B-site Fe3 Research Articles

Ambient pressure synthesis and properties of LaCu3Fe2TiSbO12: New A-site ordered ferrimagnetic quadruple perovskite

A large number of quadruple perovskites are synthesized and reported under high pressure while ambient pressure synthesized quadruple perovskites are limited in number. Recently, ambient pressure synthesis of quadruple perovskites has gained much interest.1 The A-site ordered quadruple perovskite, LaCu3Fe2TiSbO12, reported for the first time, is synthesized by high-temperature solid-state reactions at ambient pressure. Rietveld structure refinement using P-XRD data reveal formation of LaCu3Fe2TiSbO12 in body centered cubic Im-3 space group with lattice parameter of 7.4465(7) Å. The compound shows complete 1:3 order of A-site La with A′-site Cu and B/B′-site disorder of Fe with Ti and Sb instead of both A- and B-site order in CaCu3Fe2Sb2O12 (Larregola et al., 2014; Chen et al., 2013) [1,2]. The magnetic transitions observed in the compound around 160 and 60 ​K are attributed to short-range orders. The static disorder at the B-sublattice introduced by non-magnetic Ti and Sb precludes the possibility of long-range magnetic order in the compound. Lower saturation magnetization presumably supports antiferromagnetic ordering of A-site Cu2+ and B-site Fe3+ spins interrupted by the B-site disorder restricting it to a short-range phenomenon. The ferrimagnetic quadruple perovskite also show reasonably high dielectric constant in the low-frequency region and a low dielectric loss at room temperature. The stabilization of LaCu3Fe2TiSbO12 under ambient pressure despite having a fairly large global instability index (GII, calculated with SPuDS) of 0.276 v.u. is interesting and may suggest accessibility of many new quadruple perovskites at atmospheric pressure.

Iron force constants of bridgmanite at high pressure: Implications for iron isotope fractionation in the deep mantle

The isotopic compositions of iron in major mantle minerals may record chemical exchange between deep-Earth reservoirs as a result of early differentiation and ongoing plate tectonics processes. Bridgmanite (Bdg), the most abundant mineral in the Earth’s lower mantle, can incorporate not only Al but also Fe with different oxidation states and spin states, which in turn can influence the distribution of Fe isotopes between Bdg and ferropericlase (Fp) and between the lower mantle and the core. In this study, we combined first-principles calculations with high-pressure nuclear resonant inelastic X-ray scattering measurements to evaluate the effects of Fe site occupancy, valence, and spin states at lower-mantle conditions on the reduced Fe partition function ratio (β-factor) of Bdg. Our results show that the spin transition of octahedral-site (B-site) Fe3+ in Bdg under mid-lower-mantle conditions generates a +0.09‰ increase in its β-factor, which is the most significant effect compared to Fe site occupancy and valence. Fe2+-bearing Bdg varieties have smaller β-factors relative to Fe3+-bearing varieties, especially those containing B-site Fe3+. Our models suggest that Fe isotopic fractionation between Bdg and Fp is only significant in the lowermost mantle due to the occurrence of low-spin Fe2+ in Fp. Assuming early segregation of an iron core from a deep magma ocean, we find that neither core formation nor magma ocean crystallization would have resulted in resolvable Fe isotope fractionation. In contrast, Fe isotopic fractionation between low-spin Fe3+-bearing Bdg/Fe2+-bearing Fp and metallic iron at the core-mantle boundary may have enriched the lowermost mantle in heavy Fe isotopes by up to +0.20‰.

Investigation on different valence B-site ions doped Pr0.6Sr0.4FeO3-δ ceramic membrane for hydrogen production from water splitting

This study investigates the different valence B-site ions doped on perovskite-type oxygen transport membrane for hydrogen production by water splitting. Perovskite-type Pr0.6Sr0.4Fe0.9M0.1O3-δ (PSFM, M = Fe, Al, Zr, and W) are fabricated successfully by the sol-gel method and form dense membranes with orthorhombic structure. The microstructure, chemical stability, and hydrogen production performance of membranes are studied systematically. The doping of Al3+, Zr4+, and W6+ ions can enhance membranes chemical stability and long-term stability significantly, which is due to the increase of average binding energy and decrease of the valence of B-site ions. Because of higher oxygen vacancy content and lower oxygen vacancy formation energy, the PSFA membrane shows the highest hydrogen production rate of 1.07 mL min−1·cm−2 at 900 °C and stabilizes at about 1.0 mL min−1·cm−2 on long-term test. The performance degradation of PSFM membranes is attributed to the much valence variation of B-site Fe3+/4+ ions and the oxygen vacancy-related phase transition.

Structure and Mössbauer spectroscopy studies of Ni0.5Zn0.5Nd Fe2-O4 (0.00 ≤ x ≤ 0.10) ferrites

Neodymium (Nd) doped Ni0.5Zn0.5NdxFe2-xO4 (0.00 ≤ x ≤ 0.10) ferrites were synthesized by ceramic method. Structure, phase formation and cation distribution is studied by employing X-ray diffraction (XRD), FTIR and Mössbauer spectroscopy. XRD spectra confirm single (spinel cubic) phase in all ferrites. Surface morphology of the ferrites shows uniform distribution of grains with porous surface features. FTIR spectra indicate that the main absorption bands related to tetrahedral and octahedral metal-oxygen bond lying in the range 850–550 cm−1. Room temperature transmission Mössbauer spectra show weak doublet peak which indicate the existence of mixed spinel structure in all ferrites. The hyperfine field (Bhf) at A-sites is negligibly affected by the addition of Nd3+ ions. The average hyperfine field (<Bhf>) at B-sites shows a non-linear variation with increasing Nd-content. This reflects that the Nd3+ ions replace A site Fe3+ ions within spinel lattice. The isomer shift (IS) corresponding to A-site varies more rapidly as compared to B-site. This indicates that the charge distribution of A-site Fe3+ ions is more influenced by Nd3+ ions as compared to charge distribution of B-site Fe3+ ions. The change in Mossbauer parameters with addition of Nd content suggests that the Nd3+ ions preferably replace A site Fe3+ ions, taking the cation distribution form (Zn0.5Fe0.5-xNdx)A[Ni0.5Fe1.5]BO4 within the spinel lattice.

High‐spin Fe2+ and Fe3+ in single‐crystal aluminous bridgmanite in the lower mantle

AbstractSpin and valence states of iron in single‐crystal bridgmanite (Mg0.89Fe0.12Al0.11Si0.89O3) are investigated using X‐ray emission and Mössbauer spectroscopies with laser annealing up to 115 GPa. The results show that Fe predominantly substitutes for Mg2+ in the pseudo‐dodecahedral A site, in which 80% of the iron is Fe3+ that enters the lattice via the charge‐coupled substitution with Al3+ in the octahedral B site. The total spin momentum and hyperfine parameters indicate that these ions remain in the high‐spin state with Fe2+ having extremely high quadrupole splitting due to lattice distortion. (Al,Fe)‐bearing bridgmanite is expected to contain mostly high‐spin, A‐site Fe3+, together with a smaller amount of A‐site Fe2+, that remains stable throughout the region. Even though the spin transition of B‐site Fe3+ in bridgmanite was reported to cause changes in its elasticity at high pressures, (Fe,Al)‐bearing bridgmanite with predominantly A‐site Fe will not exhibit elastic anomalies associated with the spin transition.

Origin of Fe3+ in Fe-containing, Al-free mantle silicate perovskite

We have studied the ferrous (Fe2+) and ferric (Fe3+) iron concentrations in Al-free Fe containing Mg-silicate perovskite (Mg-Pv) at pressure (P), temperature (T), and oxygen fugacity (fO2) conditions related to the lower mantle using a thermodynamic model based on ab initio calculations. We consider the oxidation reaction and the charge disproportionation reaction, both of which can produce Fe3+ in Mg-Pv. The model shows qualitatively good agreement with available experimental data on Fe3+/ΣFe (ΣFe = total Fe in system), spin transitions, and equations of state. We predict that under lower-mantle conditions Fe3+/ΣFe determined by the charge disproportionation is estimated to be 0.01–0.07 in Al-free Mg-Pv, suggesting that low Al Mg-Pv in the uppermost pyrolitic mantle (where majoritic garnet contains most of the Al) and in the harzburgitic heterogeneities throughout the lower mantle contains very little Fe3+. We find that the volume reduction by the spin transition of the B-site Fe3+ leads to a minimum Fe3+/ΣFe in Mg-Pv at mid-mantle pressures. The model shows that configurational entropy is a key driving force to create Fe3+ and therefore Fe3+ content is highly temperature sensitive. The temperature sensitivity may lead to a maximum Fe3+/ΣFe in Mg-Pv in warm regions at the core–mantle boundary region, such as Large Low Shear Velocity Provinces (LLSVPs), potentially altering the physical (e.g., bulk modulus) and transport (e.g., thermal and electrical conductivities) properties of the heterogeneities.

B-Cation Order Control of Magnetism in the 1322 Perovskite CaCu3Fe2Nb2O12

Cation ordering in the 1322 perovskite CaCu3Fe2Nb2O12 has been investigated by synchrotron X-ray diffraction structure analysis, high-resolution electron microscopy, and magnetic measurements. All of the samples have full 1:3 ordering of Ca and Cu cations at the perovskite A-sites, but the Fe and Nb cations at the B sites show different degrees of ordering depending on the synthesis conditions. A slow-cooled sample has almost complete rock-salt-type 1:1 cation ordering at the B sites, but a rapidly cooled sample has no long-range B-cation arrangement. The ordered compound shows a large saturated magnetization (5.7 μB/formula unit (fu)) below 170 K due to the ferrimagnetism of the A′-site Cu2+ and B-site Fe3+ spins, whereas the B-site disordered form has a small magnetization (<1 μB/fu at 5 K). The degree of B-site cation order thus controls the magnetic behavior of 1322 CaCu3Fe2Nb2O12 perovskites.

Multiple magnetic interactions in ordered perovskite-structure oxides

Cation ordering in transition-metal oxides often drastically modifies their properties. We focus on A-and-B-site-ordered quadruple perovskite-structure oxides AA'3B2B'2O12, in which transition-metal ions are included at the A', B, and B' sites in an ordered manner. In such compounds A'-A', A'-B, A'-B', and B-B' interactions compete with each other and play important role in giving rise to unusual properties. The A-and-B-site-ordered quadruple perovskite CaCu3Fe2Sb2O12with magnetic Fe3+at the B site and nonmagnetic Sb5+at the B' site was successfully synthesized under a high-pressure and high-temperature condition. The B-site Fe3+spin sublattice adapts a tetrahedral framework and the Fe3+-Fe3+antiferromagnetic interaction causes geometrical spin frustration as seen in the double perovskite Ca2FeSbO6. With the introduction of Cu2+into the A' site, the frustration is relieved by strong antiferromagnetic A'(Cu2+)-B(Fe3+) interaction, leading to a ferrimagnetic ordering below 160 K. When B'-site Sb5+was replaced with Re5+, another A-and-B-site-ordered quadruple perovskite CaCu3Fe2Re2O12was synthesized by a high-pressure technique. The compound contains magnetic Fe3+at the B site and Re5+at the B' sites, and strong antiferromagnetic A'(Cu2+)-B'(Re5+) interaction overcomes the A'(Cu2+)-B(Fe3+) interaction, leading to a ferrimagnetism with the ferromagnetic A'(Cu2+)-B(Fe3+) spin arrangement below 550 K. More importantly, the electronic structure of CaCu3Fe2Re2O12is half metallic and the compound shows large magnetoresistance by the spin-dependent transport.

Spin crossover of iron in aluminous MgSiO3 perovskite and post-perovskite

Using density functional theory+Hubbard U (DFT+U) calculations, we investigate how aluminum affects the spin crossover of iron in MgSiO3 perovskite (Pv) and post-perovskite (Ppv), the major mineral phases in the Earth's lower mantle. We find that the presence of aluminum does not change the response of iron spin state to pressure: only ferric iron (Fe3+) in the octahedral (B)-site undergoes a crossover from high-spin (HS) to low-spin (LS) state, while Fe3+ in the dodecahedral (A)-site remains in the HS state, same as in Al-free cases. However, aluminum does significantly affect the placement of Fe3+ in these mineral phases. The most stable atomic configuration has all Al3+ in the B-site and all Fe3+ in the A-site (thus in the HS state). Metastable configurations with LS Fe3+ in the B-site can happen only at high pressures and high temperatures. Therefore, experimental observations of LS Fe3+ at high pressures in Al-bearing Pv require diffusion of iron from the A-site to the B-site and should be sensitive to the annealing temperature and schedule. In the Earth's lower mantle, the elastic anomalies accompanying the B-site HS-LS crossover exhibited in Al-free Pv are likely to be considerably reduced, according to the B-site Fe3+ population.

Spin states and hyperfine interactions of iron incorporated in MgSiO3 post-perovskite

Using density functional theory+Hubbard U (DFT+U) calculations, we investigate the spin states and nuclear hyperfine interactions of iron incorporated in magnesium silicate (MgSiO3) post-perovskite (Ppv), a major mineral phase in the Earth's D″ layer, where the pressure ranges from ~120 to 135GPa. In this pressure range, ferrous iron (Fe2+) substituting for magnesium at the dodecahedral (A) site remains in the high-spin (HS) state; intermediate-spin (IS) and low-spin (LS) states are highly unfavorable. As to ferric iron (Fe3+), which substitutes magnesium at the A site and silicon at the octahedral (B) site to form (Mg,Fe)(Si,Fe)O3 Ppv, we find the combination of HS Fe3+ at the A site and LS Fe3+ at the B site the most favorable. Neither A-site nor B-site Fe3+ undergoes a spin-state crossover in the D″ pressure range. The computed iron quadrupole splittings are consistent with those observed in Mössbauer spectra. The effects of Fe2+ and Fe3+ on the equation of state of Ppv are found nearly identical, expanding the unit cell volume while barely affecting the bulk modulus.

Charge transfer and antiferromagnetic order in the A-site-ordered perovskite LaCu3Fe4O12

High resolution neutron powder diffraction has been used to study the charge states and spin order in the A-site-ordered perovskite LaCu3Fe4O12. This undergoes a first-order phase transition between cubic Im-3 structures with charge distributions LaCu3+3Fe3+4O12 and LaCu2+3Fe3.75+4O12 at TCT ≈ 400 K. Bond valence sums confirm that these charge states are adopted in the two phases, and there are no substantial valence fluctuations near the charge transfer transition. G-type antiferromagnetic order of B-site Fe3+ spins is observed in the low temperature phase LaCu3+3Fe3+4O12 and the ordered moment at 50 K is 4.0 μB. Magnetic moment is absent at the A′-site Cu3+ cation. The thermal evolution of the ordered moment enables an intrinsic TN′ ≈ 600 K to be estimated, although the actual upper limit for the spin order is TCT. No long range magnetic ordering was found in the high temperature phase, LaCu2+3Fe3.75+4O12, showing that any ordering transition for this regime has TM < TCT.

57Fe Mössbauer and infrared studies of the system Co1−x Cd x Fe2O4

Mossbauer and infrared studies were made on samples of the ferrite system Co1−xCdxFe2O4x=0.0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1. Mossbauer spectra were taken at room temperature. The spectra of the samples withx≤0.7 showed well defined Zeeman patterns and they have been analyzed with two components, one due to A-site Fe3+ ions, and the other due to B-site Fe3+ and Fe2+ ions. The pattern due to B-site appeared to be composite and an explanation is given. The spectra withx=0.9 and 1 showed only a quadrupole splitting. The effect of cadmium substitution on the various hyperfine interactions has been discussed and the cationic distribution has been deduced for all values ofx. Far infrared spectra of the ferrite samples in the range 200–700 cm−1 were reported. Four bands were observed: the high frequency bandv1 is assigned to tetrahedral complexes, and the low frequency bandv2 to octahedral complexes, a small bandv3 is due to Co2+-O2− complexes andv4 is assigned to the lattice vibration of the system. The splitting occurred in thev1 andv2 bands atx=0.9 and inv2 atx=1, indicating the presence of Fe2+ ions in octahedral sites.

Mößbauer-Untersuchung ferrimagnetischer Spinelle des Systems CdϰNi1-ϰFe2O4 / Mößbauer Investigation of Ferrimagnetic Spineis in the System CdϰNi1-ϰFe2O4

Room temperature Mößbauer spectra of ferrimagnetic spinels in the system Cdϰ 2+Fe1-ϰ 3+[Ni1-ϰ 2+Fe1-ϰ 3+]O4 have been interpreted using the polyhedron model which is based on six different environments at the A-site for the B-site Fe3+-ions. To each environment belongs one sextet with a characteristic magnetic splitting. An additional sextet is due to Fe3+-ions at the A-site.

Mössbauer investigation of zinc ferrite particles

Mössbauer spectra of Zn0.2Fe2.3O4 particles at different temperatures have been obtained. They have been analyzed with five components, one due to the A-site Fe3+ ions, and the others due to the B-site Fe3+ and Fe2+ ions. Magnetic and electronic properties have been discussed with regard to zinc substitution and temperature dependence.

57Fe,61Ni, and121Sb M�ssbauer study of oxidic spinel ferrites CoxNi5/3?xFeSb1/3O4

Antimony substituted nickel ferrites Co x Ni5/3−xFeSb1/3O4 withx=0,1/3,1,4/3, and 5/3 were prepared by the common ceramic technique, and the single-phase structure was confirmed by x-ray powder diffraction.57Fe Mossbauer spectra were recorded at 4.2 K with and without an applied magnetic fieldHlong=60 kOe in order to determine the cation distribution and the spin structure. Utilizing the cation distributions and the Yafet-Kittel spin structures for B-site Fe3+ ions, derived from Mossbauer experiments the calculated magnetic moments are in excellent agreement with the experimental values at 4.2 K. The existence of Ni2+ ions in crystallographic A-sites was confirmed by61Ni Mossbauer spectroscopy. The magnitude of the supertransferred hyperfine field at121Sb nuclei is determined mainly by a covalent spin charge transfer from A-sites, occupied by Fe3+, Ni2+, and Co2+ ions, into the 5s orbital of Sb5+ ions. The different contributions of various magnetic A-site ions to H STHF Sb may be understood by combining the energy differences between the ionic configurations, connected with the electron transfer process and the related exchange integrals.