Octahedral Sublattice Research Articles

Atomic structure and magnetic properties of MnFe2O4 nanoparticles produced by reverse micelle synthesis

Using the aqueous cores of reverse micelles as nanoreactors, nanoparticles (d∼10 nm) of the mixed ferrite MnFe2O4 were produced. Seven processing trials were performed where the concentration of ammonium hydroxide, reaction temperature, and the oxidizing agent were varied. All trials result in Mn-ferrite particles with varying chemistry and structure. The Mn concentration in the resulting ferrite is strongly enhanced by both the presence of H2O2 as an oxidizing agent and a surplus of ammonium hydroxide. The increased Mn concentration correlates with a higher fraction of octahedrally coordinated Mn cations. When near-stoic amounts of ammonium hydroxide are used, the resulting ferrites are nearly stoichiometric with a more equitable distribution of Mn cations on the octahedral and tetrahedral sublattices. In all ferrite nanoparticles, the Mn cations have a preference for octahedral site occupancy that is larger than the 20% measured in bulk Mn-ferrite. We attribute the cation filling trends to the stabilization of excess trivalent Mn during processing.

Magnetic properties of Cr/sup 3+/ substituted BaFe/sub 12/O/sub 19/ powders grown by a sol-gel method

Cr/sup 3+/ substituted Ba-hexaferrite was fabricated by a sol-gel method. The crystallographic and magnetic properties of BaFe/sub 12-x/Cr/sub x/O/sub 19/ (0/spl les/x/spl les/7) were investigated XRD, Rutherford backscattering spectrometry, vibrating sample magnetometry and Mossbauer spectroscopy. The crystal structure was found to be magnetoplumbite, typical of M-type hexagonal ferrite. By substituting Fe/sup 3+/ in BaFe/sub 12/O/sub 19/ by Cr/sup 3+/, we have been able to attribute the Mossbauer parameters to the 5 crystallographic sites of the structure. Only the octahedral sublattices were occupied by Cr ions. The isomer shifts indicate that the valence state of the Fe ions was Fe/sup 3+/. The Curie temperatures of BaFe/sub 12-x/Cr/sub x/O/sub 19/ decreased linearly increasing Cr-substitution, at a rate of 55 K/Cr atom.

Spin-glass and reentrant spin-glass states in iron sulfospinels having dilute A and B sublattices

The magnetic and electrical properties of new compounds having spinel structure Fe1−xCr2(1−x)Sn2xS4 (0.1⩽x⩽0.33) (system 1), Fe0.67 [Fe0.165CrSn0.835]S4, and Fe0.67 [Fe0.33Cr0.67Sn]S4 has been studied. These compounds are p-type semiconductors with magnetic properties characteristic of the following magnetic-order types: ferrimagnetic (the x=0.1 composition of system 1), spin glass (the x=0.33 composition of system 1 and Fe0.67 [Fe0.165CrSn0.835]S4), and reentrant spin glass (the x=0.2 composition of system 1 and Fe0.67 [Fe0.33Cr0.67Sn]S4). For the spin-glass compositions, the dependence of the freezing temperature Tf defined as the temperature of the maximum of initial magnetic susceptibility, on temperature and magnetic field obeys the Almeida-Thouless relation, and the dependence of Tf on magnetic-field frequency is a power-law function. For the spin-glass and reentrant spin-glass compositions, a large peak in the absolute value of negative isotropic magnetoresistance was found near Tf, which becomes as high as 15% in spin glasses and 30% in reentrant spin-glass compositions. In compositions with reentrant behavior, the activation energy of conductivity in the region of Tf was found to change by about two orders of magnitude. This experimental evidence suggests that the spin-glass-paramagnet (in spin glasses) and spin-glass-long-range magnetic-order transformations are actually phase transitions, and that the spin-glass region contains ferron-type ferromagnetic clusters. These are the first spin-glasses among the chalcospinels with magnetically active ions on the tetrahedral and octahedral sublattices.

Clusters in carbon martensite: Thermodynamics and kinetics

An original method of evaluation of the cluster population in carbon martensite has been developed. Using this method, it is shown that Kurdjumov’s model of carbon redistribution within the different octahedral site sublattices can quantitatively account for both observed normal and abnormal tetragonality in carbon martensite. It is also shown that the existence of the internal strains in martensite constitutes a necessary and sufficient condition for the energetic preference of tetrahedral over the cubic lattice. The presence of the residual tetragonal distortion in the quasi-cubic phase of k-martensite is associated with the presence of the mixed clusters formed of the atoms belonging to Oc sublattice as well as to remaining ones. By using a computer simulation of the dynamical behavior of carbon martensite approaching the thermodynamical equilibrium, it was found that the ultimate state of this system is strongly beyond the thermal equilibrium. Even after long-term aging, the free energy is far beyond the minimum value allowed for this system. The reason for such a behavior and the possible aging processes proceeding in this system are discussed at the molecular level. All of the ordering parameters are affected by the aging process. The evolution proceeds in the distinctly different time intervals for different parameters. At first, the long-range ordering parameter that determines the tetragonality of martensite evolves and reaches the stable value. In the next stage, the formation and then disintegration of two-particle clusters occurs. Disintegration of two-particle clusters coincides with the stage when three-particle cluster formation occurs at a high rate. Threeparticle clusters also disintegrate when some time elapses. The same pattern repeats regarding four-, five-, six-, seven-, and eight-particle clusters. To simplify the calculations, the nine-particle clusters are assumed to be the largest possible and are identified with an existence of superstructure. The formation of 100 pct of nine-particle clusters with no contribution of free atoms in an alloy ceases all aging processes. The evolution of these processes is illustrated graphically in the time range from 16 seconds to 1500 years, as estimated on the basis of experimental data.

Magnetic Order in Li-Mn Spinels

Abstract Magnetic measurements were carried out on different samples of Lithium-Manganese spinel LiMn2O4 , great care having been taken to avoid the presence of spurious magnetic phases, such as Mn3O4 . Susceptibility data, showing deviations from paramagnetic behaviour at about 40 K, were analyzed in terms of local magnetic interactions, taking into account the structural and transport properties of these com-pounds. The magnetic response of pure and stoichiometric samples suggests that the onset of a longrange magnetic ordering is hindered by the topological frustration of the antiferromagnetic octahedral sublattice of the spinel.

Processing and cation redistribution of MnZn ferrites via high-energy ball milling

We have produced MnZn ferrites via high-energy ball milling (HEBM) of elemental oxides MnO, ZnO, and Fe2O3. X-ray diffraction (XRD) indicates a pure phase spinel forms after 21 h of HEBM. Extended x-ray absorption fine structure (EXAFS) analysis shows a nonequilibrium cation distribution, with an unusually high population of Zn cations on the octahedral sublattice. We then used EXAFS modeling to study cation site occupancy in an equilibrium MnZn-ferrite standard subjected to HEBM. We found that HEBM produces an increased preference for octahedral-site occupation among all cations for milling durations up to 300 min. After an initial improvement in magnetic properties, the magnetization diminishes steadily throughout this interval. With further increases in milling duration these structural and magnetic trends reverse, possibly due to annealing effects evidenced by XRD.

Effect of Cr3+ ions on the magnetic moment of ferrites of the system CuFe2−x CrxO4

The dependence of the magnetic moment n 0exp of samples of the system CuFe2−x CrxO4 (x=0.0, 0.2, 0.3, 1.0, 1.4, 1.6, and 2.0) on their Cr3+ content is examined here for the first time. It is found that the experimental values of the magnetic moment n 0exp are much smaller than the values calculated from the cation distribution obtained previously (n 0 theor). It is suggested that this relationship (n 0 theor>n 0 exp) is due to a decrease in the magnetic moments of the Cr3+ ions resulting both from pairing of the t 2g orbitals of these cations in the octahedral sublattice and from a transfer of spin density from the ligands to the e g orbitals of these ions. For compositions with x>1.0, the noncollinear magnetic structure also leads to an increase in the difference between n 0 theor and n 0exp.

Sonochemical Preparation of Nanosized Amorphous NiFe2O4 Particles

Nanosized amorphous NiFe2O4 powder was prepared by sonochemical decomposition of solutions of volatile organic precursors, Fe(CO)5 and Ni(CO)4, in decalin at 273 K, under an oxygen pressure of 100−150 kPa. The amorphous nature of these particles was confirmed by various techniques, such as SEM, TEM, electron microdiffraction, and X-ray diffractograms. Magnetic measurements, Mössbauer, and EPR spectral studies indicated that the as-prepared NiFe2O4 ferrite particles were superparamagnetic. The Mössbauer spectrum of the crystallized sample showed a clear sextet pattern, with hyperfine field values of 500 and 508 kOe for A (tetrahedral) and B (octahedral) sublattices, respectively, of the inverse spinel NiFe2O4. Saturation magnetization of the annealed sample (25 emu/g) was significantly lower than that for the reported multidomain bulk prticles (55 emu/g), reflecting the ultrafine nature of the sample. Thermogravimetric measurements with a permanent magnet gave Curie temperatures of 440 °C for amorphous and 560 °C for the crystallized forms.

Effect of Magnetic Field on the Temperature Variation of Internal Friction Loss of Mn–Zn Spinel Ferrites in the Manganese-Rich Region

Internal friction Q—1 of Mn–Zn spinel ferrites in the manganese-rich region, developed for their use as TV deflection coil yokes, has been studied in the temperature range from 300 to 630 K. Two distinct stress induced relaxation peaks have been observed in the internal friction versus temperature curves. The magnetic transition has been observed to change with the concentration of zinc. The effect of magnetic field on the temperature dependence of internal friction has also been studied. The increase of zinc content in this system drastically alters the acoustic relaxations. The results are discussed in the light of Jahn-Teller distortion and cationic exchange between octahedral and tetrahedral sublattices and clustering of octahedral Mn3+ ions.

Effects of temperature change and hydrogen content on titanium hydride crystal lattice volume

Temperature compressibility of hydride crystal lattice at low temperatures is found to be a function of hydrogen content. Occupation of both tetra- and octahedral sublattices by hydrogen results in a reduction of lattice compressibility with the temperature decrease.

Preparation and Magnetic Properties of LiCr1−xAlxTiO4(0 ≤x≤ 0.4)

Solid solutions LiCr1−xAlxTiO4(0 ≤x≤ 0.4) were obtained by solid state reactions in air at 1173 K. Neutron diffraction data have been used to refine the crystal structure of the spinel LiCrTiO4, which is common to all members of the series and shows a cubic symmetry (space groupFd3m). Magnetic properties are discussed and the results suggest antiferromagnetic interactions in the octahedral sublattice that provoke an inherent frustration.

Influence of processing on the Li:Mn ratio in spinel phases of the system Li 1 + xMn 2 − xO 4 − δ

The composition of the spinel phase in the system LiMnO is studied under the influence of a controlled gas atmosphere and temperature/time schedule. The existence of an extended spinel phase of composition Li[Mn 2 − x Li x ]O 4 − δ (0 ≤ x ≤ 1 3 ) is shown. The Li Mn ratio as well as the proportion between Mn 3+ and Mn 4+ is adjustable by the method of preparation. LiMn 2O 4 − δ can be prepared in a cubic and tetragonal modification. According to magnetic and spectroscopic measurement, no simple ionic model can describe the electron structure throughout the entire series of solid solution. A partial charge delocalization takes place resulting in a reduction of the total number of unpaired electrons. At lower temperatures a spin glass behaviour develops due to the inherent frustration of the octahedral antiferromagnetic sublattice. The freezing temperature depends on the Li Mn ratio and decreases with increasing x.

New magnetic semiconductors Fe1−xCr2(1−x)Sn2xS4

The magnetic and electric properties of the first Fe1−xCr2(1−x)Sn2xS4 (0.1 ≤ x ≤ 0.33) solid solution system with a spinel structure were studied. These compounds are p-type semiconductors. They have magnetic properties characteristic of the following types of magnetic order: ferrimagnetic (sample with x = 0.1), reentrant spin glass (x = 0.2) and spin glass (x = 0.33). The sample with x = 0.33 was demonstrated to have a spin glass-paramagnet phase transition. This is the first spin glass chalcospinel with magnetic ions in the octahedral and tetrahedral sublattices.

New spin glass Fe0.67Cr1.33Sn0.67S4 with magnetic ions in tetrahedral and octahedral sublattices (abstract)

To date, spin glass state is observed in chalcogenide spinels with magnetic ions either in tetrahedral A sublattice or in octahedral B sublattice. The possibility of spin glass state in chalcospinels with magnetic ions in both of A and B sublattices is not evident because the intersublattice antiferromagnetic superexhange is more higher than intrasublattice ferromagnetic superexhange. In the present paper the magnetic and electrical properties of first produced spinel Fe0.67Cr1.33Sn0.67S4 are studied. This sample is p-type semiconductor. Here B sublattice is diluted of diamagnetic Sn4+ ions and Fe2+ ions deficiency exists in A sublattice. The isotherms of the magnetization σ are not saturated in investigation region of magnetic field H≤35 kOe and of temperatures 4.2≤T≤120 K. At the same time, this sample possesses magnetic properties typical of spin glass. Thus a maximum on the temperature dependence of the magnetic susceptibility measured in a weak ac field is founded at the freezing temperature Tf. (The frequency region is 0.3≤ω≤2 kHz, H∼=0.3 Oe.) The maximum is lowered under a static magnetic field which is applied parallel to the ac field. Low-temperature magnetization in the weak field depends on the cooling process (with or without field). The Tf(ω) dependence obeys the power law and Tf(H,T) dependence obeys the Almeida–Touless relation; these facts attest that phase transition spin glass–paramagnetic exists in this sample. This is a first spin glass among chalcospinels with magnetic ions in A and B sublattices.

Cation Distribution and Magnetic Interactions in Substituted Iron-Containing Garnets: Characterization by Iron-57 Mössbauer Spectroscopy

Some new compounds with garnet-related structures have been examined by57Fe Mössbauer spectroscopy. The results show that in compounds of the type YCa2SbFe4−xGaxO12(x= 2, 3) the gallium is distributed over both the octahedral and tetrahedral sites. In these compounds, together with materials of composition Y3−2xCa2xSbxFe5−xO12(x= 1.25, 1.5) and Y3−xCaxSnxFe5−xO12(x= 1, 2), the results show evidence of more than one tetrahedral environment for the Fe3+ions. The quadrupole splitting data for the Fe3+ions in tetrahedral sites in some of these compounds are significantly larger than those previously reported for Fe3+in tetrahedral sites in other garnets. The compounds YCa2SbFe4O12and Y2CaSnFe4O12magnetically order at similar temperatures and show comparable octahedral- and tetrahedral-hyperfine magnetic fields at 18 K. The substitution of Fe3+by diamagnetic Sb5+on the octahedral sites results in a considerable lowering of the magnetic ordering temperature. The compounds Y0.5Ca2.5Sb1.25Fe3.75O12and Ca3Sb1.5Fe3.5O12show magnetic ordering on both the octahedral and tetrahedral sublattices at 18 K. A similar effect is observed in the compound YCa2SbFe3GaO12where 33% of the Fe3+ions located at the tetrahedral sites are substituted by diamagnetic Ga3+ions. The importance ofa–dantiferromagnetic superexchange interactions is demonstrated in compounds of the type YCa2SbFe2Ga2O12and YCa2SbFeGa3O12where the Ga3+ions substitute the Fe3+ions on both the octahedral and tetrahedral sites. In these compounds the dilution of the magnetic ions at the tetrahedral sites results in the frustration of magnetic ordering on both thed- anda-sublattices. The compound YCa2Sn2Fe3O12which does not contain Fe3+on the octahedral sites shows magnetic ordering on thed-sublattice in the absence of magnetic ions on theasublattice. The result demonstrates the importance ofd–dantiferromagnetic superexchange interactions.

The hyperfine field distribution in copper-cadmium ferrites studied by Mössbauer and NMR spectroscopies

Hyperfine fields have been studied at liquid nitrogen temperature in CdxCu1−xFe2O4 (0 ⩽ x ⩽ 0.5) ferrites by Mössbauer and NMR spectroscopies. The distributions of the hyperfine fields at iron sites, caused by substitution of cadmium for copper, are observed both in the tetrahedral and octahedral crystal sublattices. In both sublattices the values of the averaged hyperfine field decrease with increasing cadmium concentration. The hyperfine field distributions and size reductions are more pronounced in the octahedral sublattice than in the tetrahedral one. They are explained by different statistical distributions of the metallic cations within the tetrahedral and octahedral crystal sublattices. The Mössbauer effect and NMR data are in a good agreement each with other.

Diffusion and Percolation on Zeolite Sorption Lattices

Percolation is investigated using Monte Carlo lattice dynamics on two unusual lattices of sorption sites suggested by recent zeolite simulations. The lattices are modifications of the simple cubic and the tetrahedral lattice. We investigate the percolative behavior caused by these modifications and compare this behavior with that of the parent lattice from published simulation results for simple cubic lattices and the approximate (but analytical) effective medium approximation (EMA) theory. The modifications of the parent lattices include (1) the addition of an octahedral sublattice at each node within a simple cubic superlattice and (2) partial blocking of three adjacent bonds by a single bond-blocker on a tetrahedral lattice. For the former, we examine both random and correlated placement of superlattice bond-blockers. We also examine random placement of sub-lattice bond-blockers. We also investigate the effect of activation energies on percolation. We ascertain to what degree regular lattice percolatio...

Kinetics of order-disorder processes in spinels

Abstract In spinels, the cations are distributed over sites with octahedral and tetrahedral coordination, respectively. In crystals with a significant temperature dependence of the cation distribution, the kinetics of the cation exchange can be studied by means of temperature-jump relaxation experiments. An analysis is reported for the kinetics of the cation exchange between the tetrahedral and octahedral sublattices of spinels. Several microscopic models for the cation exchange which involve direct exchange, a vacancy mechanism, and an interstitial mechanism are discussed and compared to experimental data. These data were obtained by means of optical spectroscopy for the relaxation of the cation distribution in NiAl2O4 after temperature-jumps in the temperature range between 720 and 890°C.

Characterization of the vibrational dynamics in the octahedral sublattices of LaD2.25 and LaH2.25

Incoherent inelastic neutron scattering spectroscopy was used to characterize the optic-vibrational density of states (DOS) of the octahedrally coordinated deuterium (Do) and hydrogen (Ho) atoms in LaD2.25, LaH2.25 and LaH2.03. The DOS exhibits a temperature- and concentration-dependent behaviour consistent with that observed previously for the analogous beta -TbH2+x system. At low temperature, the Ho DOS for LaH2.03 is fairly sharp with minor spectral sidebands, indicating that the Ho atoms are predominantly isolated, with some atoms residing in short-range-ordered domains. Increasing the Ho (or Do) concentration to LaH2.25 (or LaD2.25) yields a dispersion-broadened bimodal DOS characteristic of the Ho (or Do) I4/mmm long-range order that develops in the octahedral sublattice at low temperatures and these higher Ho (or Do) concentrations. For LaH2.25 at higher temperature (340 K), a broad, somewhat asymmetric DOS is suggestive of an Ho sublattice that is now largely disordered yet still possesses some degree of short-range order.

Neutron-powder-diffraction study of the long-range order in the octahedral sublattice of LaD2.25.

Neutron-powder-diffraction patterns of the superstoichiometric rare-earth dideuteride ${\mathrm{LaD}}_{2.25}$ were measured between 15 and 400 K. Profile refinements indicated that, above \ensuremath{\sim}345 K, the ${\mathrm{LaD}}_{2.25}$ structure is cubic (Fm3m) with deuterium fully occupying the tetrahedral (t) interstices of the fcc La lattice and the excess deuterium occupying a portion of the octahedral (o) interstices with full statistical disorder. As the temperature is decreased below \ensuremath{\sim}345 K, the ${\mathrm{LaD}}_{2.25}$ structure undergoes a transformation to tetragonal symmetry concomitant with the onset of deuterium long-range order (I4/mmm) in the o sublattice. Fully developed long-range order is established near 230 K and ideally corresponds to the occupation of only those o sites within every fourth (042${)}_{\mathrm{cubic}}$ plane, in line with that reported for other rare-earth--deuterium systems possessing similar D/metal stoichiometric ratios. This ordering is accompanied by an outward expansion of the cubic ensemble of eight t-site deuterium atoms surrounding each o-site deuterium (${\mathrm{D}}_{\mathit{o}}$) atom. Moreover, the c-directed La-${\mathrm{D}}_{\mathit{o}}$ bond distances are decreased by a displacement of the La atoms toward the ${\mathrm{D}}_{\mathit{o}}$ atoms.