Spinel Polymorph Research Articles

Pressure-Induced YbFe2O4-Type to Spinel Structural Change of InGaMgO4

Spinel-type InGaMgO4 with a = 8.56615(3) Å was prepared by treating layered YbFe2O4-type InGaMgO4 at 6 GPa and 1473 K. DFT calculation and Rietveld analysis of synchrotron X-ray powder diffraction data revealed the inverse spinel structure with In3+:Ga3+/Mg2+ = 0.726:0.274 in the tetrahedral site and 0.137:0.863 in the octahedral site. InGaMgO4 spinel is an insulator with an experimental band gap of 2.80 eV, and the attempt at hole doping by post-annealing in a reducing atmosphere to introduce an oxygen defect was unsuccessful. This is the first report of the bulk synthesis of AB2O4 compounds with both YbFe2O4 and spinel polymorphs.

High-Pressure-Stabilized Post-Spinel Phase of CdFe2O4 with Distinct Magnetism from Its Ambient-Pressure Spinel Phase.

α-CdFe2O4 stabilizes its normal spinel structure due to the covalent Cd-O bond, in which all the connections between adjacent FeO6 octahedral are edge-shared, forming a typical geometrically frustrated Fe3+ magnetic lattice. As the high-pressure methods were utilized, the post-spinel phase β-CdFe2O4 with a CaFe2O4-type structure was synthesized at 8 GPa and 1373 K. The new polymorph has an orthorhombic structure with the space group Pnma and an 11.5% higher density than that of its normal spinel polymorph (α-CdFe2O4) synthesized at ambient conditions. The edge-shared FeO6 octahedra form zigzag S = 5/2 spin ladders along the b-axis dominating its low-dimensional magnetic properties at high temperatures and a long-range antiferromagnetic ordering with a high Néel temperature of TN1 = 350 K. Further, the rearrangement of magnetic ordering was found to occur around TN2 = 265 K, below which the competition of two phases or several couplings induce complex antiferromagnetic behaviors.

The effect of particle size and morphology on the rate capability of 4.7 V LiMn 1.5+ δNi 0.5− δO 4 spinel lithium-ion battery cathodes

The electrochemical impact of lithium-ion diffusivity on the discharge rate capabilities of cation ordered ( P4 332) and disordered ( Fd3 m) LiMn 1.5+ δ Ni 0.5− δ O 4 spinels were studied. Potentiostatic Intermittent Titration (PITT) measurements revealed up to 2 orders-of-magnitude lower lithium diffusion coefficient for the ordered spinel polymorph. The optimum structure of the high voltage spinel is resolved with respect to these ionic studies and our previous electronic transport studies. Modification of the morphology and pore dimensions was accomplished through the tuning of the ethylene glycol used for the synthesis of nanostructured spinel in the modified Pechini process. Glycol will be shown to play a major role on the energy and power density of LiMn 1.5+ δ Ni 0.5− δ O 4 electrodes.

Elasticity of Olivine (α), Beta(β), and Spinel (γ) Polymorphs of Germanates and Silicates

Summary Ultrasonic data for the velocities of germanate and silicate compounds in the olivine (a), beta (8) and spinel (y) crystal structures have been determined as a function of pressure to 7-5 kbar at room temperature for polycrystalline specimens hot-pressed at pressures up to 65 kbar. The a-y phase transformations are characterized by the following velocity (u)-density (p) relationships: (a) the velocity jumps are twice the percentage magnitude of the density jumps; (b) the ratio (vp/v,) of the compressional to shear velocity is either approximately constant or increases slightly across the transitions; and (c) the slopes (linear or logarithmic) on v-p diagrams for the a-y transition are comparable to those produced by isothermal compression or isobaric expansion of the low-pressure olivine phase. The behaviour of up/u, is a feature common to many phase transformations. However, the observed relationships (a) and (c), while similar to those for the quartz-coesite transition in Si02, are in marked contrast with those for the coesite-rutile, pyroxene-garnet and pyroxeneilmenite transformations. These latter transitions involve increases in cation-anion co-ordination and nearest-neighbour distances whereas the olivine-spinel and olivine-beta phase transformations do not. Such crystallographic details may be diagnostic for interpretations of the composition and mineralogy of the Earth’s transition zone. Systematic trends in the elastic properties for isostructural sequences support the concept of germanates as models for the elasticity of their silicate analogues; this scheme is applied to estimate the bulk modulus of the spinel polymorph of Mg, Si04 (K, = 2-06 0.05 Mbar). Comparison of the new elasticity data with recent Earth models in the vicinity of the 400-km discontinuity reveals that only model 1066B of Gilbert & Dziewonski is compatible with the experimental data for the u-y and a-8 transformations.

Synthesis and structure of olivine-like arsenates [formula omitted] ( [formula omitted], Fe, Co and Ni) and their high-pressure spinel-like polymorphs

High pressure has been applied to olivine-like arsenates ( Li M AsO 4 ; M=Mn, Fe, Co and Ni), some of them being reported for the first time, to transform them to the corresponding spinel-like polymorph. Below the limit of our experimental set-up (8 GPa, 1273 K) the well known olivine-spinel transformation occurred for all investigated cases but for the manganese one. This transformation is observed for the first time in arsenates and the title compounds are the first examples of arsenic-containing spinel-like oxides. The structure of the olivine and spinel polymorphs are reported.

Structure and dynamics of protonated Mg2SiO4; an ab-initio molecular dynamics study

We studied structural and dynamical properties of H 1 absorbed in Mg2SiO4 by ab-initio molecular dynamics. We first calculated the T 5 0 equation of state of pure forsterite as a function of pressure, and we determined the relative stabilities of the olivine, b-spinel, and spinel polymorphs. The results show that the ab-initio model successfully reproduces the known structural properties of the system. In the protonated phases, in agreement with experimental evidence, our computations show that H 1 is absorbed preferentially in the b-spinel phase. The most stable absorption site is located close to the O1 atom, which is coordinated by five Mg 21 cations and not directly bound to Si. In addition to this stable absorption site, the computation reveals other low-energy positions, forming an extended network of hydrogen bonds, that could play an important role in the diffusion of H 1 in b-spinel. We analyze the dependence of structure and dynamics of the pure and protonated phases as a function of temperature and pressure.

Phase Transitions Between β and γ (Mg, Fe) 2 SiO 4 in the Earth's Mantle: Mechanisms and Rheological Implications

The mechanisms of the phase transformations between the spinel (gamma) and modified spinel (beta) polymorphs of Mg(2)SiO(4) have been studied experimentally between 15 and 20 gigapascals and 800 degrees to 950 degrees C. The gamma to beta transformation occurs by a shear mechanism, whereas the beta to gamma transformation involves grain-boundary nucleation and interface-controlled growth. These contrasting mechanisms are a consequence of the number of independent slip systems that are available in the respective crystal structures. This result leads to the prediction that in subduction zones and perhaps also rising plumes in the Earth's mantle, the gamma to beta transformation should be accompanied by a transient reduction in strength.

Infrared reflectivity of γ-(spinel)Ni2SiO4

The infrared reflectance spectrum of a polycrystalline sample of the high pressure Ni 2 SiO 4 -spinel polymorph has been recorded from 100 to 1200 cm −1 . The four infrared active modes expected from symmetry analysis (F 1u ) have been observed and their approximate TO-LO frequencies are 231-234 (±2), 350-420 (±5), 500-540 (±5) and 810-955 (± 10) cm −1 . These data are compared with previous infrared studies of normal spinels with tetravalent cations (silicates and germanates). A simple harmonic model is used to infer the position of the lowest frequency infrared mode of Mg 2 SiO 4 -spinel at ≃ 300 cm −1

X-Ray Powder Structural Analysis of the Spinel Polymorph of Fe2SiO4

AbstractThe crystal structure of the spinel polymorph of Fe2SiO4, synthesized at high temperature (900°C) and high pressure (70 kbar), was studied by the Rietveld analysis of X-ray powder diffraction data collected with a Guinier-Hägg camera. The compound is cubic, space group , with cell dimension: a= 8.2413(6) Å, V= 559.8(1) Å3, Z= 8, Cell Wt. = 1630.2, Dx= 4.835 g·cm−3, Do= 4.75 g·cm−3. The figure of merit is F10= 92(0.011, 10). The final R value is RF= 0.058. The crystal has a mixed normal-inverse spinel structure. The site occupancy refinement showed that 37.9% of the silicon was found in the octahedral site (M site), while 18.9% of the iron occupied the tetrahedral site (T site). Due to the larger displacement of Si4+ion by Fe2+ion, the positional parameter of oxygen atom (0.3689) is smaller than that of X-ray single crystal structure (0.3658), and the average Si-O bond (1.697(1)Å) is longer and Fe-O bond (2.112(1)Å) is shorter than those of X-ray single crystal structure.

Observations of the influence of ion-bombardment on the microstructure of synthetic and natural ferrite crystals

It has been widely reported that spinels, prepared for TEM study by ion-beam thinning, exhibit a fine "mottled" texture on a scale of 10nm [1-6]. The observations of Frost and Gai [7] on spinels also appear to show a similar background contrast. The microstructure which produces the mottled contrast is almost certainly developed during the thinning process, because spinels exhibit no mottled contrast when they are prepared for TEM observation by crushing, yet show it strongly when prepared by ionbombardment. This mottled texture, however, is not characteristic of all ion-bombarded spinels; in particular, no strong mottled contrast appears to occur in ulvospinel, Fe2TiO4 [4-6], or in the spinel polymorph of Mg2 SiO4 [6, 8]. It should be noted that the nonoccurrence of mottling in ulvospinel remains debatable [9]. This communication concerns some new and novel observations of ion-bombardment induced microstructures in a synthetic ferrite and the natural spinelstructured ferrite minerals, jacobsite and franklinite. The synthetic material came from a manganese-zinc ferrite ([Mn-Zn]Fe204) single-crystal boule supplied by the Fuji Electrochemical Company. The same material has been the subject of an electron microscopy study of its oxidation behaviour [10]. The composition of the boule was determined by wet chemical analysis to be 55.10mo1% Fe203 (assuming all of the iron to be in the form Fd+) , 27.94mo1% MnO and 16.96mo1% ZnO. The lattice parameter of the asgrown crystal was found by means of X-ray diffraction to be 0.8487 nm [11]. Synthetic Mn-Zn ferrite samples, cut from the boule, were heat-treated at different oxygen partial pressures (Po2) to achieve different oxygen stoichiometries. The prevalent nonstoichiometry in Mn-Zn ferrite occurs through the incorporation of excess oxygen into the structure, which is reflected in the oxidation of Fe 2+ and Mn 2+ and the creation of associated cation vacancies [12, 13]. The Fe 2+ concentration was measured using a coulometric titration technique [14]. Table I gives the different types of heat-treated Mn Zn ferrite materials studied, and their estimated cation vacancy concentration, determined by means of a weight and charge balance calculation [11]. Material B was annealed at 1100 ° C in an evacuated sealed fused-silica tube for 30 days, to equilibrate the cation vacancy concentration. Materials C and D were cooled under conditions of varying Po2, in order to keep the samples on isocompositional lines (using T A B L E I The cation vacancy content of the synthetic Mn-Zn ferrite materials, heat-treated as shown

Mechanism of the olivine-spinel transformation in Co2SiO4

An experiment conducted in a 2000-ton uniaxial split-sphere apparatus (USSA-2000) utilizes large sample volume and a substantial temperature gradient to synthesize intergrowths of the olivine and spinel polymorphs of Co2SiO4. The olivine starting material consists of a finegrained fraction (<20μm) which records the stable polymorphs along the length of the sample plus large olivine grains (100–500 μm) which help decipher the mechanism of the phase transformation. At conditions near equilibrium, the olivine-spinel transformation in the large grains occurs by inward growth of a few large single crystals of spinel nucleated on the surfaces of the olivine. The overall rate of transformation is governed by the mobility of the interphase boundary, whose morphology is crystallographically controlled by the spinel. No renucleation of spinel is observed in the host olivine crystal, even in the region immediately adjacent to the olivine/spinel interface; analysis of this region with transmission electron microscopy reveals an extremely high density of dislocations induced by plastic flow accommodating the volume change associated with the phase transformation.

The lattice dynamics and thermodynamics of the Mg2SiO4 polymorphs

We use an approach based upon the Born model of solids, in which potential functions represent the interactions between atoms in a structure, to calculate the phonon dispersion of forsterite and the lattice dynamical behaviour of the beta-phase and spinel polymorphs of Mg2SiO4. The potential used (THB1) was derived largely empirically using data from simple binary oxides, and has previously been successfully used to model the infrared and Raman behaviour of forsterite. It includes ‘bond bending’ terms, that model the directionality of the Si-O bond, in addition to the pair-wise additive Coulombic and short range terms. The phonon dispersion relationships of the Mg2SiO4 polymorphs predicted by THB1 were used to calculate the heat capacities, entropies, thermal expansion coefficients and Gruneisen parameters of these phases. The predicted heat capacities and entropies are in outstandingly good agreement with those determined experimentally. The predicted thermodynamic data of these phases were used to construct a phase diagram for this system, which has Clausius-Clapeyron slopes in very close agreement with those found by experiment, but which has predicted transformation pressures that show less close agreement with those inferred from experiment. The overall success, however, that we have in predicting the lattice dynamical and thermodynamic properties of the Mg2SiO4 polymorphs shows that our potential THB1 represents a significant step towards finding the elusive quantitative link between the microscopic or atomistic behaviour of minerals and their macroscopic properties.

The α, β, γ phase relations in Fe2SiO4‐Mg2SiO4 and Co2SiO4‐Mg2SiO4: Calculation from thermochemical data and geophysical applications

On the basis of thermochemical data for olivine (α), modified spinel (β), and spinel (γ) polymorphs of Mg2SiO4, Fe2SiO4, and Co2SiO4, the binary Mg2SiO4‐Fe2SiO4 and Mg2SiO4‐Co2SiO4 phase diagrams are calculated. From Mg/Fe partitioning data between olivine‐garnet, β‐phase‐garnet, olivine‐pyroxene, and β‐phase‐pyroxene, thermochemical parameters for β‐Fe2SiO4 are estimated. The β‐Fe2SiO4 is highly metastable with respect to γ‐Fe2SiO4. The calculated binary Mg2SiO4‐Fe2SiO4 phase diagram shows a narrow field of α+β and a broad field of β+γ, in reasonable agreement with experimental phase relations. The composition range over which β phase solid solution is stable expands with increasing temperature but is generally limited to Mg/(Fe + Mg) &gt; 0.8. In the Mg2SiO4‐Co2SiO4 system, the β phase is calculated to occur only at Mg/(Co + Mg) &lt; 0.05 and Mg/(Co + Mg) &gt; 0.6, with α and γ coexisting at intermediate compositions. Using the above phase relations in Mg2SiO4‐Fe2SiO4 and assuming an adiabatic geotherm, temperatures are calculated to range from about 1573 to 1700 K at depths of 400–520 km for a mantle of Iherzolite composition.

Elasticity of the olivine and spinel polymorphs of Ni2SiO4

The single-crystal elastic moduli, cij x, of the olivine (α) and spinel (γ) polymorphs of nickel orthosilicate have been measured at atmospheric pressure and 20° C by Brillouin spectroscopy. The results are (Mbar), Ni2SiO4 olivine: c11=3.40(2), c22=2.38(2), c33=2.53(2), c44=0.71(1), c55=0.87(1), c66=0.78(1), c12=1.09(2), c13=1.10(4), c23=1.13(3), Ni2SiO4 spinel: c11=3.66(3), c44=1.06(1), c12=1.55(3). In comparing these results with extant elasticity data for olivine- and spinel-type compounds we find distinctive elastic characteristics related to crystal structure, and systematic trends due only to compositional variation. For silicate olivines, the longitudinal moduli decrease in the order c11>c33>c22, regardless of composition. The moduli c55 and c66 are approximately equal, and greater than c44. The former relationship is related to differences in polyhedral linkages along the crystallographic axes, whereas the latter may result from rotational freedom of SiO4 tetrahedra in response to different directions of shear. Composition affects elasticity most directly through the relative magnitudes of \(\bar c_{12} > \; = (c_{12} + c_{13} + c_{23} )/3\) and \(\bar c_{44} = (c_{44} + c_{55} + c_{66} )/3\). When transition-metal cations are six-coordinated by oxygen \(\bar c_{12} > \bar c_{44}\), and when alkaline-earth cations are six-coordinated \(\bar c_{44} > \bar c_{12}\).

Computer simulations of the structural and physical properties of the olivine and spinel polymorphs of Mg2SiO4

The aim of the work presented is to develop a computer simulation technique which will predict the structure and physical properties of forsterite and ringwoodite, the major mantle-forming polymorphs of Mg2SiO4. The technique is based upon energy minimization, in which all structural parameters are varied until the configuration with the lowest energy is achieved. The lattice energy and physical properties (e.g. elasticity and dielectric constants) are calculated from interatomic potentials, which generally include electrostatic and short-range terms. We investigate several types of traditional potential models, and present a new type of model which includes partial ionic charges and a Morse potential to describe the effect of covalency on the Si-O bond. This new form of potential model is highly successful, and not only reproduces the zero-pressure structural, elastic and dielectric properties of forsterite and ringwoodite, but also accurately describes their pressure dependence.

Elastic properties of the olivine and spinel polymorphs of Mg 2GeO 4, and evaluation of elastic analogues

The single-crystal elastic moduli of the olivine and spinel phases of Mg 2GeO 4 have been measured using Brillouin scattering spectra. The moduli for the olivine phase are: C 11 = 3.12, C 22 = 1.87, C 33 = 2.17, C 66 = 0.71, C 23 = 0.66, C 31 = 0.65 and C 12 = 0.60. The moduli for the spinel phase are: C 11 = 3.00, C 44 = 1.26 and C 12 = 1.18. These data are analyzed to define the best type of elastic analogue for magnesium orthosilicates. The character of the many-bodied, non-central force associated with the divalent cation is found to significantly influence the relative magnitudes of the elastic moduli. Since the nature of the many-bodied, non-central force is quite different for alkaline earth cations than for transition metal cations, we conclude that materials which contain one of these cation types is not a good analogue for materials with the other type. Magnesium orthogermanate, however, is a good analogue of magnesium orthosilicate. On the other hand, the high elastic anisotropy of the spinel phase of the germanate suggests that the germanium tetrahedron is less rigid under shear than the corresponding silicon tetrahedron. The success of the magnesium orthogermanate to model the magnesium orthosilicate is probably a result of the mechanical isolation of the tetrahedra, thus requiring the conclusions of this study to be further tested before applying them to other silicate systems.

Single‐crystal X ray study of γ Mg2SiO4

The crystal structure of the spinel polymorph of Mg2SiO4 has been refined using a single crystal synthesized at 220 kbar and 1000°C. The u parameter of the structure with a = 8.0649(1) Å is 0.3685(6). Although the refinement indicates some cation disorder in Mg2SiO4 spinel, the results are not definitive because of the similarity in the scattering factors of Mg and Si. The average Si‐O and Mg‐O distances are 1.655(4) and 2.070(4) Å, respectively. The final R and Rw values are 0.048 and 0.040 for 41 reflections up to sin θ/λ = 1.0.

A spinel to β-phase transformation mechanism in (Mg,Fe)2SiO4

Olivine of approximate composition Mg1.8Fe0.2SiO4 is a major constituent of the Earth's upper mantle. Structural changes, which occur in olivine as a function of increasing pressure and which involve the formation of the β-phase and spinel structure, γ-phase polymorphs, have therefore aroused considerable interest1–4. The mechanisms of these transformations are of particular importance because of their effect on the properties of the mantle in the zone in which they occur. Several mechanisms have been proposed4–7, but remain controversial in the absence of sufficient observational data on intergrowths of reactant and product phases. Although these phases have been produced experimentally2, direct observations of such inter-growths have generally been made only on natural material from shock-produced veins in chondritic meteorites5,6,8. Here we report observations made on the natural, high-density polymorphs of (Mg,Fe)2SiO4 found in the Peace River meteorite, which reveal that the mechanism for the spinel to β-phase transformation involves the topotactic replacement of the spinel polymorph by single, β-phase grains.

Infrared absorption spectra of the spinels Fe 2SiO 4 and Co 2SiO 4

The i.r. absorption spectra of the spinel polymorphs of Fe 2SiO 4 and CO 2SiO 4 have been measured from 200 to 4000 cm −1. Three strong absorption bands are observed at 835, 503 and 340 cm −1 and 810, 503 and 357 cm −1, respectively, and correspond to 3 of the 4 i.r. active T 1 u modes. The observed frequencies lie close to those for Ni 2SiO 4 [5] and are consistent with the normal spinel structure. The cation size effect on the i.r. frequencies and the nature of the vibrational modes are discussed for II–IV spinels.

Electron petrography of shock-produced veins in the Tenham chondrite

Electron microscopy of the minerals in, and adjacent to the black veins of the Tenham meteorite reveals that the minerals have undergone varying degrees of shock. The orthopyroxenes of the bulk of the meteorite have been transformed to clinopyroxenes in areas adjacent to the veins, and to majorite in the black vein itself. The majorite, which occurs in both equant and dendritic habit, is associated with a glassy phase from which it crystallized, and with a microcrystalline clinopyroxene aggregate produced from the majorite upon the release of pressure. The olivines of the meteorite are also deformed, and some have been transformed to the high pressure spinel polymorph. The spinel has subsequently partially inverted to the β-phase polymorph.