compressibility and substitutional site for the dominant Cr3+-centered octahedron in LaMgAl11O19 crystal

The optical spectra and the pressure-induced R-line shift of the dominant Cr(3+) center in LaMgAl(11)O(19) (LMA) crystal are studied. From the studies, the local linear compressibility d ln R/dP approximate to -1.21 x 10(-4) /kbar for the dominant Cr(3+)-centered octahedron in LMA crystal is obtained. By comparing this compressibility with that in ruby (Al(2)O(3):Cr(3+)), we confirm the assignment in the previous paper that the dominant Cr(3+) center is Cr(3+) ion at 4f site of LMA crystal. (C) 2009 Elsevier B.V. All rights reserved.

Effects of combined substitution of Dy and Mo or Si on the crystal structures and magnetic properties of Nd1−y Dy y Fe11−x TiM x (M = Mo, Si) compounds

A series of Nd1−yDyyFe11−xTiMx (M = Mo, Si) alloys have been prepared by arc melting and studied by X-ray diffraction and neutron diffraction. All samples are found to crystallize in the ThMn12-type structure. The lattice parameters a and c and unit cell volume V of Nd0.5Dy0.5Fe11−xTiMox alloys increase linearly with increasing content of Mo (x), while the lattice parameters a and c and unit cell volume V of Nd0.5Dy0.5Fe11−xTiSix alloys decrease linearly with increasing content of Si(x). In NdyDy1−yFe11−xTiMx (M = Mo, Si) compounds, Ti and Mo atoms preferentially occupy the 8i sites and Si atoms preferentially occupy the 8j and 8f sites. Magnetic measurements show that the substitution of Fe by either Mo or Si leads to a decrease in the Curie temperature.

An interpretation of the optical properties of Cr 4+-activated pyrochlores, Y 2Ti 2O 7 and Y 2Sn 2O 7

A qualitative model is proposed to explain the differences between the optical properties of the six-coordinated Cr 4+ ion in the pyrochlores, Y 2Sn 2O 7 and Y 2Ti 2O 7. It is shown that polarization of the electron density of the oxygen ions occupying the 48f sites of the pyrochlore lattice is responsible for the observed differences in the optical properties.

Pr3+ 1S0→Cr3+ energy transfer and ESR investigation in Pr3+ and Cr3+ activated SrAl12O19 quantum cutting phosphor

With the help of the Dexter’s theory, the energy transfer mechanism from Pr 3+ 1 S0 to Cr 3+ is investigated theoretically in SrAl12O19:Pr 3+ ,C r 3+ quantum cutting phosphors. The electron spin resonance (ESR) spectra of Pr 3+ and Cr 3+ -doped SrAl12O19 with magnetoplumbite structure have been studied. The Cr 3+ ion is found to enter the Al4(4f) site, which is very close to the Sr(2d) site replaced by the Pr 3+ ions in the host. The theoretical results indicate that the efficient Pr 3+ 1 S0-Cr 3+ energy transfer can only take place in the intermediate mirror planes, in which for the nearest and next-nearest Pr 3+ –Cr 3+ pairs, both dipole–dipole and dipole–quadrupole interactions can play their parts in the transfer. Finally, an overview is given about the research on the energy transfer from Pr 3+ 1 S0 to codopants in their co-doped materials.

Crystal structure and magnetic transition of MnFePGe compound prepared by spark plasma sintering

The crystal structure and magnetic transition were studied for the bulk Mn1.1Fe0.9P0.8Ge0.2 compound prepared by a simple blending and subsequent spark plasma sintering route. X-ray diffraction analysis and refinement show that the compound crystallizes in the hexagonal Fe2P-type structure, in which the Mn atoms occupy all the 3g sites and some 3f sites, the Fe atoms occupy the rest of the 3f sites, and P and the Ge atoms randomly occupy the 2c and 1b sites. Magnetic measurement indicates that the Curie temperature TC is at 253K and the thermal hysteresis of M-T curves at TC upon heating and cooling, a signature of a first-order magnetic phase transition, is about 15K. The maximum magnetic entropy change of the compound reaches as high as 49.2J∕kgK in a field change from 0to5T at 253K, while the adiabatic temperature change of the compound reaches only 1.2K in a field change from 0to1.5T at the same temperature.

Theoretical study on structural properties for rare earth intermetallic compounds RFe2Zn20-xInx

The rare earth intermetallic compounds, RFe2Zn20 （R represents rare earth）, have become one of hot spots in research field due to their excellent magnetic properties with lower concentration of rare earth atoms. The addition of quaternary could influence the structural properties. The crystal structures of ternary RFe2Zn20 and site preferences of quaternary RFe2Zn20-xInx compounds are investigated by using interatomic potentials based on the lattice inversion technique. The calculated results show that the lattice constants and cell volumes of RFe2Zn20 decline linearly with atomic weight of rare earth increasing and the linear relation is independent of whether the quaternary element is added into the compounds. The In atoms substituted for Zn atoms preferentially occupy 16c site and then 96g site, without 48f site. The sequence of site preference is coincident with the measurements and the analyses from interatomic potentials.