Cation Inversion Research Articles

High-resolution 27Al MAS NMR spectroscopic studies of the response of spinel aluminates to mechanical action

The response of the local structure of various types of spinel aluminates, ZnAl2O4 (normal spinel), MgAl2O4 (partly inverse spinel), and Li0.5Al2.5O4 (fully inverse spinel), to mechanical action through high-energy milling is investigated by means of 27Al MAS NMR. Due to the ability of this nuclear spectroscopic technique to probe the local environment of Al nuclei, valuable quantitative insight into the mechanically induced changes in the spinel structure, such as the local cation disorder and the deformation of the polyhedron geometry, is obtained. It is revealed that, independent of the ionic configuration in the initial oxides, the mechanical action tends to randomize cations over the two non-equivalent cation sublattices provided by the spinel structure. The response of the spinels to mechanical treatment is found to be accompanied by the formation of a non-uniform core–shell nanostructure consisting of an ordered crystallite surrounded by a structurally disordered interface/surface shell region. Based on the comparative NMR studies of the non-treated and mechanically treated spinels, an attempt is made to separate the surface effects from the bulk effects in spinel nanoparticles. The non-equilibrium cation distribution and the deformed polyhedra are found to be confined to the near-surface layers of spinel nanoparticles with the thickness extending up to about 0.7 nm. The cation inversion parameter of the mechanically treated spinel is compared with that of the non-treated material at non-ambient conditions.

Spinel-type solid solutions involving Mn 4+ and Ti 4+:Crystal chemistry, magnetic and electrochemical properties

This study reports the structural and magnetic properties of spinel systems Li 4Mn 5− x Ti x O 12 (“4-5-12” series) and LiNi 0.5Mn 1.5− x Ti x O 4 (“LNMTO” series), both based on Mn 4+ substitution by Ti 4+. Intermediate compositions covering the whole range of compositions (0≤ x≤5 and 0≤ x≤1.5, respectively) were prepared by solid state reaction. The 4-5-12 system forms a continuous spinel solid solution, whereas the spinel phase range in LNMTO stops before the end member “LiNi 0.5Ti 1.5O 4”, which is multi-phased with a major hexagonal phase component. Cell parameters and (Mn,Ti)–O distances increase monotonically with titanium content in both series. In the LNMTO series, the end member LiNi 0.5Mn 1.5O 4 is known to form a superstructure with Ni/Mn cation ordering. Neutron diffraction and Raman spectroscopy show that this order is lost when Ti is substituted, even at low level ( x=0.15). The LNMTO crystal chemistry is also complicated by the presence of partial cation inversion, and the presence of a secondary rocksalt-type phase that modifies the spinel stoichiometry. Magnetic properties are characterized by a competition between ferromagnetic and antiferromagnetic interactions; no magnetic ordering is achieved, in agreement with B-site cation frustration and disorder. Electrochemical measurements show that the Ti 3+/4+ and Mn 3+/4+ redox couples behave independently in the 4-5-12 series, and that titanium decreases the high-potential electrochemical redox activity of LNMTO because of its blocking character for electron transfer to and from the nickel sites in the spinel structure.

Modified magnetic ground state inNiMn2O4thin films

We demonstrate the stabilization of a magnetic ground state in epitaxial ${\text{NiMn}}_{2}{\text{O}}_{4}$ (NMO) thin films not observed in their bulk counterpart. Bulk NMO exhibits a magnetic transition from a paramagnetic phase to a collinear ferrimagnetic moment configuration below 110 K and to a canted moment configuration below 70 K. By contrast, as-grown NMO films exhibit a single magnetic transition at 60 K and annealed films exhibit the magnetic behavior found in bulk. Cation inversion and epitaxial strain are ruled out as possible causes for the new magnetic ground state in the as-grown films. However, a decrease in the octahedral ${\text{Mn}}^{4+}:{\text{Mn}}^{3+}$ concentration is observed and likely disrupts the double exchange that produces the magnetic state at intermediate temperatures. X-ray magnetic circular dichroism and bulk magnetometry indicate a canted ferrimagnetic state in all samples at low temperature. Together these results suggest that the collinear ferrimagnetic state observed in bulk NMO at intermediate temperatures is suppressed in the as grown NMO thin films due to a decrease in octahedral ${\text{Mn}}^{4+}$ while the canted moment ferrimagnetic ordering is preserved below 60 K.

Thermal annealing effect on cation inversion and particle size of Zn-ferrite

Thermal annealing effect on cation inversion and particle size of Zn-ferrite

ChemInform Abstract: Structure of Spinel

This paper reviews the crystal structure of compounds with the general formula AB 2 X 4 , which crystallize with the same atomic structure as the mineral spinel, MgAl 2 O 4 . Three degrees of freedom associated with the detailed atomic arrangements of spinels are considered here: (i) the lattice parameter, a; (ii) the anion parameter, u; and (iii) the cation inversion parameter, i. Oxide spinels are used as examples to explore the interrelationships between these parameters.

Magnetization enhancement in nanostructured random type MgFe2O4 spinel prepared by soft mechanochemical route

In this paper we report results of structural, spectroscopic, and magnetic investigations of MgFe2O4 nanoparticles prepared by soft mechanochemical synthesis. MgFe2O4 nanoparticles crystallize in Fd3¯m space group with mixed cation distribution and reduced percentage of Fe3+ at tetrahedral (8a) sites. Discrepancy in the cation distribution compared to that in the bulk Mg–ferrite is one of the highest known. X-ray line broadening analysis reveals crystallite size and strain anisotropy. The saturation magnetization, Msat=62 emu/g measured at 5 K is twice higher than that found in the bulk counterparts. Such high value of Msat is attributed to the low value of cation inversion parameter (δ=0.69), to the core/shell structure of the nanoparticles and to the surface/volume ratio. Mössbauer spectrum collected at room temperature reveals ferrimagnetic ordering between Fe3+ ions in 8a and 16d sites, while zero-field-cooled (ZFC) and field-cooled (FC) M(T) measurements were shown superparamagnetic state above 350 K.

Magnetic study of nanostructured zinc ferrite irradiated with 100 MeV O-beam

Nanostructured zinc ferrite of particle size 10 nm was synthesized by using the nitrates of appropriate cations and citric acid. This system was irradiated by 100 MeV oxygen beam with the fluence of 5×10 13 ions/cm 2. The particle size of the system remains almost same after the irradiation. We observe decrease in magnetization of the sample after irradiation at 300 and 10 K. The nature of the σ– H plot shows the presence of superparamgnetic domains at 300 K even after irradiation. The blocking temperature decreases from 276 to 63 K after irradiation. The Mössbauer spectroscopy supports the presence of superparamgnetic domains at 300 K in both the samples. The decrease in magnetization after irradiation is attributed to the decrease in cation inversion and increase in canting angle as observed from in-field Mössbauer spectroscopy.

Enhanced Néel temperature in Mn ferrite nanoparticles linked to growth-rate-induced cationinversion

Mn ferrite (MnFe2O4) nanoparticles, having diameters from 4 to 50 nm, were synthesized using a modifiedco-precipitation technique in which mixed metal chloride solutions were added todifferent concentrations of boiling NaOH solutions to control particle growth rate.Thermomagnetization measurements indicated an increase in Néel temperaturecorresponding to increased particle growth rate and particle size. The Néel temperature isalso found to increase inversely proportionally to the cation inversion parameter,δ, appearing inthe formula (Mn1−δFeδ)tet[MnδFe2−δ]octO4. These results contradict previously published reports of trends between Néel temperatureand particle size, and demonstrate the dominance of cation inversion in determining thestrength of superexchange interactions and subsequently Néel temperature in ferritesystems. The particle surface chemistry, structure, and magnetic spin configuration playsecondary roles.

Large tunability of Néel temperature by growth-rate-induced cation inversion in Mn-ferrite nanoparticles

The tuning of Néel temperature by greater than 100 K in nanoparticle Mn-ferrite was demonstrated by a growth-rate-induced cation inversion. Mn-ferrite nanoparticles, having diameters from 4 to 50 nm, were synthesized via coprecipitation synthesis. The Néel temperature (TN) increased inversely to the cation inversion parameter, δ (i.e., defined as (Mn1−δFeδ)tet[MnδFe2−δ]octO4). Concomitantly, TN increased with increased particle growth rate and particle size. These results unambiguously establish cation inversion as the dominant mechanism in modifying the superexchange leading to enhanced TN. The ability to tailor TN enables greater flexibility in applying nanoparticle ferrites in emerging technologies.

Infrared reflection spectroscopy of Zn 2SnO 4 thin films deposited on silica substrate by radio frequency magnetron sputtering

Single-phase zinc-stannate thin films of different thickness values were prepared by radio frequency magnetron sputtering on a silica substrate. Rietveld analysis of X-ray diffraction data confirmed that the films had an inverse spinel structure, with a cation inversion parameter of 0.8. Room temperature far and mid infrared reflectivity spectra were measured in the range 50–4000 cm − 1 . The reflectivity diagrams were analyzed using the four-parameter model of coupled oscillators for optical phonons with a standard multi-layer technique taking into account the thin-film layer and the substrate. The optical parameters were determined for seven oscillators belonging to the spinel structure. Their origin was discussed in relation to non-stoichiometry of the thin film and cation disorder in the crystal lattice. Born effective charges were calculated from the transversal/longitudinal splitting.

Strain-induced stabilization of new magnetic spinel structures in epitaxial oxide heterostructures

We report here on the growth of NiFe 2O 4 epitaxial thin films of different thickness (3 nm ≤ t ≤ 32 nm) on single crystalline substrates having spinel (MgAl 2O 4) or perovskite (SrTiO 3) structure. Ultrathin films, grown on any of those substrates, display a huge enhancement of the saturation magnetization: we will show that partial cationic inversion may account for this enhancement, although we will argue that suppression of antiparallel collinear spin alignment due to size-effects cannot be excluded. Besides, for thicker films, the magnetization of films on MAO is found to be similar to that of bulk ferrite; in contrast, the magnetization of films on STO is substantially lower than bulk. We discuss on the possible mechanisms leading to this remarkable difference of magnetization.

Effect of growth temperature on the magnetic, microwave, and cation inversion properties on NiFe2O4 thin films deposited by pulsed laser ablation deposition

First principles band structure calculations suggest that the preferential occupation of Ni2+ ions on the tetrahedral sites in NiFe2O4 would lead to an enhancement of the exchange integral and subsequently the Néel temperature and magnetization. To this end, we have deposited NiFe2O4 films on MgO substrates by pulsed laser deposition. The substrate temperature was varied from 700to900°C at 5mTorr of O2 pressure. The films were annealed at 1000°C for different times prior to their characterization. X-ray diffraction spectra showed either (100) or (111) orientation with the spinel structure dependent on the substrate orientation. Magnetic studies showed a magnetization value of 2.7kG at 300K. The magnetic moment was increased to the bulk value as a result of postdeposition annealing at 1000°C. The as produced films show that the ferromagnetic resonance linewidth at 9.61GHz was 1.5kOe, and it was reduced to 0.34kOe after postannealing at 1000°C. This suggests that the annealing led to the redistribution of Ni2+ ions to their equilibrium octahedral sites. Further, it is shown that the magnetically preferred direction of Ha can be aligned perpendicular to the film plane when films are grown with a fixed oxygen pressure of 5mTorr for films deposited at 700 and 900°C.

Size dependent magnetic properties and cation inversion in chemically synthesized MnFe2O4 nanoparticles

Mn Fe 2 O 4 nanoparticles with diameters ranging from about 4to50nm were synthesized using a modified coprecipitation method. X-ray diffractograms revealed a pure phase spinel ferrite structure for all samples. Transmission electron microscopy showed that the particles consist of a mixture of both spherical (smaller) and cubic (larger) particles dictated by the reaction kinetics. The Néel temperatures (TN) of MnFe2O4 for various particle sizes were determined by using high temperature magnetometry. The ∼4nm MnFe2O4 particles showed a TN of about 320°C whereas the ∼50nm particles had a TN of about 400°C. The high Néel temperature, compared with the bulk MnFe2O4 TN of 300°C, is due to a change in cation distribution between the tetrahedral and octahedral sites of the spinel lattice. Results of extended x-ray absorption fine structure measurements indicate a systematic change in the cation distribution dependent on processing conditions.

Cationic exchange in nanosizedZnFe2O4spinel revealed by experimental and simulated near-edge absorption structure

The non-equilibrium cation site occupancy in nanosized zinc ferrites (6-13 nm) with different degree of inversion (0.2 to 0.4) was investigated using Fe and Zn K-edge x-ray absorption spectroscopy XANES and EXAFS, and magnetic measurements. The very good agreement between experimental and ab-initio calculations on the Zn K-edge XANES region clearly show the large Zn2+(A)--Zn2+[B] transference that takes place in addition to the well-identified Fe3+[B]--Fe3+(A) one, without altering the long-range structural order. XANES spectra features as a function of the spinel inversion were shown to depend on the configuration of the ligand shells surrounding the absorbing atom. This XANES approach provides a direct way to sense cationic inversion in these spinel compounds. We also demonstrated that a mechanical crystallization takes place on nanocrystalline spinel that causes an increase of both grain and magnetic sizes and, simultaneously, generates a significant augment of the inversion.

A study of nanosized Ni substituted Co–Zn ferrite prepared by coprecipitation

Nanoparticles of Ni 0.25Co 0.25Zn 0.5Fe 2O 4 with different particle sizes were synthesized by chemical coprecipitation technique. The prepared nanoparticles were characterized and studied by X-ray diffraction, room temperature 57Fe Mössbauer spectroscopy and DC magnetization. Increase in annealing temperature leads to inversion of cations from their normal configurations which results in an increase in the hyperfine fields at both tetrahedral A and octahedral B sites. The decrease in saturation magnetization values with an increase in annealing temperature can be attributed to the variation in proportions of superparamagnetic phase and also the redistribution of cations in these ferrite nanoparticles. Variation of coercivity with annealing temperatures can be explained in terms of the size effect.

Direct determination of the cation disorder in nanoscale spinels by NMR, XPS, and Mössbauer spectroscopy

Quantitative microstructural information is obtained on the nonequilibrium cation arrangement in high-energy milled MgAl 2O 4, ZnFe 2O 4, and NiFe 2O 4 by means of 27Al MAS-NMR, XPS, and 57Fe Mössbauer spectroscopy, respectively. Independently of the ionic configuration in the nonactivated materials, the mechanically induced redistribution of cations was found to be directed towards the random arrangement. The cation inversion parameter of the nanosized milled spinels is compared with that of the bulk materials at high temperatures.

Nonequilibrium cation distribution in nanocrystalline MgAl 2O 4 spinel studied by 27Al magic-angle spinning NMR

Nanocrystalline magnesium aluminate (MgAl 2O 4) powders were prepared by high-energy milling of the bulk material. Due to the ability of nuclear magnetic resonance (NMR) spectroscopy to discriminate between probe nuclei on inequivalent crystallographic sites, valuable insight into the mechanically induced evolution of a local cation disorder in MgAl 2O 4 can be obtained. It, thus, was revealed for the first time that the mechanical treatment of MgAl 2O 4 tends to randomize ions over the cation spinel sublattices. Quantitative microstructural information on the nonequilibrium cation distribution provided by 27Al MAS-NMR is complemented by XRD and TEM investigations revealing the nanoscale nature of the milled material. The cation inversion parameter of the nanosized MgAl 2O 4 is compared with that of the bulk material at non-ambient conditions.

Cation-disorder-enhanced magnetization in pulsed-laser-deposited CuFe2O4 films

Copper ferrite films have been deposited on (100) MgO substrates by pulsed-laser deposition. The oxygen pressure used in deposition was varied from 1to120mTorr with the substrate temperature fixed at 700°C. Magnetization values are measured to increase with oxygen pressure, reaching a maximum value of 2480G, which is a 42% increase over the bulk equilibrium value. Extended x-ray absorption spectroscopy shows that the Cu cation inversion δ [defined as (Cu1−δFeδ)tet[CuδFe2−δ]octO4] decreases monotonically from 0.72 to 0.55 with increasing saturation magnetization.

Crystal structure and magnetic properties of the spinel compoundFe0.76In2.17S4

In the search for new materials exhibiting both ferromagnetic and semiconductingproperties, approximately 10% Fe has been incorporated into the large gap semiconductorIn2S3, resulting in a newspinel compound Fe0.76In2.17S4. Magnetic measurements show properties of a canonical spin glass state below a freezing temperatureof TF = 5 K.Standard powder x-ray diffraction at room temperature revealed the cubic spinel structure. Furtherstructural investigations by high-resolution neutron powder diffraction in the temperature range1.6 K ≤T≤280 K confirmed the spinel structure with an almost complete cation inversion corresponding toan inversion parameter of 0.94. A mutually consistent description of the neutron diffractiondata, EDX analysis and magnetic measurements show that the present compound may bewritten as , with denoting vacancies. No magnetic intensities could be observed, in agreement with a spinglass state.

Magnetic and structural properties of pulsed laser deposited CuFe2O4 films

Highly crystal textured copper ferrite films have been grown on (100) MgO substrates using pulsed laser deposition. Deposition temperatures were varied from 600–900°C, with deposition oxygen pressure varied from 1mTorr to 120mTorr. Saturation magnetization was measured to increase monotonically with substrate temperature approaching the bulk value of 1700G near 900°C. Magnetization was also shown to increase with oxygen pressure with a maximum value of 2481G obtained at a pressure of 90mTorr. Although divalent Cu prefers octahedral sites (i.e., 85% under equilibrium conditions), cation inversion was measured to decrease with increasing oxygen pressure and magnetization.