Occupation Of Octahedral Sites Research Articles

Low temperature synthesis and influence of Ce3+ substitution on structural and magnetic properties of nanocrystalline cobalt ferrite using citrate precursor sol-gel method

Rare earth element Ce3+ substituted spinel cobalt ferrite, CoFe2-xCexO4 for (x = 0.0, 0.05, 0.10, 0.15, 0.20) is synthesized using an economical sol–gel technique. The X-ray diffraction analysis reveals that all samples have spinel symmetry indexed to Fd3m space group. Fullprof Rietveld analysis is performed to find out lattice parameters that increase monotonically due to induced lattice strains in samples. The crystallite size was determined to be 54.9 nm using Williamson-Hall plots. The morphological analysis of samples has been performed using SEM imaging. FTIR spectrum confirms octahedral and tetrahedral site occupancy by metal–oxygen bonds for all samples. The magnetic properties of samples refine with Ce3+ doping at iron site, with magnetization ranging from 23.59 to 57.68 emu/g and retentivity 17.14 to 24.08 emu/g. The Coercive field values drastically increased from 739 to 1912 Gauss. Thus, we have explored the influence of rare earth Ce3+ (1.14 Å) substitution at the Fe3+ site (0.55 Å) in CoFe2O4 crystal lattice for improvement of its structural and magnetic properties in this paper.

The role of Li site occupancy on the Li-ion conductivity of Ta-doped Li6.75La3Zr1.75Ta0.25O12 solid electrolyte materials with high Li concentrations

Garnet-type Li7La3Zr2O12 has attracted much attention because of its promising properties of high ionic conductivity, good chemical stability against Li metal, and good mechanical strength. To better understand the Li conduction mechanism, the Li site occupancy and Li-ion conductivity are studied with a high Li concentration in the range of 6.70–7.15 mol per formula unit. Aluminum and silicon, which presumably were transferred to the sample from the Al2O3 crucible during processing, were included in the cubic garnet structure for the Rietveld refinements. The Li-ion conductivity increases with the occupancy of the octahedral Li site (Occupancy: 0.78–0.87, Li-ion conductivity: 3–5 × 10−4 S/cm). As the lithium content increases, the portion of lithium occupying the 96 h octahedral site increases and that occupying the 24d tetrahedral site decreases.

Understanding of intriguing metal to semiconductor transition in Ni0.5Zn0.5Fe2O4 nanoparticulates

Temperature-dependent electrical transport characteristics of un-milled and high-energy ball-milled samples (3 h (70 nm), 6 h (55 nm) and 9 h (45 mm)) of Ni0.5Zn0.5Fe2O4 spinel ferrite were explored. A well-defined metal to semiconductor transition exhibited by all the samples has been construed in view of direct and superexchange cationic interactions and delocalization to localization of charge carriers on increasing temperature. The peak temperature (Tmax) was found to shift towards a higher temperature side on milling principally governs by the lattice vibration scattering and intrinsic excitation. The crystallite size reduction, enhancement in strain and sudden decrease in the formation and octahedral site occupancy of Fe 2+ ions on milling found responsible for the prodigious rise (~ 250 times) in normalized resistivity values for the sample comminuted for 9 h. The spectrum of energies corresponds to charge trapping centers that cause small bump (3 h milled sample) and sharp cusp (9 h milled sample) for T>Tmax. These materials may be found suitable for thermal cutoff switching applications.

Distorted octahedral site occupation-induced high-efficiency broadband near-infrared emission in LiScGe2O6:Cr3+ phosphor

Benefiting from a proposed distorted octahedral site occupation strategy using Cr3+, we demonstraed a high-efficiency broadband NIR-emitting phosphor LiScGe2O6:Cr3+ peaking at 886 nm with a full width at half maximum of 160 nm and a record external quantum efficiency of ∼40%.

Configurational and communal entropies of formation of non-stoichiometric PdHx at the glass transition temperature and the critical temperature

Non-stoichiometric palladium hydride (PdHx) with vacancies for hydrogen exhibits a glass transition temperature (Tg), resulting in generation of both configurational and communal entropies. The communal entropy was evaluated under a given configurational entropy with a suitable hydrogen occupation of tetrahedral and octahedral sites of Pd using the standard molar entropy of formation (ΔfSm(exp.)°) calculated from the equilibrium pressures in pressure–composition–temperature (PCT) curves and the spectroscopic entropies of molecular hydrogen and PdHx.PCT curves with little hysteresis between hydrogenation and dehydrogenation processes were obtained just below the critical temperature Tc, at which the lattice volume mismatch between α and β phases of PdHx is almost negligible. The ΔfSm(exp.)° value calculated from the PCT curves was −83.4 ± 1.3 J/(K·molH2) in the hydrogenation process and −87.3 ± 1.7 J/(K·molH2) in the dehydrogenation process, showing only a relatively small discrepancy between the two.Based on data from the literature, we assumed that the α phase, with its lower hydrogen concentration, was PdH0.006, and that the β phase, with its higher hydrogen concentration, was PdH0.6 in the equilibrium hydrogenation reaction between α and β phases. We then estimated the difference in communal entropy (ΔScom.°) between PdH0.006 and PdH0.6 in terms of the ΔfSm(exp.)° value, the hydrogen molar entropy (Sm,H2°), the vibrational entropy (ΔSvib.°), the electronic entropy (ΔSele.°), and given configurational entropy (ΔSconf.°). We found that near Tc, H atoms occupied both the octahedral and tetrahedral sites in PdH0.6, with ΔScom.° approaching the value of the gas constant R. On the other hand, ΔScom.° approached 0 J/(K·molH) near the glass transition temperature Tg, with H atoms occupying only all the octahedral sites in PdH0.6.

Synthesis and characterisation of Ca1-xCexZrTi2-2xCr2xO7: Analogue zirconolite wasteform for the immobilisation of stockpiled UK plutonium

A series of Ca1-xCexZrTi2-2xCr2xO7 zirconolite ceramics (0 ≤ x ≤ 0.35) were reactively sintered in air at 1350 °C for 20 h. Single phase zirconolite-2M was formed for x ≤ 0.15, with Cr2O3 and an undesirable Ce-bearing perovskite phase present above x = 0.20. Electron diffraction analysis confirmed that the zirconolite-2M polytype was maintained over the solid solution. X-ray absorption near edge structure (XANES) data determined that between 10–20% Ce was speciated as Ce3+, and Cr was present uniformly as Cr3+ with near edge features consistent with occupation of octahedral sites within the zirconolite-2M structure. A sample corresponding to x = 0.20 was processed by reactive spark plasma sintering (RSPS), with a rapid processing time of less than 1 h. XANES data confirmed complete reduction to Ce3+ during RSPS, promoting the formation of a Ce-bearing perovskite, comprising 19.3 ± 0.4 wt. % of the phase assemblage.

Hydrogen as a probe for defects in materials: Isotherms and related microstructures of palladium-hydrogen thin films

Metal-hydrogen systems offer grand opportunities for studies on fundamental aspects of alloy thermodynamics. Palladium-hydrogen (Pd-H) thin films of nano crystalline, multi-oriented and epitaxial microstructures, electrolytically charged with hydrogen, serve as model systems. In these films thermodynamics of hydrogen absorption is modified by interface effects related to mechanical stress and to microstructural defects. Since in this respect hydrogen can be utilized to reveal the microstructural constituents of the films, we aim to investigate the distribution of sites (DOS) hydrogen occupies in the films’ solid solution regime. A σDOS model is proposed, taking the measured substrate-induced stress contribution to the chemical potential into account. This enables the determination of the different sites’ volume fractions and of pure site energy distributions by fitting measured isotherms. Interstitial sites, grain/domain boundary sites and deep traps are distinguished. Dislocations and vacancies are shown to have a minor impact on the films’ trapping of hydrogen atoms, while deep traps are related to the films’ surface. Enhanced binding energies in nano crystalline films can be ascribed to the tensile strain effect of grain boundaries acting on the grains. Measured surface trapping energies fit to the respective bulk values, while the trapping of hydrogen in grain/domain boundaries of the films is significantly increased. This can be interpreted with different grain/domain boundary structures. Different from octahedral interstitial site occupation, tetrahedral site occupation is suggested for grain/domain boundaries of the films.

Unexpected photoluminescence from Mn2+ in LiZnNbO4

In this paper the synthesis of the LiNbZnO4 doped with Mn2+ by solid state method and its photoluminescence results are presented. The structural analysis was performed by X-ray diffraction and Rietveld refinement, while the emission of the impurity ion into this structure was investigated by photoluminescence and excitation measurements. The photoluminescence spectrum presented a broad band centered at 700 nm, associated to the transition of Mn2+ from the excited state 4T1 (4G) to the 6A1(6S) fundamental state. Crystal field parameter Dq and Racah parameters B and C were estimated using the transition energies observed in the excitation spectrum. Photoluminescence results indicate that the emission is related to an unexpected octahedral site occupation of Mn2+ in the LiZnNbO4 lattice.

Structural, magnetic, optical properties and cation distribution of nanosized Co0.7Zn0.3TmxFe2-xO4 (0.0 ≤ x ≤ 0.04) spinel ferrites synthesized by ultrasonic irradiation.

This study expressed the influence of Tm substitution on the structural, optical and magnetic properties of Co-Zn spinel ferrites (Co0.7Zn0.3TmxFe2-xO4 (0.0 ≤ x ≤ 0.04)). The different compositions were synthesized by sonochemical method using Qsonica ultrasonic homogenizer, frequency: 20 kHz and power: 70 W for 60 min. XRD patterns proved the presence of single-phase spinel ferrites with crystallites size in the 8-10 nm range. Cation distribution approved the occupancy of octahedral (B) site by Tm. The morphology and the elements stoichiometry are obtainable through FE-SEM, EDX and elemental mapping. Optical band gap (Eg) values were estimated via DR % (percent diffuse reflectance) investigations and Kubelka-Munk theory. Tauc plots revealed that direct Eg values are ranging between 1.49 and 1.68 eV. The analyses of magnetization versus magnetic field, M(H), were performed. The following magnetic parameters like saturation magnetization Ms, squareness ratio (SQR = Mr/Ms), magnetic moment nB, coercivity Hc and remanence Mr have been evaluated. M(H) curves revealed the superparamagnetic (SP) at RT and ferromagnetic property at 10 K. It was showed that the Tm3+ substitutions significantly affect the magnetic properties of host spinel ferrites. An increasing trend in the Ms, Mr, Hc, and nB values was noticed for lower Tm3+ substitution content.

Fluorine-controlled composition of biotite in granulites of Madagascar: the effect of fluorine on thermometry of biotite–garnet gneisses

Because of the strong preference of fluorine (F) for “hydrous” magnesian silicate minerals, the temperature T(0F) given by the reaction (Bi–Gt) of Fe–Mg exchange between biotite and garnet [phlogopite (Phl) + almandine (Alm) = annite (Ann) + pyrope (Py)] can be dramatically underestimated when the reciprocal effects between octahedral and hydroxyl site occupancies in biotite are ignored. In the granulites of southeastern Madagascar, widespread Mg metasomatism was associated with F-rich fluids. It is mainly manifested by the regional occurrence of diopside skarns hosting F-phlogopite and thorianite mineralisation. At first, biotite- and garnet-bearing granites and leucogneisses appear unaffected by this event. However, their biotites are rich in F [up to 6 wt%; XF = XF/(F + OH) = 0.69) and in Mg (XMg = Mg/(Mg + Fe) up to 0.8], which shows the tendency of equilibrium with the regional fluids. This equilibration leads to low values of Fe/Mg, which in turn cause strong underestimation of the temperature: T(0F) as low as 500 °C, whereas the temperature T(Ti) given by the TiO2 content of biotite is approximately 750 °C. The slightly modified thermodynamic model of Zhu and Sverjensky (Geochim Cosmochim Acta 56:3435–3467, 1992) for reciprocal solid solution is applied to a set of 19 samples to correct this underestimation and to quantify the effect of F on thermometry. The reciprocal activity coefficients for phlogopite (λ(Phl)) and annite (λ(Ann)) calculated by the model are added to the winTWQ software to obtain corrected values T(F) of the temperature. Similar values of ΔT = T(F) − T(0F) are obtained by (1) the linear fit of the whole set of the samples and (2) RTlnKλ = RTlnλ(Ann) − RTlnλ(Phl) = 33.417XF − 0.1092 (r2 = 0.9994) equation applied to a median sample: ΔT = 30–40, 60–70, 95–100, 160, and 200 °C for XF = 0.1, 0.2, 0.3, 0.5, and 0.7, respectively. The same calculations based on the Thermocalc thermodynamic data set lead to values of ΔT 1.5 times higher than those given by winTWQ. Thus, the results of these calculations confirm (for the first time from a set of natural samples) the validity of the thermodynamic model of reciprocal solid solution and the necessity of accounting for F in thermometry based on Fe–Mg exchanges involving biotite.

Interstitial atom ordering in fcc-based Ni4X with X = N and C

Interstitial transition metal nitrides and carbides based on simple crystal structures with partial occupation of certain sites by N and C frequently show long-range ordering. In this work the focus is on the important group of superstructures based on a face-centered cubic (fcc)-based arrangement of the metal atoms and octahedral site occupation by N or C. The ordering tendencies are explored for various Ni4N and Ni4C superstructures using density-functional-theory. The ordering tendencies differ strikingly as revealed by the low-energy superstructures encountered for Ni4N and for Ni4C: in the case of Ni4N 2nd-next-neighbor N-N pairs appear favorable whereas in Ni4C 1st-next neighbor C-C pairs are preferred. This can be explained by an interplay between elastic interactions favoring 1st-next neighbor pairs of N and C atoms in an fcc-based arrangement of metal atoms and chemical/Coulomb interactions favoring 2nd-next neighbor (i.e. more distant) pairs. While the elastic interactions are expected to be similar for N and C, the chemical/Coulomb interactions are stronger for N-N as compared to C-C and are thus able to explain the different ordering tendencies.

Mechanochemical Induced Structure Transformations in Lithium Titanates: A Detailed PXRD and 6Li MAS NMR Study

Lithium titanates are used in various applications, such as anode materials for lithium intercalation (Li4Ti5O12) or breeding materials in fusion reactors (Li2TiO3). Here, we report the formation of nano-crystalline lithium titanates by a mechanochemical approach and present a deeper insight into their structural characteristics by X-ray diffraction (XRD) and solid-state NMR spectroscopy. The compounds were synthesized in a high-energy planetary ball mill with varying milling parameters and different grinding tools. NaCl type Li2TiO3 (α-Li2TiO3) was formed by dry milling of lithium hydroxide with titania (rutile or anatase) and by a milling induced structure transformation of monoclinic β-Li2TiO3 or spinel type Li4Ti5O12. Heating of mechanochemical prepared α-Li2TiO3 induces a phase transformation to the monoclinic phase similar to hydrothermal reaction products, but a higher thermal stability was observed for the mechanochemical formed product. Microstructure and crystallographic structure were characterized by XRD via Rietveld analysis. Detailed phase analysis shows the formation of the cubic phase from the various educts. A set of two lattice parameters for α-Li2TiO3 was refined, depending on the presence of OH− during the milling process. An average crystallite size of less than 15 nm was observed for the mechanochemical generated products. The local Li environment detected by 6Li NMR revealed Li defects in the form of tetrahedral instead of octahedral site occupation. Subsequent adjustment of the structural model for Rietveld refinement leads to better fits, supporting this interpretation.

Structural, magnetic and dielectric properties of Gd3+ substituted NiFe2O4 nanoparticles

NiGdxFe2-xO4 nanoparticles with x = 0.0, 0.03, 0.05, 0.07, and 0.10 were synthesized by co-precipitation method and effect of Gd3+ substitution on various properties of nanocrystalline NiFe2O4 has been studied. Formation of single phase cubic mixed spinel structure is confirmed by X-ray diffraction and Raman spectroscopy and cations distribution has also been proposed from Rietveld refined data. Room temperature Mössbauer spectra showed two ferrimagnetic Zeeman sextets with one superparamagnetic doublet. Octahedral site occupancy of Gd3+ ions in these nanoparticles is suggested by Mössbauer parameters. Within the framework of X-ray photoelectron spectroscopy analysis, the ratio of cation concentration at tetrahedral and octahedral sites has been found to be consistent with proposed cation distribution. Magnetic measurements at room temperature exhibit the reduction in the saturation magnetization from 36 emu/gm to 4.9 emu/gm for x = 0.0 to 0.10 samples. The discrepancy in measured and calculated magnetic moment may be ascribed to the nonzero Yafet–Kittel angles of B site spins, which weaken A–B interaction. The coercivity decreases from 121 Oe to 48 Oe for x = 0.0 to 0.07 samples and again increases to 153 Oe for x = 0.10 sample. The degree of inversion calculated by Raman analysis varies with Gd3+ concentration which is supported by the variation of coercivity. The calculated spin numbers from electron spin resonance spectra decreases with Gd3+ doping which confirms the reduction in magnetization. Both dielectric constant and dielectric loss are found to decrease with the Gd3+ substitution because of the reduced hopping rate for Fe3+ at the octahedral sites in doped nanoparticles. Lower values of dielectric loss suggest that Gd3+ doped NiFe2O4 nanoparticles are suitable for high frequency microwave device applications.

Synthesis, structure and properties of the layered CuxTiS2 compounds

A low-temperature layered phase CuxTiS2 (0 ≤ x ≤ 0.7) has been prepared. The occupation of both octahedral and tetrahedral sites by the copper atoms in the interlayer gap of the TiS2 lattice is revealed. The presence of two concentration regions of 0 ≤ x ≤ 0.3 and 0.5 ≤ x ≤ 0.7, which differ in the concentration dependence of the unit cell volume, has been observed. In the first region the unit cell volume increases with x, in the second region it decreases. This difference may be associated with the competition between Cu-Ti and Cu-S covalent centres. The electrical and magnetic properties of the materials are in a good agreement with this assumption. The electronic structure of CuxTiS2 in the low copper concentrations region has been studied using X-ray photoelectron, resonant photoelectron and absorption spectroscopy techniques. It was found that, in the low copper concentrations region, the charge transfer from Cu to the conduction band of TiS2 along with the formation of covalent centres is observed.

First-Principles Study of Spinel MgTiS2 as a Cathode Material

Spinel intercalation hosts are well known to to facilitate high rate capability and high voltage Li-ion batteries. A recent experimental study has shown that Mg can reversibly intercalate in spinel TiS2, demonstrating the viability of Li intercalation host crystal structures for Mg-ion batteries [Sun, X.; Energy Environ. Sci. 2016, 9, 2273−2277]. We report on a first-principles statistical mechanics study of Mg insertion into spinel TiS2, accounting for occupancy on both octahedrally and tetrahedrally coordinated interstitial sites. In contrast to Li-containing spinels, we predict mixed octahedral and tetrahedral site occupancy at nondilute Mg concentrations consistent with the recent experimental study of Sun et al. The onset of mixed occupancy is correlated with an increase in the spinel volume upon Mg insertion, which is more pronounced in MgxTiS2 than in its Li counterpart. The results in this study suggest that the degree of mixed occupancy could be controlled through the volume of the host with addi...

2D-3D transition in Cu-TiS2 system.

The phase diagram of the quasi-binary Cu-TiS2 system has been studied in situ in the temperature-concentration plane for the first time. Phase transitions between 2D (layered) and 3D (cubic) phases in CuxTiS2 (x = 0-0.5) intercalation compounds have been studied by the X-ray diffraction technique in the temperature range 20-1000 °C. It has been found that the stability of the layered phase is determined by the distribution of copper atoms between the octahedral and tetrahedral crystallographic sites. The occupation of octahedral sites dominates at low temperatures. Upon heating, tetrahedral site occupation is limited due to elastic lattice distortion and the layered phase becomes unstable and transforms to the spinel. Further heating allows the distribution of copper between octahedral and tetrahedral sites; the layered phase becomes stable again.

Origin of clay minerals in Early Eocene volcanic paleosols on King George Island, Maritime Antarctica

The paleoclimate during the Early Eocene in Maritime Antarctica is characterized by cool conditions without a pronounced dry season. Soils formed on volcanic material under such climate conditions in modern analogue environments are usually Andosols rich in nanocrystalline minerals without pedogenic smectite. The paleosols formed on volcanic material on King Georges Island are covered by basalts, dated by 6 new 40Ar/39Ar datings to 51–48 Ma, and are rich in smectite. A pedogenic origin of the smectites would suggest a semi-arid rather than a wet non-seasonal humid paleoclimate. To investigate the origin of the smectites in these paleosols we used X-ray diffraction and microscopic techniques. Minor mineralogical changes between the volcanic parent material and the paleosols and a homogenous distribution of smectites throughout the paleosol horizons indicate that these smectites were mainly inherited from the pyroclastic parent material, which was altered prior to surficial weathering. Nevertheless, the mineralogical properties, such as degree of crystallinity and octahedral site occupancy, of these smectites were modified during the ancient soil formation. Our findings highlight that trioctahedral smectites were a product of deuteric alteration of pyroclastic rocks and were progressively transformed to dioctahedral smectites during weathering in a soil environment on King George Island.

Cation distribution correlated with magnetic properties of cobalt ferrite nanoparticles defective by vanadium doping

Nanoparticles cobalt ferrite, vacancies defective through vanadium substitution for iron, were synthesized by a sol-gel method. Two systems CoFe2-xVxO4 (0.0≤x≤0.25) and CoFe2-1.67xVxO4 (x=0.1, 0.2) were prepared. The crystal structure, microstructure and magnetic properties were investigated using XRD, SEM and VSM magnetometer. The occupancy of tetrahedral and octahedral sites by different cations was determined by Rietveld analysis and correlated with magnetic measurements. Vanadium resides at octahedral sites up to x=0.10, while for higher values it resides mainly at octahedral sites with a lesser amount at the tetrahedrons. Upon increasing the vanadium content, the cell parameter decreases and the bond lengths of the tetrahedral and octahedral sites change opposite to each other. The change in the coercivity and saturation magnetization is correlated with cation distribution. For the same amount of doping x, the iron deficient samples CoFe2-1.67xVxO4 have saturation magnetization obviously reduced than the corresponding samples in CoFe2-xVxO4. The spin canting between cations in A- and B- sites was discussed in details based on Yafet–Kittel triangular arrangement model.

Effect of Ni/Al molar ratio on the performance of substoichiometric NiAl2O4 spinel-based catalysts for partial oxidation of methane

The viability of a series of nickel aluminate-based spinel catalysts with varying Ni deficiency (corresponding to a Ni/Al molar ratio in the 0.13–0.50 range) was explored for the partial oxidation of methane under different operation conditions in terms of temperature, volume hourly space velocity, O/C molar ratio and time on stream. Thus spinel-type catalysts with a Ni loading between 11 and 31wt.% were prepared by coprecipitation. A wide number of techniques including WDXRF, XRD, N2 physisorption, Raman spectroscopy, XPS, UV–vis-NIR DRS, H2-TPR, TEM and TGA-MS were used to characterise the calcined, reduced and post-run samples.With respect to the reference stoichiometric sample (Ni/Al=0.50) alumina excess in the precursor oxide provoked notable changes in the surface area, structural properties connected with the relative cation distribution between tetrahedral and octahedral coordination and reducibility of the resultant spinel phase. It was found that the catalytic performance of these non-stoichiometric samples could be optimised for a Ni/Al molar ratio of 0.25, which corresponded to a metal loading of 19wt.%Ni. The oxidation activity was associated with the remarkable intrinsic activity of nickel particles derived from Ni2+ cations with a preferential occupancy of octahedral sites in the lattice of the oxide. The promising catalytic behaviour of this sample was further proven by the notable activity and stability shown under severe reaction conditions with a reduced loss of yield of hydrogen with time on stream.

Concentration dependence of hydrogen diffusion in clamped vanadium (0 0 1) films

The chemical diffusion coefficient of hydrogen in a 50 nm thin film of vanadium (0 0 1) is measured as a function of concentration and temperature, well above the known phase boundaries. Arrhenius analysis of the tracer diffusion constants reveal large changes in the activation energy with concentration: from 0.10 at 0.05 in H V−1 to 0.5 eV at 0.2 in H V−1. The results are consistent with a change from tetrahedral to octahedral site occupancy, in that concentration range. The change in site occupancy is argued to be caused by the uniaxial expansion of the film originating from the combined hydrogen induced expansion and the clamping of the film to the substrate.