Occupation Of Tetrahedral Sites Research Articles

Tuning a Colloidal Synthesis to Control Co2+ Doping in Ferrite Nanocrystals

Cobalt−ferrite nanocrystals were synthesized using a high-temperature organometallic decomposition scheme in the presence of surfactant molecules. The influence of the addition of cosurfactant molecules of polyol type on the resulting nanocrystals was examined. The properties of the nanocrystals were studied using electron microscopy, and magneto-optical and Raman spectroscopies. The addition of the cosurfactants was found to influence the growth mechanism of the nanocrystals, resulting in a significant reduction in the concentration of the Co2+ ions incorporated into the ferrite lattice up to a factor of 4 and an increase in the size of the synthesized nanocrystals. In addition, control over the occupation of octahedral versus tetrahedral coordination sites by the cobalt ions was demonstrated.

Investigation of synthetic Mg1.3V1.7O4 spinel with MgO inclusions: Case study of a spinel with an apparently occupied interstitial site

A magnesium vanadate spinel crystal, ideally MgV2O4, synthesized at 1 bar, 1200 °C and equilibrated under FMQ + 1.3 log fO2 condition, was investigated using single-crystal X-ray diffraction, electron microprobe, and electron backscatter diffraction (EBSD). The initial X-ray structure refinements gave tetrahedral and octahedral site occupancies of T(Mg0.966◻0.034) and M(V3+0.711V4+0.109Mg0.180), respectively, along with the presence of 0.053 apfu Mg at an interstitial octahedral site (16c). Back-scattered electron (BSE) images and electron microprobe analyses revealed the existence of an Mg-rich phase in the spinel matrix, which was too small (≤3 μm) for an accurate chemical determination. The EBSD analysis combined with X-ray energy dispersive spectroscopy (XEDS) suggested that the Mg-rich inclusions are periclase oriented coherently with the spinel matrix. The final structure refinements were optimized by subtracting the X-ray intensity contributions (~9%) of periclase reflections, which eliminated the interstitial Mg, yielding a structural formula for spinel TMgM(V3+1.368V4+0.316Mg0.316)O4. This study provides insight into possible origins of refined interstitial cations reported in the literature for spinel, and points to the difficulty of using only X-ray diffraction data to distinguish a spinel with interstitial cations from one with coherently oriented MgO inclusions.

Hydrogen-induced and A-site substitution-dependent structural properties of AB 2-type (Ho 1− xA x)Co 2–hydrogen system (A = Mm and Ti)

The structural changes of the Laves phases pseudo-binary (Ho 1− x A x )Co 2 alloys (A = Mm and Ti) upon hydrogen absorption have been investigated by means of powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The hydrogenation of alloys at different hydrogenation pressure and temperature has been performed using Sieverts-type apparatus. The effect of dehydrogenation and formation of hydride phases on the structural properties have been studied and results are interpreted with respect to their unhydrogentaed alloys. The pressure and temperature-dependent hydrogen-induced transformation from crystalline to amorphous and decomposition into constituents have been studied and discussed. The preferential occupancy of tetrahedral interstitial sites for hydrogen and relative stability of crystalline, amorphous and elemental hydrides have been reported. Further, the effect of Mm and Ti substitution upon amorphization conditions has been discussed.

Structure and Lithium Extraction Mechanism in LiNi0.5Mn1.5O4 after Double Substitution with Iron and Titanium

The simultaneous partial replacement of Ni and Mn by Fe and Ti in the 5-V LiNi 0 . 5 Mn 1 . 5 O 4 spinel was studied. The spinel structure is preserved after double substitution of iron and titanium, while partial tetrahedral site occupancy by iron ions was observed. The best capacity retention in lithium cells was achieved by double substitution for the LiFe 0 . 1 0 Ti 0 . 1 0 Ni 0 . 4 5 Mn 1 . 3 5 O 4 composition, associated with the particular single-phase mechanism of lithium extraction combined with the structural stabilization induced by titanium and tetrahedral iron. In addition, the capacity is higher for smaller particle sizes, while a slightly higher capacity retention is found for larger, regularly shaped particles.

Structure and electrochemical performance of Li 2MnSiO 4 and Li 2FeSiO 4 as potential Li-battery cathode materials

Recently, preparation and preliminary testing of Li 2FeSiO 4, a representative of a new class of Li storage materials, has been reported [A. Nyten, A. Abouimrane, M. Armand, T. Gustaffson, J.O. Thomas, Electrochem. Commun. 7 (2005) 156]. In the present paper, we report preparation of another material from this class: Li 2MnSiO 4. To the best of our knowledge, the existence of this compound has not been reported so far. Similarly as in the case of the LiMPO 4 materials family (M = Fe, Mn), the Mn analogue shows considerably poorer electrochemical performance. Quite unexpectedly, however, the Mn analogue shows a better stability, especially under harsh conditions. This property appears to be crucial for determination of detailed structural features of this class of materials. The obtained structure reveals partial occupation of alternate tetrahedral sites by Li and Mn, thus implying a high ionic conductivity of these materials. The poor electrochemical performance is most likely due to poor electron wiring.

Synthesis and Characterization of Spinel‐Type Gallia‐Alumina Solid Solutions

AbstractBy precipitation with ammonia of ethanolic solutions containing the appropriate proportions of gallium and aluminium nitrate, following by calcination of the resulting gels at 773 K, mixed Ga2O3/Al2O3 oxides having Ga:Al ratios of 9:1, 4:1, 1:1, 1:4 and 1:9 were obtained. Powder X‐ray diffraction showed that these mixed metal oxides form a series of solid solutions having the spinel‐type structure; also shown by γ‐Al2O3 and γ‐Ga2O3. The specific surface area (determined by nitrogen adsorption at 77 K) was found to range from 160 m2 g−1 for the mixed oxide having Ga:Al = 9:1 up to 370 m2 g−1 for that having Ga:Al = 1:9. High resolution MAS NMR showed that Ga3+ and Al3+ ions occur at both tetrahedral and octahedral sites in the spinel‐type structure of the mixed metal oxides, although there is a preferential occupation of tetrahedral sites by Ga3+ ions. A proportion of penta‐coordinated Al3+ ions was also found. IR spectra of carbon monoxide adsorbed at 77 K showed that the mixed metal oxides have a considerable Lewis acidity, related mainly to tetrahedrally coordinated metal ions exposed at crystal surfaces. The characteristic infrared absorption band of coordinated (adsorbed) CO appears in the range 2205–2190 cm−1, and its peak wavenumber is nearly independent of Ga:Al ratio in the mixed gallia‐alumina oxides.

Insights from calculated phonon dispersion curves for an overlayer of H on Pt(1 1 1)

We have calculated the dispersion curves of H vibrational modes on Pt(1 1 1), using first-principles, total energy calculations based on a mixed-basis set and norm-conserving pseudopotentials. Linear response theory and the harmonic approximation are invoked. For 1 ML coverage with H atoms in the fcc hollow sites, we find modes at 73.5 meV and 142.6 meV, polarized, respectively, parallel and perpendicular to the surface, at the Γ point of the surface Brillouin zone. The degeneracy of the parallel mode is lifted at the zone boundaries, yielding modes at 69.6 meV and 86.3 meV, at the M point, and at 79.4 meV and 80.8 meV, at the K point. The changes in Pt surface force constant on H adsorption are found to shift the frequencies of the substrate surface modes from their calculated values for clean Pt(1 1 1). We also consider the case of subsurface adsorption for 2 ML of H and present vibrational frequencies of H atoms adsorbed in several subsurface sites. The appearance of new vertically polarized H modes in the range 98–106 meV (octahedral site occupancy) and 124–162 meV (tetrahedral site occupancy) is discussed in the context of experimental data.

Preparation of LiNi 0.8Co 0.2O 2-based cathode materials for lithium batteries by a co-precipitation method

LiNi 0.8Co 0.2− x Al x O 2 (0 ≤ x ≤ 0.1) cathode powders were prepared with a novel co-precipitation method followed by heat-treating at temperatures between 500 and 900 °C. The electrochemical properties as cathode material for lithium batteries were predicted with XRD patterns and examined with capacity retention studies. Among the prepared samples, 800 °C heat-treated un-substituted LiNi 0.8Co 0.2O 2 sample showed the best cycling performance. However, the initial specific discharge capacity decreases with increase amount of Al-substitution for the LiNi 0.8Co 0.2− x Al x O 2 powders prepared with the same process. It may due to the increase in the amount of electrochemical inactive Al 3+ and the increase in the hindrance of Li + ions diffusion by increasing Al 3+ occupancy of interstitial tetrahedral 6c sites in the Al-substituted cathode materials.

Combined Neutron Diffraction, NMR, and Electrochemical Investigation of the Layered-to-Spinel Transformation in LiMnO2

The conversion that occurs from layered LiMnO2 to spinel on electrochemical cycling has been studied by neutron diffraction and NMR. Neutron diffraction results indicate that the tetrahedral sites in the Li layers that share faces with octahedral sites in the transition metal layers are occupied even following the first charge to 4.6 V. NMR results are consistent with the conversion from the monoclinic, Jahn−Teller distorted, to the rhombohedral, layered phase on charging. On subsequent discharging, clear evidence for the monoclinic phase is seen by NMR indicating the presence of Jahn−Teller distorted domains in the material at 3.5 V and below. The fraction of monoclinic phase decreases gradually as a function of cycle number and disappears by 35 charge−discharge cycles. Diffraction patterns obtained as a function of cycle number were refined with a structural model that included both a layered phase (with octahedral and tetrahedral site occupancy) and a spinel phase, with the fraction of spinel increasing from 0.12 (5 cycles) to 0.93 following 92 cycles. Both diffraction and NMR results indicate that the spinel phase that nucleates from the layered material is stoichiometric and does not contain Li occupancy on the Mn sites. An additional site is seen by NMR which reaches a maximum in cycles 25−50, which is assigned to tetrahedrally coordinated Li in a local environment intermediate between that of the spinel and the layered phase. The observation of this site by NMR is associated with two characteristic peaks in the incremental capacity plot at 3.75 and 3.9 V on charge and discharge. The data indicate that the mechanism for Li insertion and removal into these local environments is complex and involves simultaneous structural rearrangements. The intermediate environment decreases in concentration on subsequent cycling as the concentration of the spinel phase continues to grow.

Incorporation of sodium into the chlorite structure: the crystal structure of glagolevite, Na(Mg,Al)6[Si3AlO10](OH,O)8

The crystal structure of glagolevite, Na(Mg,Al) 6 [Si 3 AlO 10 ](OH,O) 8 , a new mineral from the Kovdor alkaline massif, Kola peninsula, Russia, has been solved and refined to R 1 = 0.117, calculated from 1503 unique observed reflections (F o ≥ 4σF o ), and to R 1 = 0.118 for all 1550 unique reflections. The specimen contained several polytypes; the crystal studied yielded triclinic space group C1, a = 5.3580(11), b = 9.2810(19), c = 14.574(3) Å, α = 90.00(3), β = 97.08(3), γ = 90.00(3)°, V = 719.2(3) Å 3 . Glagolevite is the first mineral of the chlorite group that contains Na atoms located between the 2:1 layers and the interlayer octahedral sheets. The Na atoms are in sevenfold coordination. The Na(O,OH) 7 polyhedron is a trigonal prism with one of the triangular bases extended to a square. The tetrahedral sheets show disorder owing to polytype intergrowths. Tetrahedral site occupancies correlate with the occupancies of the Na sites. The polytypic composition of the crystal studied is estimated at 57% IIb-6, 31% IIb-2, and 12% IIb-4.

A theoretical study of H surface and subsurface species on Pt(1 1 1)

Adsorption of H at a coverage ranging up to 2 monolayers with occupation of fcc and hcp sites and insertion in subsurface sites of Pt(111) is studied with use of a DFT-based ab initio method. It is shown that no important repulsion between the adatoms grows up to 1 ML, justifying the usual wisdom that 1 H atom adsorbs per Pt surface atom. Beyond this coverage, the surface occupation of both the fcc and the hcp sites become disfavoured and H starts to occupy subsurface sites. Both occupation of octahedral and tetrahedral insertion sites were studied. It is concluded that configurations involving fcc surface sites associated with tetrahedral subsurface sites are favoured. The evolution of the Pt local density of states (LDOS) with the coverage is reported. It is shown that the presence of H in the subsurface decrease the surface Pt chemical reactivity. Finally, stability curves of the Pt–H system as a function of H2 pressure and temperature are given, showing that subsurface H is likely to be present under usual catalytic reaction conditions.

A theoretical study of H surface and subsurface species on Pt(111)

Adsorption of H at a coverage ranging up to 2 monolayers with occupation of fcc and hcp sites and insertion in subsurface sites of Pt(1 1 1) is studied with use of a DFT-based ab initio method. It is shown that no important repulsion between the adatoms grows up to 1 ML, justifying the usual wisdom that 1 H atom adsorbs per Pt surface atom. Beyond this coverage, the surface occupation of both the fcc and the hcp sites become disfavoured and H starts to occupy subsurface sites. Both occupation of octahedral and tetrahedral insertion sites were studied. It is concluded that configurations involving fcc surface sites associated with tetrahedral subsurface sites are favoured. The evolution of the Pt local density of states (LDOS) with the coverage is reported. It is shown that the presence of H in the subsurface decrease the surface Pt chemical reactivity. Finally, stability curves of the Pt–H system as a function of H2 pressure and temperature are given, showing that subsurface H is likely to be present under usual catalytic reaction conditions.

Preparation of zinc ferrite by high-energy ball-milling and microstructure characterization by Rietveld’s analysis

Nanocrystalline zinc ferrite is synthesized at room temperature by high-energy ball-milling the stiochiometric (1:1mol%) mixture of ZnO–α-Fe2O3. Nucleation of nanocrystalline Zn-ferrite particle takes place by solid-state diffusion between nanocrystalline ZnO and α-Fe2O3 particles. Both the particle size and collision temperature favors the formation mechanism. The process of mechanosynthesis of zinc ferrite at room temperature led to the formation of metastable inverse spinel structure. The formation of normal and inverse spinel structures is noticed after 30min and 2.5h ball-milling, respectively, and the degree of inversion increased with increasing milling time. The structural and microstructural evolution of ZnFe2O4 caused by milling is investigated by X-ray powder diffraction. The relative phase abundances of different phases, particle size, rms strain, lattice parameter change, etc. have been estimated from Rietveld powder structure refinement analysis of XRD data. A significant amount (wt.%) of nanocrystalline ZnFe2O4 is formed within 2.5h ball-milling with a very compressed lattice parameter. However, lattice parameter of 10h-milled sample annealed at 1273K for 1h agreed well the literature value and surprisingly, inverse spinel structure is completely absent in the annealed material. This leads to the conclusion that inverse spinel structure is a metastable one, formed under high-energy impact with a very small particle size (∼6nm) and high temperature annealing even for a shorter time is sufficient to reduce the disorderness in occupancy of tetrahedral and octahedral sites by Zn2+ and Fe3+ cations.

Characterization of a natural magnetite

Single crystals of a rock magnetite were separated from steatite cobbles collected in a geological site near the city of Serro (18° 36′ 47′′ S 43° 22′ 46′′ W), Minas Gerais, Brazil. A typically well-shaped magnetite single crystal was characterized by chemical analysis, 57Fe Mossbauer spectrometry at 300, 77 and 4 K and under an applied magnetic field of 6 T at 10 K, magnetization measurements and electronic microprobe. From Mossbauer data, the sample is stoichiometric with a tetrahedral and octahedral site occupancy ratio of 1:2. Elemental chemical analysis and point-to-point electron microscope probing show some inclusions of lamellar ilmenite (≤ 1 mass%) randomly distributed throughout the magnetite matrix, and also that the magnetite matrix is constituted only by Fe2+ and Fe3+, with no isomorphic substitution. Results are discussed on the basis of the magnetization curve and of the temperature dependence of the AC magnetic susceptibility. The Verwey transition occurs in the temperature range of 100–115 K, observed by a sudden change in the temperature dependence of the magnetization.

Low-temperature Snoek-type relaxation of hydrogen interstitial atoms in Nb0.8Mo0.2

Neutron spectroscopy on H-doped b.c.c. Nb0.8Mo0.2 alloys demonstrated recently (i) a complete suppression of low-temperature H precipitation up to 5 at.% H, and (ii) an exclusive H occupation of tetrahedral interstitial sites, like pure b.c.c. Nb. We studied by mechanical spectroscopy Nb0.8Mo0.2 alloys containing 0.85 and 3 at.% H and found a H-induced low-temperature relaxation at 2 kHz around 80 K. Activation energy and reciprocal pre-exponential relaxation time are ∼0.054 eV and ∼2.8·107 s−1, respectively. The relaxation peak is about 80% broader than for an ideal Debye relaxation, which reveals a spectrum of relaxation times. The height of the relaxation (Q−1) peaks is approximately linear to H concentration, thus indicating a Snoek-type relaxation of single H atoms. The heights of the peaks yield a tetragonality |λ1–λ2| of about 0.04, which is an order of magnitude smaller than that of the Snoek effect of O in Nb. The results suggest that the observed relaxation reflects low-temperature diffusive jumps of single H atoms between tetrahedral interstitial sites, with a jump rate above ∼1.2·104 s−1 at 80 K.

Ab initiostudies of the passive film formed on iron

Passive metals are protected from the environment by a thin (3\char21{}5 nm) oxide film that forms on their surface. The corrosion rate of these materials is typically of the order of $0.1\ensuremath{\mu}\mathrm{m}/\mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r},$ and depends on the electronic structure of the oxide layer. Here we present ab initio total-energy calculations of the passive oxide film that forms on iron at anodic potentials in weakly alkaline solutions. Surface x-ray diffraction studies have revealed that this nanocrystalline passive film has a spinel structure with a fully occupied oxygen lattice [M. F. Toney, A. J. Davenport, L. J. Oblonsky, M. P. Ryan, and C. M. Vitus, Phys. Rev. Lett. $79,$ 4282 (1997)]. However, the octahedral and tetrahedral iron site occupancies are found to be reduced (approximately 80% and 66%, respectively) and partial occupancy of octahedral interstitial sites is also observed (approximately 12%). We have used total-energy pseudopotential calculations to study the energetics and electronic structure of these defects and the interrelationships between site occupancies. The calculations suggest that film is metastable and may be semi-conducting. The calculations also suggest a correlation between octahedral interstitials and tetrahedral vacancies. Finally, an estimation of energy barriers in the film suggests that cation migration through the tetrahedral sublattice dominates film growth.

Refinement of the Crystal Structure of Cronstedtite-2H2

AbstractThe crystal structure of cronstedtite-2H2 was refined in a hexagonal cell, space group P63, Z = 2, using two acicular crystals from Wheal Maudlin, Cornwall, England, and from Pribram, Czech Republic. The Wheal Maudlin sample has the chemical composition (Fe2.2912+Fe0.7093+)(Si1.298Fe0.7073+Al0.004)O5(OH)4 and the Příbram sample has the composition (Fe2.2692+Fe0.7313+)(Si1.271Fe0.7243+Al0.005)O5(OH)4. The results of refinements are as follows: a = 5.500(1), c = 14.163(2) Å, V = 371.08(8) Å3, R = 3.83%, from 381 independent reflections, and a = 5.4927(1), c = 14.1481(2) Å, V = 369.70(4) Å3, R = 4.77%, from 1088 independent reflections for the Wheal Maudlin and Příbram samples, respectively. The best Fovs.Fc agreement was achieved when the structure was interpreted as merohedral twin; several possible twinning laws are discussed. The cronstedtite layer consists of one tetrahedral sheet and one octahedral sheet. There is one octahedral (M1) position, occupied by Fe only, and two tetrahedral (T1, T2) positions in the structure. Refinement of occupancy of tetrahedral sites led to values Si:Fe = 0.45:0.55(1) (Wheal Maudlin) and 0.432:0.568(8) (Příbram) in T1, and Si: Fe = 0.99:0.01(1) (Wheal Maudlin) and 0.888:0.112(7) (Příbram) in 72. Whereas the size of T1 is reasonable (average dT1-O = 1.693 Å (Wheal Maudlin), 1.691 Å (Příbram)), T2 is unusually large: (dT2-O= 1.740 Å (Wheal Maudlin), 1.737 Å (Příbram)) with respect to the small or almost zero Fe content. As an explanation, an alternative structure model comprising a certain amount of vacancies in T2 is presented. The tetrahedral rotation angle α is highly positive (+12.1° and +12.5° for the Wheal Maudlin and Příbram samples, respectively), and the layer belongs to the Franzini type A. Distortion parameters of octahedra and tetrahedra are given for both samples. One hydrogen atom engaged in the hydrogen bond was located in the Wheal Maudlin sample.

Site Occupancies by Iron in Nontronites

AbstractTwelve nontronites and two ferruginous smectites have been characterized with respect to Fe3+ occupancy of tetrahedral sites. The techniques used were near infrared, Fe-K X-ray absorption near-edge and X-ray absorption fine-structure spectroscopies, along with two X-ray diffraction techniques. The results show that calculations of the structural formulae of many nontronites should be adjusted to include Fe3+ in tetrahedral sites. The nontronite from Spokane County, Washington, (∼44% Fe2O3) is essentially an end-member with its non-siliceous tetrahedral sites occupied by Fe3+. Samples with chemical compositions similar to Garfield nontronite (∼36.5% Fe2O3) may have small amounts (<5% of total Fe3+) of tetrahedral Fe3+. Tetrahedral Fe3+ is unlikely to be present in samples containing less than ∼34% Fe2O3.

Infrared Spectroscopic Study of LiCoO2 Thin Films

Site disorder of Co3+ ions in sputtered films of lithium cobaltite has been examined using infrared spectroscopy. Both transmission and reflectance modes have been used to characterize the nature of IR absorption. It is found that Co3+ in the sputtered films occupy two types of octahedral sites that differ in the nature of second-neighbor environment. Li+ ions exhibit two bands, which may arise from tetrahedral and octahedral site occupancies or from the presence of disordered regions in the films.

Simulation models of defects in MgAl2O4:Fe2+, Fe3+ spinels

A static computer simulation using the GULP code has been used to study dipole defects in iron doped spinels in normal and inverse structures. The occupation of tetrahedral (Mg2+) and octahedral (Al3+) sites in the normal structure was simulated and the results indicate a preferential replacement of Mg2- by an Al3+ justified by the observed stoichiometric deviation in synthetic spinels. Besides this, Al2O3 excess produces Mg2+ vacancies and favours the hole centre production. Antisite disorder of the form [Mg2+]Al 3+ and [Al3+]Mg 2+ is one of the possible mechanisms responsible for the existence of dipole defects in these materials. The obtained energy for the interchange between Al3+ and Mg2+ was 0.52eV. The calculated energies indicate that Fe2+ replaces Mg2+ and Fe3+ replaces Al3+. The presence of Fe3+ in octahedral sites takes into account the oxidation process of Fe2+ in these sites. Computer simulation has also been carried out on the inverse structure where the stability of the lattice was obtained for the Imma space group.