4f Sites Research Articles

Effect of oxygen sublattice order on conductivity in highly defective fluorite oxides

The study of ionic transport in cubic bismuth oxides is important technically because these materials exhibit the highest oxygen–ion conductivity of any material known to date. From a scientific point of view, the study of ionic transport in cubic bismuth oxides also provides an understanding of how anion transport in oxides with the fluorite structure is influenced by high vacancy concentration and how this is influenced by local structure. Bismuth oxide doped with isovalent rare earth cations retains the high temperature defective fluorite structure upon cooling down to room temperature. However, these doped materials undergo an order–disorder transition of the oxygen sublattice at about 600 °C. When annealed at temperatures less than the transition temperature the oxygen sublattice continues to order, and consequently oxygen ion conductivity undergoes a decay. However, the conductivity activation energies of the ordered structures after extended aging at 500 °C were observed to be lower than those of the structures prior to aging. Modeling of ordered structures based on TEM diffraction patterns indicates a<111> vacancy ordering in the anion sublattice (occupancy ordering). Neutron diffraction studies show additional structural changes in the oxygen sublattice due to positional ordering. These studies indicate that the ionic conductivity is dependent on the distribution of oxygen ions between the regular 8c sites and the interstitial 32f sites in the fluorite structure. Based on the TEM and neutron diffraction studies and conductivity of ordered and disordered structures the influence of local structure on conductivity is described. These results indicate that ordering of anion vacancies in <111> is common to fluorite oxides at high vacancy concentrations. Further, that the tendency to order depends on the dopant radii and polarizability.

Magnetic and electronic properties of strontium hexaferrite SrFe12O19from first-principles calculations

The magnetic and electronic structure of strontium hexaferrite SrFe12O19has been investigated using density functional theory within the local spin densityapproximation. The calculations show that in this ferrite all the Fe3+ ions are fully spinpolarized with S = 5/2.The electronic structure of this ferrite is strongly influenced bythe exchange interactions between the iron ions. The most stableform of the ferrite is a ferrimagnet with the Fe3+ ions atthe 4f sites having their spin polarization anti-parallel to the rest of the Fe3+ions, in agreement with Gorter’s prediction. The ferrite SrFe12O19is calculated to be a semiconductor with both the top of the valence band and thebottom of the conduction band being dominated by Fe 3d states. A stronganisotropy was found for the conductive charge carriers. The calculatedresults are in good agreement with the available experimental data.

Cation Distribution and Melting Behavior of La3Ga5M4+O14 (M = Si, Ti, Ge, Zr, Sn, and Hf) Crystals

The crystal structure of La3Ga5M4+O14 (M = Si, Ti, Ge, Zr, Sn, and Hf) was refined from its powder x-ray diffraction data. These compounds were confirmed to be isostructural to Ca3Ga2Ge4O14 (P321(No.150), Z = 1) and the site preference of M atoms was determined. Si and Ge atoms occupy only the smaller tetrahedral (2d) sites, Ti atoms occupy both octahedral (1a) and tetrahedral (3f) sites, and the others (Zr, Sn, and Hf) all occupy the octahedral (1a) sites. Single crystals were grown by a slow cooling method using raw materials with stoichiometric compositions. An analysis of growing process showed that the La3Ga5M4+ O14 (M = Si and Ge) crystal congruently melts, and that the other compounds incongruently melt and are peritectic compounds.

A Mössbauer study of the spin reorientation transition in DyFe 11Mo

The spin reorientation transition in DyFe 11Mo around the spin reorientation temperature (220 K) is investigated by Mössbauer spectrometry. The temperature dependence of the hyperfine parameters for each Fe site reveals an obvious discontinuity of the hyperfine field. The magnitude of the discontinuity is more important for the 8f site than for the 8i and 8j sites, indicating that the most prominent contribution to the overall anisotropy in the Fe sublattice should be from the Fe ion at the 8f site. This is attributed to the 3d(Fe(8f))–3d(Mo(8i)) hybridization, which may play a quite important role in R(Fe,Mo) 12 compounds.

57Fe Mössbauer spectroscopy and magnetization study of YFe xAl 12− x (4.4⩽ x⩽5)

YFe x Al 12− x in the composition range 4.4⩽ x⩽5 was prepared by induction melting followed by annealing in vacuum at 1270 K. Magnetization data below 150 K show complex magnetic behaviour dependent on applied field, composition and temperature. The transition temperature T c, corresponding to the main maximum of the magnetization vs. temperature curves and below which magnetic interactions are observed for a significant fraction of the Fe atoms in the Mössbauer spectra, decreases from 180 K for x=4 down to 100 K for 4.2⩽ x⩽4.7 and rises again up to 160 K for x=5. The analysis of the spectra obtained at 5 K is consistent with full occupation of the 8f sites by Fe atoms and sharing of the 8j sites by Fe and Al as deduced from the Rietveld analysis of X-ray powder diffraction data. The Mössbauer spectra further show a dependence of magnetic hyperfine fields and isomer shifts on the crystallographic site and on the number of the Fe nearest neighbours similar to that observed in UFe x Al 12− x (4⩽ x⩽6) and RFe x Al 12− x (R=Y, Lu, x=4, 4.2). The magnetic properties of the UFe x Al 12− x and YFe x Al 12− x series are compared and the magnetic interactions between the different Fe sublattices are discussed.

Sc2TiO5, an entropy-stabilized pseudobrookite-type compound

Discandium(III) titanate(IV), Sc2TiO5, prepared by the flux method, has a pseudobrookite-type structure. The Sc and Ti atoms are partially disordered on the two octahedrally coordinated metal (M) sites (Wyckoff designations 8f and 4c in space group Cmcm). The most probable cation distribution suggests the approximate structural formula [8f](Sc0.6,Ti0.4)2[4c](Sc0.8Ti0.2)O5, in agreement with earlier observations of a preference of Ti for the more distorted 8f site in isotypic AIII2BIVO5 compounds. The average M—O bond lengths for the 8f and 4c sites are 2.059 and 2.095 A, respectively. The strongly distorted octahedra share edges to form trioctahedral units, which are linked into infinite double chains along c. Further sharing of octahedral edges results in a three-dimensional framework. All atoms are on special positions. An Al-bearing variety of Sc2TiO5 has Al preferentially incorporated on the 4c site.

The Magnetism of Rare Earth Intermetallics RFe4Al8

The YFe4Al8 and ErFe4Al8 intermetallics were studied by57Fe Mossbauer spectroscopy between 2 K and room temperature. The spectra are consistent with iron occupying 8f sites only. There is hardly any difference between corresponding spectra of YFe4Al8 and ErFe4Al8. The Neel temperatures (≈185 K) and saturation hyperfine fields (≈11 T) are practically the same, confirming that the presence of the additional rare earth magnetic sublattice has hardly any influence on the magnetism of the Fe sublattice. From the saturation hyperfine field a low moment of ≈0.7μ B on the Fe ions is deduced. The Mossbauer spectra show that atT N not all Fe ions order magnetically, a fraction of Fe ions remaining in a magnetically non-ordered state. This non-ordered fraction diminishes when the temperature decreases, but never vanishes completely. This peculiar behaviour seems to be characteristic for allx=4 members of the RFe x Al12−x series of alloys. Its origin is not understood at this stage. It might be connected with the complex Fe spin structure exhibited by thex=4 intermetallics and needs to be considered in interpretations of magnetic measurements.

Structure and Curie temperature of Y2(Fe1-x-y,Coy,Crx)17 compounds

The structure and Curie temperature of Y2(Fe1-x-y,Coy ,Crx)17 c ompounds have been investigated by means of x_ray diffraction and magnetization measurement. The Y2(Fe1-x-y,Coy,Crx) 17 compounds have a hexago nal Th2Ni17_type structure. With increasing x, the unit_ce ll volumes of Y2Fe17-xCrx and Y2CoFe16-xCrx compounds have a small decreas e initially, followed by a greater decrease. It is proved that the magnetic inte raction of Fe atoms at 4f sites of Y2Fe17 compound is anti ferromagnetic by studying the variation of Curie temperature of Y2(Fe1-x-y,Coy,Crx )17 compounds. The result also shows that a few Co and Cr atoms subst ituti ng for Fe atoms can enhance Curie temperature of Y2(Fe1-x-y,Coy,Crx)17 obviously.

A Structural Investigation of La 2(GeO 4)O and Alkaline-Earth-Doped La 9.33(GeO 4) 6O 2

The structures of the oxyorthogermanate La 2(GeO 4)O and the apatite-structured La 9.33(GeO 4) 6O 2 have been refined from powder neutron diffraction data. La 2(GeO 4)O crystallizes in a monoclinic unit cell ( P2 1/c ) and is cation stoichiometric in contrast to previous reports. La 9.33(GeO 4) 6O 2 crystallizes in a hexagonal unit cell ( P6 3/m ) and the powder diffraction data show anisotropic peak broadening that is observed in electron diffraction patterns as incommensurate diffuse spots at hkq reciprocal planes (with q=1.6–1.7) and can be attributed to a correlated disorder in the “apatite channels”. This compound was doped up to a nominal composition close to M 2La 8(GeO 4) 6O 2 with M=Ca, Sr, Ba. The dopant ions preferentially occupy the 4f sites as the number of La vacancies decreases. The measured ionic conductivity of La 9.33(GeO 4) 6O 2 is about 3 orders of magnitude larger than for La 2(GeO 4)O at high temperatures and decreases with increasing dopant content from the highest value of about 0.16 S cm −1 at 1160 K.

First Principles Calculations of the Adsorption Properties of CO and NO on the Defective TiO2(110) Surface

First-principles calculations based on spin-polarized density functional theory (DFT) and the generalized gradient approximation (GGA) have been used to study the adsorption of CO and NO molecules on the rutile (TiO2) (110) surface in the presence of oxygen vacancy sites. The calculations employ slab geometry and periodic boundary conditions with full relaxation of all atomic positions. We have identified several possible adsorption configurations at both Ti five-coordinated (Ti(5f)) and four-coordinated (Ti(4f)) sites, finding that adsorption binding energies are dependent on the defect density. Among these configurations the most stable have been found at the Ti(4f) sites in the case of the surface with missing bridging-oxygen rows. In this case both CO and NO molecules can bind either on-top of Ti(4f) atoms or in vertical or tilted bridge configurations to neighbor Ti(4f) sites. The highest binding energies we have determined are 36 kcal/mol for CO and 87.15 kcal/mol for NO, respectively, and correspond to tilted bridge molecular configurations along the [001] direction. The large increase of the binding energies on the defective surface relative to the full oxidized surface indicates that the adsorption on vacancy defect sites takes place through a predominantly chemisorption mechanism. Additional calculations performed for N2O and N2O2 molecules indicate that on the defective surface the adsorption at Ti(4f) is also preferred with maximum adsorption energies of 51.2 and 126.7 kcal/mol.

Luminescent Properties of Eu3 + in Defect Apatite, Sr3La6 ( SiO4 ) 6

is one of the defect silicate apatites. -doped i.e., solid solutions, were synthesized and their luminescent properties were studied. The emission intensity increased with increasing concentration up to and decreased to At it raised again dramatically. In addition, the lattice parameter, a, decreased quickly with increasing concentration between and This fact was explained by the change of site occupancies of and ions and defects on the 4f and 6h sites in © 2002 The Electrochemical Society. All rights reserved.

A neutron powder diffraction study of electrochemically lithiated R-Li3+xFe2(PO4)3 for x = 1.8

The electrochemically lithiated form of R-Li3Fe2(PO4)3, synthesised by ion exchange of fine powders of Na3Fe2(PO4)3 in a concentrated aqueous solution of LiNO3 at a slightly elevated temperature (40 °C), has been investigated by a combination of ex situ X-ray and neutron diffraction to probe particularly the lithium-ion distribution in the structure. Li3+xFe2(PO4)3 samples were extracted from discharged electrochemical cells with a Li-metal anode; their structure was refined by the Rietveld method. Approximately 1.8 Li per formula unit can be inserted reversibly into the structure, corresponding to the reduction of almost all Fe3+ to Fe2+. Ex situ X-ray powder diffraction shows the Fe2(PO4)3 framework to remain intact during lithiation. The Li(18f) site in R-Li3Fe2(PO4)3 (space group: R) is totally vacated in R-Li3+xFe2(PO4)3, 0 ≤ x ≤ 1.8, with the lithium ions moving into two new general (18f) positions, Li(1) and Li(2), with 74(4)% and 86(4)% occupation, respectively. This corresponds to the Li+ ions moving cooperatively from pairs of layers (alternate separations: 2.2 and 5.4 A) for x = 0 to equally spaced layers (separation: 3.8 A) for x = 1.8.

Neutron diffraction analysis of melt spun 2:14:1 type (NdPr)–Fe–B compounds with Ti and Zr additions

There has been continued technological and research interest in Nd2Fe14B-based compounds due to their excellent intrinsic magnetic properties [(BH)max*∼64 MGOe,Ha∼73 kOe]. It is found that Ti and Zr substitution in Nd2Fe14B leaves its magnetic properties largely unaffected. We have carried out neutron diffraction studies on Zr and Ti substituted 2:14:1 compounds. Three specimens of melt spun alloys of compositions (Nd0.75Pr0.25)12Fe80B8, (Nd0.75Pr0.25)8.4Fe79.7Ti4.7B7.2 and (Nd0.75Pr0.25)9.2Fe79.2Ti2Zr2.2B7.4 were studied. Rietveld refinements reveal that the 4f site is equally occupied by Nd and Pr while the 4g site is occupied only by Nd. In the Ti-containing sample, the lattice expands and Ti is found on the j1, j2 sites normally occupied by Fe, (4% and 8%, respectively). This accounts for about 30% of the added Ti. In the third sample, the lattice contracts, which could occur only if Zr replaces the larger rare-earth atoms. However, due to the possibility of four different species occupying the 4f site (i.e., Nd, Pr, Ti, and Zr), it is not possible to determine their relative concentrations from neutron diffraction data alone. The balance of Ti and Zr in this sample must be locked in the grain boundaries. Despite peak broadening associated with the fine grain structure, robust refinements were achieved.

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.

Magnetic structure and anisotropy of YFe 6 Ga 6 and HoFe 6 Ga 6

The magnetic structure of RFe6Ga6 intermetallic compounds with R = Y, Ho have been determined by neutron powder diffraction, 57Fe Mossbauer spectroscopy, AC susceptibility, TGA (Thermo-Gravimetric Analysis) and magnetization measurements. Both compounds crystallize in the tetragonal ThMn12 structure (space group I4/mmm) with the magnetic structure of YFe6Ga6 consisting of a simple ferromagnetic alignment of Fe moments in the basal plane with a Curie temperature of 475(5) K. Gallium atoms are found to fully occupy the 8i site, with Fe and Ga atoms equally distributed over the 8j site, whilst Fe atoms fully occupy the 8f site. The average Fe moments are 1.68(10) and 1.46(10) at 15 and 293 K, respectively. The average room temperature Fe magnetic moments determined by neutron diffraction are in overall agreement with the average Fe moment deduced from Mossbauer spectroscopy and bulk magnetization measurements on this compound. The magnetic anisotropy of the compound HoFe6Ga6 is also planar in the temperature range 6-290 K, with Ho magnetic moments of 9.28(20) and 2.50(20) at 6 K and 290 K, respectively, coupled anti-ferromagnetically to the Fe sublattice and a Curie temperature of 460(10) K. The magneto-crystalline anisotropies of both compounds are comparable at low temperatures.

Electrical Properties of Proton-Conducting Ca2 + -Doped La2Zr2 O 7 with a Pyrochlore-Type Structure

The electrical conductivity and the mean transport number of pyrochlore-type 0.03, 0.05) and were systematically studied as a function of temperature under wet hydrogen and oxygen atmospheres Under wet hydrogen atmosphere, pure proton conduction was observed below 873 K for both and systems. The proton conductivities at 873 K were and for and respectively. The proton conductivity of was approximately three times higher than that of although the number of positive charges introduced by doping was equal for the two samples. The three times higher proton conductivity of than could be attributed to the higher proton concentration dissolved in than in This may have been because the oxygen and/or its vacant sites responsible for the proton dissolution were different between and systems, i.e., the 8b and the 48f oxygen sites for and the 48f oxygen site for may have contributed to the proton dissolution. As a result, the proton conductivity of -doped depended on both the doping level and the doping site. Above 873 K under a wet hydrogen atmosphere, the mean transport number of oxygen ions increased with increasing temperature. Under a wet oxygen atmosphere, the positive hole was a dominant carrier for both systems, in contrast to under the wet hydrogen atmosphere. © 2001 The Electrochemical Society. All rights reserved.

Mössbauer spectra and magnetic properties of mechanically alloyed SmFe 11− xCo xTi

The effect of Co substitution on coercivity ( H C) and 57Fe hyperfine parameters of mechanically alloyed SmFe 11− x Co x Ti ( x⩽2) were systematically studied. H C exhibits a maximum at 1123 K resulting from the competition between surface defect effect and domain size. It is shown from the isomer shift ( δ) assigned on the basis of Wigner–Seitz cell volumes that Co atoms occupy preferentially 8f sites of the ThMn 12-type structure. The mean hyperfine field ( B HF) increases with substitution.

Oxygen migration in A2B2O 7 pyrochlores

Atomic scale computer simulation has been used to predict activation energies for oxygen migration. In total, 54 compounds with the A 2 B 2O 7 pyrochlore structure were simulated. In each case, oxygen migration was assumed to proceed via an oxygen vacancy mechanism with oxygen ions hopping between 48f sites. For some compounds the unoccupied 8a interstitial position played an important role in the migration mechanism. The results were analyzed using a contour map of activation energy versus A cation radius along the ordinate and B cation radius along the abscissa. This identified areas of similar cation radii, which exhibit lower activation energy. Results compare favourably with available experimental data.

Effect of oxygen sublattice ordering on interstitial transport mechanism and conductivity activation energies in phase-stabilized cubic bismuth oxides

Bismuth oxide doped with isovalent rare earth cations retains the high temperature defective fluorite structure upon cooling down to room temperature. However, these doped materials undergo an order–disorder transition of the oxygen sublattice at about 600°C. When annealed at temperatures less than the transition temperature, the oxygen sublattice continues to order, and consequently oxygen ion conductivity undergoes a decay. However, the conductivity activation energies of the ordered structures after extended aging at 500°C were observed to be lower than those of the structures prior to aging. Modeling of ordered structures based on TEM diffraction patterns indicates a 〈111〉 vacancy ordering in the anion sublattice. Neutron diffraction studies show additional structural changes in the oxygen sublattice due to ordering. These studies indicate that the ionic conductivity is dependent on the distribution of oxygen ions between the regular 8c sites and the interstitial 32f sites in the fluorite structure. Based on the TEM and neutron diffraction studies and conductivity activation energies of the ordered and disordered structures, a transport mechanism for oxygen ions through interstitial positions is proposed.

Real disordered crystal structure and Curie temperature of intermetallic compounds Y 2Fe 17− xM x (M=Si or Al)

The crystal structure of intermetallic compounds Y 2Fe 17, Y 2Fe 15.3Al 1.7 and Y 2Fe 15.3Si 1.7 was studied by powder X-ray and neutron diffraction. The real disordered structure of the compounds was found to be a disordered variant of the Th 2Ni 17-type structure, in which the Y atoms from the 2b sites substitute in part the Fe “dumbbells” on 4f sites and the Fe atoms occupy part of vacant 4e sites. Besides, the Y atoms implant in 2c sites in the iron plane of the compounds, which causes plane distortions and splitting of the 12j Fe sites into two subsites. The substitution of Fe by Si or Al was found to have a preferential character for sites in the order: 12k, 4f, 4e, 6g and 12j. Partial substitution of Al or Si for Fe is established as resulting in an increase in the atomic Fe–Fe spacing in the 4f “dumbbell” site. The Curie temperature of the compounds Y 2Fe 17, Y 2Fe 15.3Al 1.7 and Y 2Fe 15.3Si 1.7 is proportional to the atomic Fe–Fe spacing in the “dumbbell” site.