Search

The metal-insulator transition driven by electronic correlations is one of the most fundamental concepts in condensed matter. In mixed-valence compounds, this transition is often accompanied by charge ordering (CO), resulting in the emergence of complex phases and unusual behaviors. The famous example is the archetypal mixed-valence mineral magnetite, Fe3O4, exhibiting a complex charge-ordering below the Verwey transition, whose nature has been a subject of long-time debates. In our study, using high-resolution X-ray diffraction supplemented by resistance measurements and DFT+DMFT calculations, the electronic, magnetic, and structural properties of recently synthesized mixed-valence Fe4O5are investigated under pressure to ∼100 GPa. Our calculations, consistent with experiment, reveal that at ambient conditions Fe4O5is a narrow-gap insulator characterized by the original Verwey-type CO. Under pressure Fe4O5undergoes a series of electronic and magnetic-state transitions with an unusual compressional behavior above ∼50 GPa. A site-dependent collapse of local magnetic moments is followed by the site-selective insulator-to-metal transition at ∼84 GPa, occurring at the octahedral Fe sites. This phase transition is accompanied by a 2+ to 3+ valence change of the prismatic Fe ions and collapse of CO. We provide a microscopic explanation of the complex charge ordering in Fe4O5which "unifies" it with the behavior of two archetypal examples of charge- or bond-ordered materials, magnetite and rare-earth nickelates (RNiO3). We find that at low temperatures the Verwey-type CO competes with the "trimeron"/"dimeron" charge ordered states, allowing for pressure/temperature tuning of charge ordering. Summing up the available data, we present the pressure-temperature phase diagram of Fe4O5 © 2022 American Chemical Society. All rights reserved. EAR-1634415; National Science Foundation, NSF: EAR-1606856; U.S. Department of Energy, USDOE: DE-FG02-94ER14466; Office of Science, SC; Argonne National Laboratory, ANL: DE-AC02-06CH11357; Deutsche Forschungsgemeinschaft, DFG: OV-110/3-2; Russian Foundation for Basic Research, РФФИ: 20-42-660027; Israel Science Foundation, ISF: 1552/18, 1748/20; Russian Science Foundation, RSF: 19-72-30043; 122021000039-4 We thank L. S. Dubrovinsky, I. A. Abrikosov, and V. Prakapenka for their interest in this research and B. Lavina for fruitful discussions about in situ DAC synthesis. We are grateful to M. Hanfland for the assistance in using beamline ID-15B of ESRF, Grenoble, France. Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation-Earth Sciences (Grant EAR-1634415) and Department of Energy-GeoSciences (Grant DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Use of the COMPRES-GSECARS gas loading system was supported by COMPRES under NSF Cooperative Agreement EAR-1606856 and by GSECARS through NSF Grant EAR-1634415 and DOE Grant DE-FG02-94ER14466. The work was partly supported by the Israel Science Foundation (Grants No. 1552/18 and 1748/20) and the Deutsche Forschungsgemeinschaft Grant No. OV-110/3-2. The theoretical analysis was supported by Russian Foundation for the Basic Research (Project No. 20-42-660027). The DFT calculations were supported by the state assignment of Minobrnauki of Russia (Theme “Electron” No. 122021000039-4). The DFT+DMFT calculations were supported by the Russian Science Foundation (Project No. 19-72-30043).

The pressure-induced Mott insulator-to-metal transitions are often accompanied by a collapse of magnetic interactions associated with delocalization of 3d electrons and high-spin to low-spin (HS-LS) state transition. Here, we address a long-standing controversy regarding the high-pressure behavior of an archetypal Mott insulator FeBO

The honeycomb lattice iridates ${A}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ ($A=\mathrm{Na}$, Li) are spin-orbit assisted Mott insulators proximate to Kitaev's quantum spin liquid. The insulating state as well as the magnetic properties are believed to arise due to a delicate balance of several energy scales. We report on high-pressure electrical transport and x-ray-diffraction measurements on ${A}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ ($A=\mathrm{Na}$, Li) in an attempt to study their structural and electronic evolution with pressure. We found that while $\mathrm{L}{\mathrm{i}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ undergoes a structural phase transition into the dimerized state at a pressure of $P$ \ensuremath{\sim} 4 GPa, in $\mathrm{N}{\mathrm{a}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ the conservation of the original $C2/m$ structure up to at least 58 GPa is observed. In addition, $\mathrm{L}{\mathrm{i}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ undergoes a sluggish structural rearrangement at the pressure range 20--40 GPa coinciding with a significant decrease in resistance. Despite dissimilar structural evolution and different mechanisms of the electrical conductivity, Arrhenius conductivity for $\mathrm{N}{\mathrm{a}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$ and Mott variable-range hopping in $\mathrm{L}{\mathrm{i}}_{2}\mathrm{Ir}{\mathrm{O}}_{3}$, both systems show a very similar $R$($P$) behavior. Namely, after a nonmonotonic decrease of the resistance $R$ and the charge gap \ensuremath{\Delta}, the \ensuremath{\Delta} stabilizes at about 45 GPa and even increases slightly with pressure; the $R$($T$) shows insulating behavior up to the highest pressure measured, 80 and 55 GPa, respectively. This resilient nonmetallic behavior of the studied iridates suggests a formation close to a localized-itinerant crossover of unusual electronic states, whose possible features are discussed. Unforeseeably, the $R$($P$) behavior is not dependent on the buffer element $A$, which seems essential for understanding the nature of the electrical conductivity in iridates.

We report high-pressure x-ray diffraction, x-ray absorption spectroscopy, and electrical transport measurements on $\mathrm{Cu}M{\mathrm{O}}_{2}$ ($M=\mathrm{Cr}$, Mn) delafossitelike oxides in an attempt to study their structural and electronic evolution with pressure. Recent studies of the similar $\mathrm{CuFe}{\mathrm{O}}_{2}$ delafossite has revealed a pressure-induced breaking of the unusual high axial anisotropy resulting in a structural phase transition coinciding with the metal-metal intervalence charge-transfer phenomenon. The present study revealed other possible scenarios responsible for the collapse of the high axial anisotropy and evolution of the O-Cu-O bonds in delafossitelike materials under pressure. Thus in $\mathrm{CuMn}{\mathrm{O}}_{2}$, the O-Cu-O dumbbells tilt with respect to the $c$ axis at $Pg13$ GPa, but in contrast to $\mathrm{CuFe}{\mathrm{O}}_{2}$, the tilting is continuous with pressure increase, justifying a second-order phase transition within the C2/m structure. Meanwhile in $\mathrm{CuCr}{\mathrm{O}}_{2}$ ($R\overline{3}m$) the first-order structural phase transition to the monoclinic structure ($P{2}_{1}/m$) is observed at about 26 GPa characterized by the discontinuous bending of the O-Cu-O bond in contrast to the tilting in the case of $\mathrm{CuFe}{\mathrm{O}}_{2}$ and $\mathrm{CuMn}{\mathrm{O}}_{2}$. In both studied systems, we did not find clear evidence of valence transformations, similar to that observed in $\mathrm{CuFe}{\mathrm{O}}_{2}$.

Structural and electronic transformation taking place in α-FeOOH goethite have been studied by Fe K-edge x-ray absorption spectroscopy at pressures up to 50 GPa. These studies have shown the symmetrization of FeO6 octahedra coinciding with the Fe3+ high to low spin transition at pressure above ~45 GPa. Our data are in excellent agreement with the results of recent single crystal XRD and Mossbauer spectroscopy studies (Xu et al 2013 Phys. Rev. Lett. 111 175501), supporting the H-bonds symmetrization in iron oxyhydroxide, resulting from the Fe3+ high-to-low spin crossover at above 45 GPa. Our study shows an applicability of the x-ray absorption spectroscopy in a further study of the H-bonds symmetrization phenomenon.

The electronic ground state in many iridate materials is described by a complex wave-function in which spin and orbital angular momenta are entangled due to relativistic spin-orbit coupling (SOC). Such a localized electronic state carries an effective total angular momentum of $J_{eff}=1/2$. In materials with an edge-sharing octahedral crystal structure, such as the honeycomb iridates Li2IrO3 and Na2IrO3, these $J_{eff}=1/2$ moments are expected to be coupled through a special bond-dependent magnetic interaction, which is a necessary condition for the realization of a Kitaev quantum spin liquid. However, this relativistic electron picture is challenged by an alternate description, in which itinerant electrons are confined to a benzene-like hexagon, keeping the system insulating despite the delocalized nature of the electrons. In this quasi-molecular orbital (QMO) picture, the honeycomb iridates are an unlikely choice for a Kitaev spin liquid. Here we show that the honeycomb iridate Li2IrO3 is best described by a $J_{eff}=1/2$ state at ambient pressure, but crosses over into a QMO state under the application of small (~ 0.1 GPa) hydrostatic pressure. This result illustrates that the physics of iridates is extremely rich due to a delicate balance between electronic bandwidth, spin-orbit coupling, crystal field, and electron correlation., 7 pages, 5 figures, additional supplemental material included

Undoped and calcium substituted hematite (α-Fe₂O₃) nanoparticles are synthesized by surfactant-directed co-precipitation and post annealing method. The annealed nanoparticles were found to be in single phase in nature and crystallize in the rhombohedral structure with space group R3c as confirmed by Rietveld refinement of the X-ray diffraction (XRD) data. Average crystallite sizes are calculated to be 20 to 30 nm and 50 to 60 nm for the nanoparticles annealed at 400 and 600 °C respectively. Mossbauer spectra for all the nanoparticles could be fitted with a sextet corresponding to the single magnetic state of the iron atoms in its Fe³⁺ state in the hematite matrix. The FTIR and Raman spectra of all the samples correspond to specific modes of α-Fe₂O₃. UV-Vis spectra of annealed samples showed broad peaks in the range of 525-630 nm resulting from spin-forbidden ligand field transition together with the spin-flip transition among the 2t₂g states. The estimated band gap energies were in the range of 1.6 to 1.9 eV which are much lower than the reported values for nano hematite. From the room temperature magnetic hysteresis loop measurements, weak ferromagnetic behavior is observed in all undoped and Ca²⁺ doped hematite samples. Morin temperature (T(M)) is calculated to be 257 and 237 K for 1.45% doped samples with particle size 54 and 27 nm respectively. The sample with Ca content of 1.45 wt% when annealed at 400 °C showed that the particles were of different shapes which included both quasi spherical and rod shaped. On annealing the same sample at 600 °C, the nanorods collapsed to form bigger spherical and ellipsoidal particles.

Mn-doped NiO nanoparticles of the series Ni 1− x Mn x O ( x =0.00, 0.02, 0.04 and 0.06) are successfully synthesized using a low temperature hydrothermal method. Samples up to 6% Mn-doping are single phase in nature as observed from powder x-ray diffraction (XRD) studies. Rietveld refinement of the XRD data shows that all the single phase samples crystallize in the NaCl like fcc structure with space group Fm-3m . Unit cell volume decreases with increasing Mn-doping. Pure NiO nanoparticles show weak ferromagnetism, may be due to nanosize nature. Introduction of Mn within NiO lattice improves the magnetic properties significantly. Room temperature ferromagnetism is found in all the doped samples whereas the magnetization is highest for 2% Mn-doping and then decreases with further doping. The ZFC and FC branches in the temperature dependent magnetization separate well above 350 K indicating transition temperature well above room temperature for 2% Mn-doped NiO Nanoparticle. The ferromagnetic Curie temperature is found to be 653 K for the same sample as measured by temperature dependent magnetization study using vibrating sample magnetometer (VSM) in high vacuum.

The nano-porous LiFe₀.₉M₀.₁P₂O₇ (M = Fe, Co, Mn, Ni) particles were successfully prepared by simple microwave assisted combustion method and studied its detailed physico-chemical properties. The phase purity, crystallinity, functional group identification was revealed through X-ray diffraction (XRD), Fourier Transform Infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis. The presence of nanoporous was identified through transmission electron microscopic (TEM) images. The electrical conductivity results illustrated that LiFe₀.₉Ni₀.₁P₂O₇ has higher conductivity (2.85 x 10⁻⁷ S cm⁻¹) among the studied systems owing to their negligible grain boundary effect. The normal dielectric behaviour was observed for all the LiFe₀.₉M₀.₁P₂O₇ (M = Fe, Co, Mn, Ni) materials. The paramagnetic behaviour and the Fe³⁺ state of LiFe₀.₉M₀.₁P₂O₇ were obtained from VSM and Mossbauer spectral analysis respectively. The cyclic voltammogram suggested that the good electrochemical lithium intercalation/de-intercalation behaviour of LiFe₀.₉M₀.₁P₂O₇ (M = Fe, Co, Mn, Ni) electrodes in aqueous electrolytes. The obtained diffusion coefficient value is comparable with carbon based materials.

Samarium orthoferrite (SmFeO3) is synthesized by a simple combustion method using aspartic acid as fuel. Phase purity and functional groups are analyzed via X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) analysis, which confirms the single phase formation of orthorhombic SmFeO3. Approximately spherical particles with size range 150-300 nm is revealed by scanning electron microscope (SEM). The conductivity of the material is identified by the single semicircle obtained in the solid state impedance spectra at elevated temperatures. The calculated electrical conductivity increases with increasing temperature, inferring the semiconducting nature of SmFeO3. A magnetic study at room temperature revealed weak ferromagnetic behaviour in SmFeO3 due to Dzyaloshinsky-Moriya antisymmetric exchange interaction mechanism. Mossbauer analysis confirmed the +3 oxidation state of iron and magnetic ordering of the sample at room temperature.

Here, we report on the enhancement in the magnetic properties of polycrystalline Bi1−xBaxFeO3 (x = 0.0, 0.05, 0.1 and 0.2) ceramic samples as a consequence of mechanical activated solid-state-reaction in comparison to standard solid-state-reaction processed samples. Our results suggest that mechanical activation has pronounced effect on the temperature of phase evolution as well as magnetic characteristics of Ba doped BiFeO3. The temperature, at which perovskite phase crystallizes is lower for mechanical activation assisted samples than for standard solid-state-reaction processed samples, attributed to pre-alloying during mechanical activation process. All the samples crystallize in a distorted perovskite structure with space group R3c and no structural change is found up to highest Ba doping. Mechanical activation also leads to significantly altered magnetic properties, particularly in higher Ba-doping samples which show about 50% increase in the magnetization and more than three fold increase in the coercive field. Local magnetic behavior investigated by 57Fe Mossbauer spectroscopy rules out any valence fluctuations of Fe and the hyperfine field corroborates the magnetization data.

Cd1−xMnxO (x=00–0.07) nanostructures were synthesized and their structural, optical and magnetic properties were investigated. The shift of diffraction peaks towards lower angle side with increase of Mn content indicates the incorporation of Mn2+ ions into the CdO lattice. The values of optical band gaps were calculated at each value of Mn concentration. The values of band gap were increased by increasing the Mn concentrations as a direct consequence of the quantum confinement effect. The undoped CdO [Cd1−xMnxO (x=0.00)] nanostructure shows weak ferromagnetic nature at room temperature. The ferromagnetic nature increases consistently with increase of Mn concentrations from x=0.01 to 0.05 and then slightly drops for x=0.07. As there were no magnetic impurities present in the samples, we assume that the origin of ferromagnetism in the undoped CdO nanostructures could be due to formation of CdO structure in triplet state (S=1). However, the consistent increase of magnetic nature with Mn doped (x=0.01–0.05) CdO nanostructures might be attributed to the ferromagnetic coupling between the spins and enhancement of spin concentrations due to entering of Mn atom into the lattice. The sudden drop of ferromagnetic nature at x=0.07 may be due to the presence of anti-ferromagnetic coupling.

Nanocrystalline spinel cobalt ferrite particles are synthesized by simple combustion method using aspartic acid and glycine as fuels. The single phase cubic structure of CoFe 2 O 4 is revealed through X-ray diffraction analysis (XRD). Further the Rietveld refinement confirms the formation of inverse spinel structure of CoFe 2 O 4 . The characteristic functional groups of Co–O and Fe–O are identified from Fourier Transform Infrared (FT-IR) analysis. Uniform distribution of of nearly spherical particles with the size range of 40–80 nm is identified through field emission scanning electron microscope (FESEM) images. The calculated DC conductivity is 1.469 × 10 −7 and 2.214 × 10 −8 S cm −1 , for CoFe 2 O 4 synthesized using aspartic acid and glycine, respectively. The dielectric behavior obeys the Maxwell–Wagner interfacial polarization. The ferromagnetic behavior of CoFe 2 O 4 is identified using VSM analysis and the calculated coercivity is 27 Oe and saturation magnetization is 68 emu/g.

We report systematic investigations on structural and magnetic properties of nanosized NiO powders prepared by the ball milling process followed by systematic annealing at different temperatures. Both as-milled and annealed NiO powders exhibit face centered cubic structure, but average crystallite size decreases (increases) with increasing milling time (annealing temperature). Pure NiO exhibits antiferromagnetic nature, which transforms into ferromagnetic one with moderate moment at room temperature with decreasing crystallite size. The on-set of ferromagnetic behavior in the as-milled powders was observed at higher temperatures (>750 K) as compared to bulk Ni (~630 K). On the other hand, annealing of as-milled powders showed a large reduction in magnetic moment and the rate of decrease of moment strongly depends on the milling conditions. The observed properties are discussed on the basis of crystallite size variation, defect density, oxidation/reduction of Ni and interaction between uncompensated surfaces and particle core with lattice expansion.

X-ray diffraction pressure studies at room temperature demonstrate that the spinel $\mathrm{FeA}{\mathrm{l}}_{2}{\mathrm{O}}_{4}$ transforms to a tetragonal phase at \ensuremath{\sim}18 GPa. This tetragonal phase has a highly irregular unit-cell volume versus pressure dependence up to \ensuremath{\sim}45 GPa, after which a transformation to a Cmcm postspinel phase is onset. This is attributable to pressure driven Fe\ensuremath{\leftrightarrow}Al site inversion at room temperature, corroborated by signatures in the $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy pressure data. At the tetragonal\ensuremath{\rightarrow}postspinel transition, onset in the range 45--50 GPa, there is a concurrent emergence of a nonmagnetic spectral component in the M\"ossbauer data at variable cryogenic temperatures. This is interpreted as spin crossover at sixfold coordinated Fe locations emanated from site inversion. Spin crossover commences at the end of the pressure range of the tetragonal phase and progresses in the postspinel structure. There is also a much steeper volume change \ensuremath{\Delta}$V/V$ \ensuremath{\sim} 10% in the range 45--50 GPa compared to the preceding pressure regime, from the combined effects of the structural transition and spin crossover electronic change. At the highest pressure attained, \ensuremath{\sim}106 GPa, the M\"ossbauer data evidence a diamagnetic Fe low-spin abundance of \ensuremath{\sim}50%. The rest of the high-spin Fe in eightfold coordinated sites continue to experience a relatively small internal magnetic field of \ensuremath{\sim}33 T. This is indicative of a magnetic ground state associated with strong covalency, as well as substantive disorder from site inversion and the mixed spin-state configuration. Intriguingly, magnetism survives in such a spin-diluted postspinel lattice at high densities. The $R$ (300 K) data decrease by only two orders of magnitude from ambient pressure to the vicinity of \ensuremath{\sim}100 GPa. Despite a \ensuremath{\sim}26% unit-cell volume densification from the lattice compressibility, structural transitions, and spin crossover, $\mathrm{FeA}{\mathrm{l}}_{2}{\mathrm{O}}_{4}$ is definitively nonmetallic with an estimated gap of \ensuremath{\sim}400 meV at \ensuremath{\sim}100 GPa. At such high densification appreciable bandwidth broadening and gap closure would be anticipated. Reasons for the resilient nonmetallic behavior are briefly discussed.

We show that polycrystalline $\mathrm{GeS}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{4}$ in the fcc phase (f-GST), which is an insulator at low temperature at ambient pressure, becomes a superconductor at elevated pressures. Our study of the superconductor-insulator transition versus pressure at low temperatures reveals a second-order quantum phase transition with linear dependence of the transition temperature on the pressure above the critical zero-temperature pressure. In addition, we demonstrate that at higher pressures the f-GST goes through a structural phase transition via amorphization to bcc GST (b-GST), which also becomes superconducting. We also find that in the pressure regime where an inhomogeneous mixture of amorphous and b-GST exists, there is an anomalous peak in magnetoresistance, and we suggest an explanation for this anomaly.

The present work discusses corrosion behavior of newly developed bainitic steels made by isothermal heat treatment of a new steel composition (0.71 pct C, 1.15 pct Mn, 0.20 pct Ni, 0.59 pct Cr, 0.40 pct Cu, 0.35 pct Si, 0.026 pct S, 0.027 pct P, and rest Fe (weight percent)). Corrosion behavior of the pearlitic steel made by normalization is also studied. Electrochemical polarization and salt fog tests are carried out in 0.6 M NaCl. Steel rusts after salt fog tests are analyzed. Modified composition, finer microstructures, and compact rust morphology attribute to better corrosion resistance of the bainitic steels. Corrosion mechanisms for the pearlitic and bainitic steels are discussed.

X-ray diffraction studies to $\ensuremath{\sim}90\phantom{\rule{4.pt}{0ex}}\text{GPa}$ at room temperature show that $\mathrm{F}{\mathrm{e}}_{2}\mathrm{Ti}{\mathrm{O}}_{4}$ ferrous inverse spinel undergoes the following sequence of structural transitions: $\mathrm{cubic}\phantom{\rule{4pt}{0ex}}(Fd\overline{3}m)\stackrel{\ensuremath{\sim}8\phantom{\rule{4.pt}{0ex}}\text{GPa}}{\ensuremath{\rightarrow}}\mathrm{tetragonal}\phantom{\rule{4pt}{0ex}}(I{4}_{1}/amd)\stackrel{\ensuremath{\sim}16\phantom{\rule{4.pt}{0ex}}\text{GPa}}{\ensuremath{\rightarrow}}\mathrm{orthorhombic}\phantom{\rule{4pt}{0ex}}(Cmcm)\stackrel{\ensuremath{\sim}55\phantom{\rule{4.pt}{0ex}}\text{GPa}}{\ensuremath{\rightarrow}}\mathrm{orthorhombic}\phantom{\rule{4pt}{0ex}}(Pmma),$ at the indicated onset transition pressures. Within the Cmcm phase, site-specific spin crossover is initiated and involves only highly distorted octahedral sites constituting $\ensuremath{\sim}25%$ of all Fe locations. This is manifest as a steeper volume decrease of $\mathrm{\ensuremath{\Delta}}V/{V}_{0}\ensuremath{\sim}3.5%$ beyond $\ensuremath{\sim}40\phantom{\rule{4.pt}{0ex}}\text{GPa}$ and an emergent diamagnetic component discerned in $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy at variable cryogenic temperatures. A subsequent $Cmcm\ensuremath{\rightarrow}Pmma$ Fe/Ti disorder-order reconfiguration is facilitated at sixfold coordinated (octahedral) sites. The rest of the high-spin Fe in sixfold and eightfold coordinated sites ($\ensuremath{\sim}75%$ abundance) in the Pmma phase exhibit average saturation internal magnetic fields of ${H}_{hf}\ensuremath{\sim}42\phantom{\rule{4.pt}{0ex}}\text{T}$ to $\ensuremath{\sim}90\phantom{\rule{4.pt}{0ex}}\text{GPa}$, typical of spin-only (orbitally quenched) Fermi-contact values. By contrast, average ${H}_{hf}\ensuremath{\sim}20\phantom{\rule{4.pt}{0ex}}\text{T}$ values, signifying unquenched orbital moments, occur below the $40--45\phantom{\rule{4.pt}{0ex}}\text{GPa}$ spin-crossover initiation regime in the Cmcm phase. Therefore, site-specific spin crossover invokes a cooperative lattice response and polyhedral distortions at the rest of the high-spin Fe sites, translating to $3d$ level (sub-band) changes and consequential orbital moment quenching. Near $\ensuremath{\sim}90\phantom{\rule{4.pt}{0ex}}\text{GPa}, \mathrm{F}{\mathrm{e}}_{2}\mathrm{Ti}{\mathrm{O}}_{4}$ is a partially spin-converted chemically ordered Pmma post-spinel having a persistent charge gap of $\ensuremath{\sim}100\phantom{\rule{4.pt}{0ex}}\text{meV}$. Despite structural symmetry changes, partial spin crossover and lattice compressibility, resulting in a $\ensuremath{\sim}33%$ total reduction in unit-cell volume and corresponding $3d$ bandwidth broadening, strong electron correlations persist at high densification.

Lithium-based ferrites are promising and potential magnetic materials for microwave applications. They possess a spinel (AB 2 O 4 ) type crystal structure, where the distributions of metal cations over the tetrahedral (A) and octahedral (B) voids play a crucial role for exhibiting different physical properties. Among various parameter of synthesis, pH is an important factor that influences the cation distribution over A and B voids, overall growth of the nanoparticles and different physical properties. In the present work single phase Cr substituted lithium ferrite nanoparticles have been synthesized by the sol–gel method at different pH. The phase identification and crystallite size have been probed by X-ray diffraction studies. The crystallite size changes by 44.2–48.8 nm upon varying the pH from 3.5 to 11.5. In order to investigate the cations distribution at A/B sites, Mossbauer spectroscopic measurements were done. The values of magnetic hyperfine field obtained from the Mossbauer data for the A and B sites are ≈49.5 T and 51 T, respectively. Moreover, it is observed that the area ratio of B site to A site increases with increasing the pH. This observation further suggests that the B site is more preferable for Fe 3+ cations at higher pH than the A site. The magnetic parameter such as saturation magnetization ( M s ), remanent magnetization ( M r ), coercive field ( H C ) and squareness ( S ) are determined by vibrating sample magnetometer (VSM) measurements, which show a consistent increase with increasing pH. The reason for the variation in magnetic properties has been explained on the basis of increased Fe 3+ cation occupancies at the B site and size effect, which is well supported by Mossbauer spectroscopic and XRD studies.

In this work, we have explored the effect of surface functionalization in ZnO:Fe nanostructures. The ZnO:Fe nanostructures (NS), synthesized through hydrothermal route, have been surface functionalized (capped) by Tri-n-butylamine (amine). These samples have been characterized by X-ray diffraction (XRD), photoluminescence (PL) and Fourier transform infrared spectroscopy (FTIR). Field emission scanning electron microscopy (FESEM) and transmission electron microscope (TEM) illustrate that obtained nanostructures are high aspect ratio nanorods. Room temperature Mossbauer spectra (MS) of the samples indicate the presence of paramagnetic iron state i.e. Fe3+ in ZnO host. The ferromagnetic (FM) behavior of the samples was investigated by vibrating sample magnetometer (VSM) which shows that the significant enhancement of saturation magnetization in amine capped ZnO:Fe samples. This observed/enhanced ferromagnetism has been explained in terms of modification in the ZnO conduction band. The enhanced ferromagnetism due to surface functionalization has been justified by XRD, PL, FTIR and MS.

Using two different fuels such as aspartic acid and glycine, the spinel zinc ferrite nanoparticles were synthesized by the combustion method at different pH values. The thermochemical calculations for both the fuel assisted materials and its adiabatic flame temperature were calculated. The X-ray diffraction (XRD) pattern revealed the formation of single phase ZnFe 2 O 4 with high crystallinity. The characteristic functional groups of Fe3O and Zn3O were identified through FTIR analysis. Uniform size distribution of spherical particle in the average size range of 35–100 nm was inferred from SEM images. The room temperature DC conductivities of ZnFe 2 O 4 particles prepared by using aspartic and glycine are in the order of 10 −7 and 10 −8 respectively. The dielectric spectral analysis inferred that the obtained dielectric constant is high at low frequency and decreases with increase in frequency. This dielectric behavior is in accordance with the Maxwell–Wagner interfacial polarization. VSM and Mossbauer analysis revealed that the prepared material exhibits paramagnetic behavior and Fe 3+ state of iron content in ZnFe 2 O 4 at room temperature.

The present investigation describes the evolution of nano iron oxide/oxyhydroxide phases synthesized under identical conditions of precipitation using different starting reagents. Depending on the Fe(III) source and the neutralizing agent, single-phased nanogoethite, single-phased ferrihydrite and a combination of goethite, hematite and ferrihydrite phases were evolved in the synthesized samples. The prepared precursors were annealed at 400, 600 and 800 °C to study the changes in the morphology, optical and magnetic properties of single phase α-Fe2O3. Detailed characterization studies of the precursors and hematite obtained by annealing the samples were carried out using X-ray diffraction (XRD), Transmission electron microscopy (TEM), Raman and Mossbauer spectroscopy. Raman spectra of α-Fe2O3 obtained upon annealing the precursors showed changes in peaks' positions and intensities. These changes have been attributed to surface defects, the phonon confinement effect, the particle shape, and surface morphology. TEM images of the as-synthesized and annealed samples clearly showed differences in the shape, size and crystallinity. The direct and indirect band gap energies of annealed samples estimated from UV-vis absorption spectra showed these values to be smaller as compared to the bulk value for α-Fe2O3. The band gaps were not much affected by the nature of the precursor for the formation of α-Fe2O3 but did depend on annealing temperature. Very interesting trends were observed for the magnetic behaviour of α-Fe2O3. The coercivity values varied in the range of 28.7 to 2966 Oe. Probable mechanisms of formation of different phases have been outlined.

We report systematic investigations on structural, magnetic and optical properties of NiO nanoparticles prepared by mechanical alloying. As-milled powders exhibit face centred cubic structure, but average particle size decreases and effective strain increases for the initial periods of milling. Lattice volume increases monotonically with a reduction in particle size. Antiferromagnetic NiO particles exhibit significant room temperature (RT) ferromagnetism with modest moment and coercivity. A maximum moment of 0.0147 μB/f.u at 12 kOe applied field and a coercivity of 160 Oe were obtained for 30 h milled NiO powder. Exchange bias decreases linearly with a decrease in NiO particle size. Thermo-magnetization data reveal the presence of mixed magnetic phases in milled powders and shifts magnetic phase transition towards high temperature with increasing milling. Annealing of milled NiO powder and photoluminescence studies show a large reduction in RT magnetic moment and blue-shifting of band edge emission peak. The observed properties are discussed on the basis of finite size effect, defect density, oxidation/reduction of Ni, increase in number of sublattices, uncompensated spins from surface to particle core, and interaction between uncompensated surfaces and particle core with lattice expansion.

Here we report the discovery of superconductivity in multiple phases of the compressed $\mathrm{GeS}{\mathrm{b}}_{2}\mathrm{T}{\mathrm{e}}_{4}$ (GST) phase change memory alloy, which has attracted considerable attention for the last decade due to its unusual physical properties with many potential applications. Superconductivity is observed through electrical transport measurements, both for the amorphous $(a$-GST) and for the crystalline $(c$-GST) phases. The superconducting critical temperature ${T}_{c}$ continuously increases with applied pressure, reaching a maximum ${T}_{c}=6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at $P=20\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ for $a$-GST, whereas the critical temperature of the cubic phase reaches a maximum ${T}_{c}=8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at 30 GPa. This material system, exhibiting a superconductor-insulator quantum phase transition, has an advantage over disordered metals since it has a continuous control of the crystal structure and the electronic properties using pressure as an external stimulus.

The interplay between lattice, orbital, and spin degrees of freedom in iron chromite (spinel $[\mathrm{Fe}]{\mathrm{C}{\mathrm{r}}_{2}}{\mathrm{O}}_{4})$ has been investigated to near megabar pressures. The cubic-to-tetragonal transition, from static Jahn-Teller distortions at the Fe locality, rises from ${T}_{\mathrm{J}\text{\ensuremath{-}}\mathrm{T}}\ensuremath{\sim}135\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at ambient pressure to 300 K by $\ensuremath{\sim}12\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. The tetragonal distortion progressively increases and orbital moment quenching is triggered beyond $\ensuremath{\sim}24\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, as monitored by the magnetic hyperfine field ${H}_{hf}$ at Fe sites. In the range 30--60 GPa, original $\mathrm{F}{\mathrm{e}}^{2+}$ tetrahedral sites with an unquenched orbital moment $({H}_{hf}\ensuremath{\sim}20\phantom{\rule{0.16em}{0ex}}\mathrm{T})$ coexist with newly evolved $\mathrm{F}{\mathrm{e}}^{2+}$ at tetrahedral sites having the orbital moment quenched and a resultant large ${H}_{hf}\ensuremath{\sim}35\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. Additionally, new $\mathrm{F}{\mathrm{e}}^{2+}$ sites having distinguishable orbital moment quenching signatures $({H}_{hf}\ensuremath{\sim}42\phantom{\rule{0.16em}{0ex}}\mathrm{T})$ are discerned. Those sites also have other Fe nuclear hyperfine interaction parameter values typical of $\mathrm{F}{\mathrm{e}}^{2+}$ in octahedral coordination. There is a concurrent change to a steeper decrease of unit-cell volume as pressure rises above $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. These electronic and lattice responses are interpreted as signatures of progressive partial spinel inversion from high-spin $\mathrm{Fe}\ensuremath{\leftrightarrow}\mathrm{Cr}$ tetrahedral/octahedral site exchange, triggered near $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Beyond 60 GPa a new diamagnetic low-spin M\"ossbauer spectral component emerges. This is preceded by an inflection and discontinuity in the pressure dependence of the resistance and tetragonal unit-cell volume, respectively. By $\ensuremath{\sim}93\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, half of the iron is low spin in octahedral sites from evolved tetragonal-spinel inversion processes. The remainder reside in tetrahedral high-spin sites with ${H}_{hf}\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{T}$. The charge gap from electron correlations does not close, despite an anticipated appreciable band broadening from a $\ensuremath{\sim}30%$ unit-cell volume reduction upon pressurization to $\ensuremath{\sim}93\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. This is attributable to an increase in effective on-site repulsion (Hubbard ${U}_{\mathrm{eff}}$) with increasing pressure or at spin crossover, specific to the $\mathrm{C}{\mathrm{r}}^{3+}({d}^{3})$ and $\mathrm{F}{\mathrm{e}}^{2+}({d}^{6})$ electronic configurations, respectively. Thus a $\ensuremath{\sim}200\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ ``Mott'' gap prevails in the mixed spin-state of the partially inverted tetragonal phase $[\mathrm{F}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{r}}_{x}]{\mathrm{F}{\mathrm{e}}_{x}\mathrm{C}{\mathrm{r}}_{2\ensuremath{-}x}}{\mathrm{O}}_{4}\phantom{\rule{0.28em}{0ex}}(x\ensuremath{\sim}0.5)$ up to near a megabar (100 GPa) densification.

Ultrafine (2-5 nm) particles of amorphous Ca-Mg co-doped ferrihydrite have been synthesized by surfactant mediated co-precipitation method. The evolution of the amorphous ferrihydrite by Ca-Mg co-doping is quite different from our earlier investigations on individual doping of Ca and Mg. Amorphous phase of ferrihydrite for the present study has been confirmed by X-ray diffraction (XRD) and Mössbauer spectroscopy at room temperature and low temperatures (40 K and 20 K). Hematite nanoparticles with crystallite size about 8, 38 and 70 nm were obtained after annealing the as-prepared samples at 400, 600 and 800 degrees C respectively in air atmosphere. Superparamagnetism has been found in 8 nm sized hematite nanoparticles which has been confirmed from the magnetic hysteresis loop with zero remanent magnetization and coercive field and also from the superparamagnetic doublet of its room temperature Mössbauer spectrum. The magnetic properties of the 38 and 70 nm sized particles have been studied by room temperature magnetic hysteresis loop measurements and Mössbauer spectroscopy. The coercive field in these hematite nanoparticles increases with increasing particle size. Small amount of spinel MgFe2O4 phase has been detected in the 800 degrees C annealed sample.

Multiferroic nano-composite (1-x) BiFeO3–x Li0.5Fe2.5O4 (x=0.25 and 0.5) have been successfully synthesized by mixing the two phases, prepared independently by two different methods followed by annealing at 600 0 C. Existence of the two phases in the composite is confirmed by x-ray diffraction pattern. Average particle size is calculated to be about 45 nm for both of these phases. The saturation magnetization, remnant magnetization and coercive field increases linearly with increasing ferromagnetic phase (Li0.5Fe2.5O4) as investigated by VSM measurement. Local magnetic behaviors have been investigated by 57 Fe Mossbauer spectroscopic studies. Large dielectric constant of the order of 10 3 -10 4 has been observed in these composites. Copyright © 2013 VBRI press

Structural, magnetic, and electrical-transport properties of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{LiFe}{\mathrm{O}}_{2}$, crystallizing in the rocksalt structure with random distribution of Li and Fe ions, have been studied by synchrotron x-ray diffraction, $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy, and electrical resistance measurements at pressures up to 100 GPa using diamond anvil cells. It was found that the crystal structure is stable at least to 82 GPa, though a significant change in compressibility has been observed above 50 GPa. The changes in the structural properties are found to be on a par with a sluggish $\mathrm{F}{\mathrm{e}}^{3+}$ high- to low-spin (HS-LS) transition $(S=5/2\ensuremath{\rightarrow}S=1/2)$ starting at 50 GPa and not completed even at \ensuremath{\sim}100 GPa. The HS-LS transition is accompanied by an appreciable resistance decrease remaining a semiconductor up to 115 GPa and is not expected to be metallic even at about 200 GPa. The observed feature of the pressure-induced HS-LS transition is not an ordinary behavior of ferric oxides at high pressures. The effect of $\mathrm{F}{\mathrm{e}}^{3+}$ nearest and next-nearest neighbors on the features of the spin crossover is discussed.

Undoped and calcium substituted hematite (α-Fe₂O₃) nanoparticles are synthesized by surfactant-directed co-precipitation and post annealing method. The annealed nanoparticles were found to be in single phase in nature and crystallize in the rhombohedral structure with space group R3c as confirmed by Rietveld refinement of the X-ray diffraction (XRD) data. Average crystallite sizes are calculated to be 20 to 30 nm and 50 to 60 nm for the nanoparticles annealed at 400 and 600 °C respectively. Mössbauer spectra for all the nanoparticles could be fitted with a sextet corresponding to the single magnetic state of the iron atoms in its Fe³⁺ state in the hematite matrix. The FTIR and Raman spectra of all the samples correspond to specific modes of α-Fe₂O₃. UV-Vis spectra of annealed samples showed broad peaks in the range of 525-630 nm resulting from spin-forbidden ligand field transition together with the spin-flip transition among the 2t₂g states. The estimated band gap energies were in the range of 1.6 to 1.9 eV which are much lower than the reported values for nano hematite. From the room temperature magnetic hysteresis loop measurements, weak ferromagnetic behavior is observed in all undoped and Ca²⁺ doped hematite samples. Morin temperature (T(M)) is calculated to be 257 and 237 K for 1.45% doped samples with particle size 54 and 27 nm respectively. The sample with Ca content of 1.45 wt% when annealed at 400 °C showed that the particles were of different shapes which included both quasi spherical and rod shaped. On annealing the same sample at 600 °C, the nanorods collapsed to form bigger spherical and ellipsoidal particles.

The nano-porous LiFe₀.₉M₀.₁P₂O₇ (M = Fe, Co, Mn, Ni) particles were successfully prepared by simple microwave assisted combustion method and studied its detailed physico-chemical properties. The phase purity, crystallinity, functional group identification was revealed through X-ray diffraction (XRD), Fourier Transform Infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis. The presence of nanoporous was identified through transmission electron microscopic (TEM) images. The electrical conductivity results illustrated that LiFe₀.₉Ni₀.₁P₂O₇ has higher conductivity (2.85 x 10⁻⁷ S cm⁻¹) among the studied systems owing to their negligible grain boundary effect. The normal dielectric behaviour was observed for all the LiFe₀.₉M₀.₁P₂O₇ (M = Fe, Co, Mn, Ni) materials. The paramagnetic behaviour and the Fe³⁺ state of LiFe₀.₉M₀.₁P₂O₇ were obtained from VSM and Mössbauer spectral analysis respectively. The cyclic voltammogram suggested that the good electrochemical lithium intercalation/de-intercalation behaviour of LiFe₀.₉M₀.₁P₂O₇ (M = Fe, Co, Mn, Ni) electrodes in aqueous electrolytes. The obtained diffusion coefficient value is comparable with carbon based materials.

Single phase BiFeO3 nanoparticles have been successfully synthesized for the first time by a novel citrate combustion method without using any solvent. Well mixed metal nitrates along with citric acid which is used as fuel combust to give BiFeO3 nanoparticles after annealing. These particles are single phase in nature and crystallize in the rhombohedral distorted perovskite structure (space group-R3c) which has been confirmed by the Rietveld refinement of the room temperature powder x-ray diffraction data. Nearly spherical particles of average particle size 47 nm have been seen from transmission electron micrograph. Room temperature magnetic hysteresis measurement shows weak ferromagnetism though the magnetization does not saturate upto 1.75 T applied field. The coercive field value is calculated to be 180 Oe which is 3 times higher than that prepared by solvent free combustion method using glycine. 57 Fe Mossbauer spectrum can be fitted with a sextet corresponding to single magnetic state of hyperfine field about 49.5 T corresponding to Fe 3+ state of the iron atom. The dielectric relaxation and ac conductivity as a function of frequency have been discussed. High dielectric permittivity has not been found in these nanoparticles like other reported BiFeO3 ceramics. Copyright © 2012 VBRI press.

Reported herein is a hitherto unknown family of diiron(III)-μ-hydroxo bisporphyrins in which two different spin states of Fe are stabilized in a single molecular framework, although both cores have identical molecular structures. Protonation of the oxo-bridged dimer (2) by using strong Brønsted acids, such as HI, HBF(4), and HClO(4), produce red μ-hydroxo complexes with I(3)(-) (3), BF(4)(-) (4), and ClO(4)(-) (5) counterions, respectively. The X-ray structure of the molecule reveals that the Fe-O bond length increases on going from the μ-oxo to the hydroxo complex, whereas the Fe-O(H)-Fe unit becomes more bent, which results in the smallest known Fe-O(H)-Fe angles of 142.5(2) and 141.2(1)° for 3 and 5, respectively. In contrast, the Fe-O(H)-Fe angle remains unaltered in 4 from the corresponding μ-oxo complex. The close approach of two rings in a molecule results in unequal core deformations in 3 and 4, whereas the cores are deformed almost equally but to a lesser extent in 5. Although 3 was found to have nearly high-spin and admixed intermediate Fe spin states in cores I and II, respectively, two admixed intermediate spin states were observed in 4. Even though the cores have identical chemical structures, crucial bond parameters, such as the Fe-N(p), Fe-O, and Fe⋅⋅⋅Ct(p) bond lengths and the ring deformations, are all different between the two Fe(III) centers in 3 and 4, which leads to an eventual stabilization of two different spin states of Fe in each molecule. In contrast, the two Fe centers in 5 are equivalent and assigned to high and intermediate spin states in the solid and solution states, respectively. The spin states are thus found to be dependent on the counterions and can also be reversibly interconverted. Upon protonation, the strong antiferromagnetic coupling in the μ-oxo dimer (J, -126.6 cm(-1)) is attenuated to almost zero in the μ-hydroxo complex with the I(3)(-) counterion, whereas the values of J are -36 and -42 cm(-1), respectively, for complexes with BF(4)(-) and ClO(4)(-) counterions.

We report a combination of a surfactant-directed approach and calcium ion incorporation during synthesis of nano iron oxide/oxyhydroxide that will result in selective phase formation of goethite (α...

The ZnO:Fe nanoparticles of mean size 3–10 nm were synthesized at room temperature by simple co-precipitation method. The crystallite structure, morphology and size estimation were performed by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM). The wurtzite structure of ZnO gradually degrades with the increasing Fe doping concentration. The magnetic behavior of the nanoparticles of ZnO with varying Fe doping concentration was investigated using a vibrating sample magnetometer (VSM). Initially these nanoparticles showed strong ferromagnetic behavior, however at higher doping percentage of Fe, the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Fe–Fe ions suppressed the ferromagnetism at higher doping concentrations of Fe. Room-temperature Mossbauer spectroscopy investigation showed Fe 3+ nature of the iron atom in ZnO matrix.

BiFe1-xCrxO3 (x=0.0, 0.05 and 0.1) nanoparticles are prepared by a combustion method without using any solvent. All the synthesized nanoparticles are single phase in nature, nearly spherical in shape and crystallize in distorted perovskite structure and space group R3c with an average crystallite size of the order of 40 nm. The room temperature magnetization observed in BiFeO3 nanoparticles is larger than that in the bulk. Increasing Cr doping leads to increase in the magnetization and coercivity. Strong superexchange interaction between Fe3+ and Cr3+ atoms is likely to give rise to such increase in magnetization with Cr-doping. M\"ossbauer data of these nanoparticles show ordered magnetic state in which Fe atoms are in 3+ oxidation states., Comment: 18 pages. 5 figers and 2 tables

The pure and Fe-doped CuO nanoparticles of the series Cu(1-x)Fe(x)O (x = 0.00, 0.02, 0.04, 0.06 and 0.08) were successfully prepared by a simple low temperature sol-gel method using metal nitrates and citric acid. Rietveld refinement of the X-ray diffraction data showed that all the samples were single phase crystallized in monoclinic structure of space group C2/c with average crystallite size of about 25 nm and unit cell volume decreases with increasing iron doping concentration. TEM micrograph showed nearly spherical shaped agglomerated particles of 4% Fe-doped CuO with average diameter 26 nm. Pure CuO showed weak ferromagnetic behavior at room temperature with coercive field of 67 Oe. The ferromagnetic properties were greatly enhanced with Fe-doping in the CuO matrix. All the doped samples showed ferromagnetism at room temperature with a noticeable coercive field. Saturation magnetization increases with increasing Fe-doping, becomes highest for 4% doping then decreases for further doping which confirms that the ferromagnetism in these nanoparticles are intrinsic and are not resulting from any impurity phases. The ZFC and FC branches of the temperature dependent magnetization (measured in the range of 10-350 K by SQUID magnetometer) look like typical ferromagnetic nanoparticles and indicates that the ferromagnetic Curie temperature is above 350 K.

Maghemite (&#947-Fe 2 O 3 ) nanoparticles are synthesized by chemical co-precipitation technique in AOT-microemulsion with a view to have possible application for biotagging. The investigations by means of X-ray diffraction, isothermal magnetization M (H) and 57 Fe Mossbauer Spectroscopy show that the particles are nonspherical, mostly of rod shape. The inter-particle interaction is so large that even a powder sample of thickness about 40 mg/cm 2 shows preferential orientation of magnetic moments in the plane of the sample. The saturation magnetization is much lower than the expected values for maghemite. Keywords : Maghemite, Magnetic nanoparticles, Magnetic Ordering, Mossbauer spectroscopy International Journal of Engineering, Science and Technology , Vol. 2, No. 8, 2010, pp. 33-39

A set of Ni 0.5 Zn 0.5 Fe 2 O 4 samples were prepared by citrate precursor route to investigate the growth mechanism and its effect on cationic distribution. Following the information from DTA–TGA analysis, samples were annealed at 550 &#176C, 700 &#176C and 750 &#176C. Magnetization and Mossbauer studies suggest that initially the cationic distribution deviates from its normal preferences but it gets back to the normal preference at a temperature around 675 &#176C. It has been found that size onset for having the bulk cation configuration exclusively depends on the composition. Keywords : Mossbauer Spectroscopy; Ferrite; Nanoparticles, Cationic distribution International Journal of Engineering, Science and Technology , Vol. 2, No. 8, 2010, pp. 104-109

10% Co‐doped BiFeO3 nanoparticles have been prepared by a low temperature sol‐gel method. XRD result shows that particles are single phase in nature and crystallizes in distorted perovskite structure (space group R3c) with average particle size of 60 nm. Room temperature ferromagnetism along with saturation magnetization of about 2.24 emu/g and high coercive field of 1260 Oe has been found by VSM measurements. Mossbauer data can be fitted with single magnetic sextet of isomer shift about 0.35 mm/s and hyperfine field about 49 T indicating Fe3+ nature of Fe atom present in Co‐doped BiFeO3 nanoparticles.

Mixed nano iron oxides powder containing goethite (α-FeOOH), hematite (α-Fe(2)O(3)) and ferrihydrite (Fe(5)HO(8)·4H(2)O) was synthesized through surfactant mediation-precipitation route using cetyltrimethyl ammonium bromide (CTAB). The X-ray diffraction, FTIR, TEM, Mossbauer spectroscopy were employed to characterize the sample. These studies confirmed the nano powder contained 77% goethite, 9% hematite and 14% ferrihydrite. Fluoride adsorption onto the synthesized sample was investigated using batch adsorption method. The experimental parameters chosen for adsorption studies were: pH (3.0-10.0), temperature (35-55°C), concentrations of adsorbent (0.5-3.0 g/L), adsorbate (10-100 mg/L) and some anions. Adsorption of fluoride onto mixed iron oxide was initially very fast followed by a slow adsorption phase. By varying the initial pH in the range of 3.0-10.0, maximum adsorption was observed at a pH of 5.75. Presence of either SO(4)(2-) or Cl(-) adversely affected the adsorption of fluoride in the order of SO(4)(2-)>Cl(-). The FTIR studies of fluoride loaded adsorbent showed that partly the adsorption on the surface took place at surface hydroxyl sites. Mossbauer studies indicated that the overall absorption had gone down after fluoride adsorption that implies it has reduced the crystalline bond strength. The relative absorption area of ferrihydrite was marginally increased from 14 to 17%.

The potential of a mono-organophosphate ester in assembling low to medium nuclearity iron complexes with novel topological architectures has been investigated. Reaction of 2,6-diisopropylphenyl dihydrogen phosphate (dipp-H(2)) with ferrous acetate under an inert atmosphere resulted in the formation of mononuclear, [Fe(II)(dipp-H)(2)(py)(4)] (1) (py = pyridine), dinuclear [Fe(III) (2)O(dipp-H)(4)(3,5-dmpz)(3)(thf)](3,5-dmpz)(thf)(3) (2) (3,5-dmpz = 3,5-dimethylpyrazole, thf = tetrahydrofuran), and trinuclear [Fe(III) (2)Fe(II)O(dipp-H)(6)(thf)(3)](collidine) (3) complexes by changing the ancillary amine used in the reaction. Use of a preformed mu(3)-oxo bridged trinuclear complex, [Fe(III) (3)O(O(2)CR)(6)(H(2)O)(3)]X (X = Cl and NO(3)), as the precursor yielded two tetranuclear iron phosphates, [Fe(III) (4)O(dipp)(3)(py)(4)(PhCOO)(4)](toluene)(3) (4) and [Fe(III) (4)O(dipp)(3)(OAc)(4)(py)(4)](py)(2) (5), having a core structure similar to those found in tetranuclear iron phosphonates. When FeCl(3)6H(2)O was used as the iron precursor, an unprecedented pentanuclear iron phosphate complex, [Fe(III) (5)O(dipp)(6)(py)(4)Cl(2)][pyH] (6) with a novel structure was isolated. Another pentanuclear complex having a similar core structure, [Fe(5)O(dipp)(6)(HO(2)CPh)(3)(CH(3)CN)(3)Cl] (7) was isolated on treatment of dipp-H(2) with the triangular complex, [Fe(III) (3)O(O(2)CPh)(6)(H(2)O)(3)]Cl in the absence of any added amine co-ligand. Compounds 1-7 have been characterized by analytical techniques, spectroscopic studies, single crystal X-ray diffraction studies, and magnetic measurements. The Mössbauer studies carried out at room temperature support the formulation of all the compounds.One to five, one at a time: The choice of iron precursors and the amine co-ligand are employed as switches to control the nuclearity in iron phosphate molecular clusters. Using this control, it is possible to isolate selectively either a mono, di, tri, tetra, or pentanuclear cluster, for a given set of reactants. The potential of a mono-organophosphate ester in assembling low to medium nuclearity iron complexes with novel topological architectures has been investigated. Reaction of 2,6-diisopropylphenyl dihydrogen phosphate (dipp-H