Octahedral 16d Sites Research Articles

Crystallographic and magnetic structure in spinel system FeGa xCr 2−xS 4 by neutron diffraction

The samples of FeGa x Cr 2− x S 4 ( x=0.1, 0.3) were prepared by solid reaction method. The crystallographic structure and the magnetic properties of the obtained compounds were investigated by X-ray and neutron powder diffraction, vibrating sample magnetometer (VSM), and Mössbauer spectroscopy. The Cr ions occupy 16(d) site exclusively, while mixed spinel behaviors are developed with increase of Ga ions. Finally, it induces a large quadrupole splitting in octahedral (16d) sites. Neutron diffraction on FeGa x Cr 2− x S 4 above 10 K shows that there is no crystallographic distortion and reveals antiferromagnetic ordering. The magnetic moment of Fe 2+ (3.45 μ B) are found to be aligned antiparallel to Cr 3+ (−2.89 μ B), in the sample x=0.1. Below the Néel temperature, magnetic peaks exist on the crystal diffraction peaks. It proves that spin structure of the same species are aligned parallel.

Neutron powder diffraction study on Li1.8Na0.2MgCl4

Abstract The crystal structure of inverse spinel-type (space group Fd-3m, Z = 8), quaternary lithium chloride Li1.8Na0.2MgCl4 has been determined by neutron powder diffraction methods (a = 1044.16(7) pm, 114 reflections, R exp = 5.23%). The sodium ions are solely placed at the octahedral 16d site together with the Mg and a part of the Li+ ions. These results are in agreement with analogous studies on the manganese compound (Lutz et al., 1989) but being in contradiction with the results of 6Li and 7Li MAS NMR experiments (Nagel et al., 2002). The reason of this discrepancy is discussed.

Cation Distribution and Size-Strain Microstructure Analysis in Ultrafine Zn−Mn Ferrites Obtained from Acetylacetonato Complexes

The ultrafine ZnFe2O4, MnFe2O4, and cation deficit Zn−Mn ferrites were obtained by thermal decomposition of appropriate mixed complex compounds with acetylacetone (2,4-pentadione) ligands ([M(AA)x]) at 500 °C. In ZnFe2O4 cation distribution is partially inverse with 14% of Zn2+ ions at octahedral 16d sites, while MnFe2O4 is a normal spinel. Cation distribution in nonstoichiometric (Zn,Mn,Fe)3-δO4 (δ = 0.18−0.30) is found to be (ZnxMny□ε)8a[Fez□ν]16d, with a random distribution of vacancies. Nonstoichiometry in Zn−Mn ferrites is accompanied by a cation valence change, i.e., partial oxidation of Mn2+ to Mn4+. Microstructure size-strain analysis shows isotropic X-ray line broadening due to the crystallite size effect and anisotropic X-ray line broadening due to the crystallite strain effect. In binary ferrites anisotropic X-ray line broadening due to the strain effect is higher in ZnFe2O4 than in MnFe2O4, while in ternary cation-deficient Zn−Mn ferrites it decreases as the vacancy concentration δ increases.

Structure and electrochemical behavior of LiCr yMn 2− yO 4 compositions

Partial substitution of certain cations for manganese is an attractive route to improve the electrochemical property of LiMn 2O 4. In this paper, we focus on the effect of chromium substitution. LiCr y Mn 2− y O 4 (y=0.00, 0.03, 0.05, 0.08, 0.12, 0.16, 0.20) has been prepared and their structure and electrochemical characteristics have been investigated. Rietveld refinements of the XRD patterns indicated that with increasing Cr content, the occupation of Mn 3+ (or Cr 3+) in octahedral 16d sites increased and the crystal structure improved. At the same time, in the 32e ( z, z, z) sites, which are occupied by oxygen, the value of z also increases and result in decreasing the bond lengths of O–M (Mn, Cr) and Mn–Mn and increasing the bond length of O–Li. The optimal composition was LiCr y Mn 2− y O 4 ( y=0.05), whose initial discharge capacity was 121.6 mAh g −1.

The effect of pressure on the structure of NiAl2O4

NiAl2O4 is known to transform from a normal spinel, (Ni2+)[Al23+ ]O4,to an inverse spinel, (Al3+)[Ni2+Al3+]O4, and vice versa. In this process,the larger Ni2+ ions which occupy the tetrahedral (8a) sites in normalspinel move to the octahedral (16d) sites in inverse spinel while halfof the smaller Al3+ ions move in the opposite direction. The extentof this move is measured by the disorder or inversion parameter,I(the fraction of tetrahedral sites occupied by the Al ions; I = 0 for normalspinel and I = 1for inverse spinel). Previous studies suggest that the lattice constant of spinel candecrease as the disorder parameter increases to better accommodate the Ni ions.In situ neutron diffraction studies performed by us indicate that this process isalso occurring during the reduction of NiAl2O4 to Ni and Al2O3. It is possiblethat the compressive residual stresses generated during reduction play a role inthe structural evolution of NiAl2O4. To systematically investigate the effect ofpressure on the structure of NiAl2O4, x-ray diffraction studies at the X17beamline of the National Synchrotron Light Source were performed. The pressure(up to 35 GPa) was applied via a diamond anvil cell and the experiments wereconducted using a polychromatic x-ray beam. By comparing the relativeintensities of certain spinel reflections that are sensitive to cationic disorder, atrend toward inverse spinel as a function of pressure was observed. Theresults are presented in comparison to previous studies on this material.

Synthesis, cation distribution and crystal structure of the spinel type solid solution Ga xCoFe 1− xCrO 4 (0⩽ x⩽1)

Solid solutions of Ga x CoFe 1− x CrO 4 ( x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were obtained by solid-state reactions in air at 1623 K. X-ray and neutron powder diffraction measurements have been performed on the materials in order to characterize them. Rietveld refinement of neutron powder diffraction data showed cubic symmetry corresponding to the space group Fd3m for all samples of the series. The results reveal that Ga has been substituted for Fe selectively. Ga exclusively occupies the octahedral site up to x=0.8, while at x=1.0 a small amount of Ga (5%) enters into the tetrahedral site in the compound. Cr, Fe and Co cations are distributed between the tetrahedral 8a and octahedral 16d sites. Their proportion at each site varies with composition. The fractional coordinates of oxygen are almost the same in all the samples while lattice parameter shows a slight decrease with increasing Ga content. The magnetic structure at room temperature was ferrimagnetic for the composition x=0.0 and 0.2.

6Li and 7Li MAS NMR Studies on Fast Ionic Conducting Spinel-Type Li2MgCl4, Li2–xCuxMgCl4, Li2–xNaxMgCl4, and Li2ZnCl4

6Li and 7Li MAS NMR spectra including 1D-EXSY (exchange spectroscopy) and inversion recovery experiments of fast ionic conducting Li2MgCl4, Li2–xCuxMgCl4, Li2–xNaxMgCl4, and Li2ZnCl4 have been recorded and discussed with respect to the dynamics and local structure of the lithium ions. The chemical shifts, intensities, and half-widths of the Li MAS NMR signals of the inverse spinel-type solid solutions Li2–xMIxMgCl4 (MI=Cu, Na) with the copper ions solely at tetrahedral sites and sodium ions at octahedral sites and the normal spinel-type zinc compound, respectively, confirm the assignment of the low-field signal to Litet of inverse spinel-type Li2MgCl4 and the high-field signal to Lioct as proposed by Nagel et al. (2000). In contrast to spinel-type Li2–2xMg1+xCl4 solid solutions with clustering of the vacancies and Mg2+ ions, the Cu+ and Na+ ions are randomly distributed on the tetrahedral and octahedral sites, respectively. The activation energies due to the various dynamic processes of the lithium ions in inverse spinel-type chlorides obtained by the NMR experiments are Ea=6.6–6.9 and ΔG*>79 KJ mol−1 (in addition to 23, 29, and 75 kJmol–1 obtained by other techniques), respectively. The largest activation energy of >79 KJ mol−1 corresponds to hopping exchange processes of Li ions between the tetrahedral 8a sites and the octahedral 16d sites. The smallest value of 6.6–6.9 KJ mol−1, which was derived from the temperature dependence of both the spin-lattice relaxation times T1 and the correlation times τC of Litet, reveals a dynamic process for the Litet ions inside the tetrahedral voids of the structure, probably between fourfold 32e split sites around the tetrahedral 8a site.

Stability of LiMn 2O 4 and new high temperature phases in air, O 2 and N 2

We investigate the reversible and irreversible transformations, the Li, Mn spinel undergoes under different atmospheres (air, O 2, and N 2) when heated up to 1050 °C. In air and O 2, a substantial reversible cation exchange occurs. For T≥800 °C, Mn 2+ ions substitute Li + at the tetrahedral (8a) position, and Li + shifts to interstitial octahedral (16c) site. Charge balance is achieved by a decrease in the Mn 3+ fraction, which is partially reduced to Mn 2+ in the regular octahedral (16d) site, according to the charge distribution (Li 1−2 y +Mn 2 y 2+) 8a[Li 3 y +] 16c[Mn 5 y 2+Mn 1−5 y 3+Mn 1 4+] 16dO 4 ( y increases with T). Under N 2 flow, a first decomposition occurs between 600 and 800 °C and yields Mn 3O 4, orthorhombic LiMnO 2 and O 2. A minor O 2 release occurs when heating above 900 °C, which is accompanied by the transformation of o-LiMnO 2 into a cubic Li x Mn 1− x O solid solution ( x≤0.5), consistent with a decrease in the average oxidation state of Mn ions. This cubic phase is stable at high temperature and decomposes upon cooling (T≤800 ° C) leaving just the Mn 3O 4 and o-LiMnO 2 phases.

Magnetic and Transport Properties of Mechanosynthesized FeCr<sub>2</sub>S<sub>4</sub> Sulfospinel

The FeCr2S4 sulfospinel has been synthesized from the elemental components, by highenergy ball milling. The magnetic properties of the resulting crystallites (mean diameter d = 120±10 nm) have been studied by means of ac and dc susceptibility, Mossbauer and transport measurements. Magnetization curves showed a parato ferrimagnetic transition of FeCr2S4 phase at TC = 165(3) K, and a cusp at TL ≈ 70 K. The evolution of Mossbauer spectra at different temperatures indicates the existence of a disordered surface layer, and its thickness was estimated to be t = 8(2) nm. Transport measurements show that the onset of the insulator-metal (IM) transition at T =241 K, a ~30% higher temperature than previous reports. In contrast, the 'colossal' magnetoresistance (CMR) effect observed in bulk FeCr2S4 is greatly reduced. Introduction Sulfospinels have attracted attention from decades due to their complex magnetic and transport properties [1-3]. For some transition metals like Cu, Cr, Cd, the corresponding sulfospinel compound have revealed several unique properties such as metal-to-insulator transition at high pressure, and the ability to form both metallic and insulating spin glasses [4,5]. As found in most spinels, the crystal structure of FeCr2S4 is cubic, space group Fd3m, with a normal ionic distribution, i.e., with Fe and Cr ions occupying the tetrahedral 8a and octahedral 16d sites respectively. However, Mossbauer data on different samples has shown that Fe can be also present, yielding partially inverted configurations [6]. The system is a p-type semiconductor, showing a parato ferrimagnetic transition at TC ≈ 170 K, and also spin-glass behavior has been reported below 60 K [7] Recently, FeCr2S4 spinel has been revisited since it presents colossal magnetoresistance near TC from different origin than the double-exchange (DE) mechanism previously found in manganites [8]. This compound has been described as a non-DE system because, even for samples with small amounts of Fe, no evidence of site exchanging has been reported so far. There is still no consensus on which are the microscopic mechanisms that govern the CMR effect in manganites, pyrochlores and sulfospinels, nor even on whether the CMR effect observed in these compounds have the same origin. Since the magnetic properties are intimately associated to their transport properties, we have attempted to study the influence of magnetic disorder on the transport properties. We present a study on the magnetic and transport properties of the obtained FeCr2S4 phase, by means of a.c. and d.c. magnetization, resistivity and Mossbauer spectroscopy measurements. † E-mail address: goya@macbeth.if.usp.br 2 Journal of Metastable and Nanocrystalline Materials 10 20 30 40 50 60 70 80 90 3.5 39 3.0 15 2.5 00 2.0 41 1.9 25 1.7 70 1.5 80 1.5 24 1.4 39 1.3 15 1.2 51 1 .17 6 1.1 19 1.1 02 1.1 19 1.1 76 1.2 51 1.3 15 1.4 39 1.5 24 1.5 80 1.7 70 1.9 25 2.0 41 2.5 00 3.0 15 3.5 39 + + + + + ∗ + FeCr2S4 (CrFe)3S4 Int en sit y ( a.u .)

A New Silicon-Doped Cation-Deficient Thiospinel, Cu5.52(8)Si1.04(8) □1.44Fe4Sn12S32: Crystal Structure, Mössbauer Studies, and Electrical Properties

Starting from pure metals and sulfur in evacuated silica tubes, single crystals of Cu5.52(8)Si1.04(8)□1.44Fe4Sn12S32 have been obtained by quenching from 680°C. The above cation-deficient thiospinel crystallizes in the Fd3m space group with a=10.3322(6) Å. Si doping leads to additional vacancies in the copper site. 119Sn Mössbauer data show the presence of Sn in II as well as IV oxidation states and all the Sn is present in the octahedral 16d sites. 57Fe Mössbauer studies show Fe to be present in the octahedral sites in both II and III oxidation states. The above thiospinel shows semiconducting behavior with resistivity of ∼1×102 Ω-cm at room temperature and a small band gap of ∼0.1 eV.

Infrared spectroscopic studies on spinel-type chromium indium sulfide solid solutions

Far infrared reflection spectra (near normal incidence) of metastable, quenched spinel-type cobalt chromium indium sulfide solid solutions CoCr2−xIn2xS4 were recorded and analysed in order to elucidate the behaviour of the four IR allowed phonon modes on going from (Co)[Cr2]S4 with normal metal atom distribution to (In)[CoIn]S4 with an inverse one. The energies of the transversal and longitudinal optic phonon modes were determined from dielectric dispersion relations ϵ″ and −Im(1/ϵ), respectively, via Kramers Kronig analyses. The highest-energy phonon mode exhibits one-mode behaviour whereas the three low-energy phonons reveal two-mode behaviour in contrast to M1−xM′xCoCr2S4 spinel-type solid solutions (with M and M′ bivalent metal atoms at the tetrahedral 8a site), which mainly display one-mode behaviour. This different behaviour of CoCr2−xIn2xS4 solid solutions is owing to the change of the metal atom distribution mentioned above in addition to the different masses of the atoms involved. The one-mode behaviour of the highest-energy phonon mode at 389 and 401cm−1 (CoCr2S4) and at 323 and 350cm−1 (CoIn2S4) is due to the fact that in the eigenvectors of this phonon almost only motions of the non-metal atoms are involved. The spectra of the solid solutions under investigation, which decompose on annealing at elevated temperatures to chromium and indium rich specimens, reveal further that there is a random distribution of the atoms at the octahedral 16d sites not only in the X-ray but also in the vibration spectroscopic size scale.

Cation distribution in Ga-doped Li 1.02Mn 2O 4

The structure of Ga-doped spinel oxides with the general formula, Li 1.02Mn 2− x Ga x O 4, has been studied by neutron and X-ray powder diffraction. Rietveld refinement of the neutron data confirms that the Ga 3+ ions selectively occupy the octahedral (16d) sites of the lattice. The absence of gallium ions from the tetrahedral (8a) or the additional octahedral (16c) is thought to be an important factor in the material's increased performance as a cathode in a rechargeable lithium ion battery.

Structure and cation disorder of hydrous ringwoodite, γ-Mg 1.89 Si 0.98 H 0.30 O 4

The structure of a single crystal hydrous ringwoodite, Mg1.89Si0.98H0.30O4 synthesized at conditions of 1300 °C and 20 GPa has been analyzed. Crystallographic data for hydrous ringwoodite obtained are; Cubic with Space group: Fd3m (no. 227). a= 8.0693(5) A, V=526.41(9) A3, Z=8, Dcalc= 3.48 g cm−3. The results of site occupancy refinement using higher angle reflections showed the existence of a small degree of Mg2+-Si4+ disorder in the structure such as (Mg1.84Si0.05□0.11)(Si0.93Mg0.05□0.02)H0.30O4. The IR and Raman spectra were measured and OH vibration spectra were observed. A broad absorption band was observed in the IR spectrum and the maxima were observed at 3160 cm−1 in the IR and at 3165 cm−1 and 3685 cm−1 in relatively sharp Raman spectra, which suggest that locations between O-O pairs around the octahedral 16c and 16d sites are possible sites for hydrogen.

Ion Exchange in Manganese Dioxide Spinel: Proton, Deuteron, and Lithium Sites Determined from Neutron Powder Diffraction Data

The structures of deuterated, protonated, and relithiated forms of MnO2 spinel, obtained by ion exchange of Li ions with D or H in a well-crystallized spinel Li1.27Mn1.73O4 compound, have been determined by powder neutron diffraction. Refinements were carried out using the Rietveld method of powder profile analysis in the space group Fd3m. Li ions are removed from both tetrahedral 8a and octahedral 16d sites when Li1.27Mn1.73O4 is contacted with DCl or HCl solution. The neutron diffraction data show that deuterium and hydrogen ions are predominantly incorporated into the crystal as −OD and −OH by bonding to lattice oxygen atoms without any other major change to the MnO2 spinel structure. The −OD and −OH groups are directed into the interstitial space of the 8a tetrahedra, but with an orientation which suggests that the deuteron or proton site is favored for occupation only when the nearest neighbor octahedral 16d site contains no Mn ion. When the protonated compound is contacted with LiOH solution, reinse...

Decomposition of Spinel-Type Nickel Chromium Indium Sulfides—X-Ray Powder Structure Determination

The phase diagram of the quaternary system NiCr2S4–NiIn2S4–Cr2S3–In2S3has been studied at 873 and 1223 K by X-ray powder structure determination of quenched samples. At high temperatures, spinel-type Niy(Cr2−2xIn2x)1−yS3−2ysolid solutions are formed in a large range of composition (y: 0–0.5,x: 0.3–1). At temperatures below 1183 K, these solid solutions decompose to Cr-rich and In-rich spinel-type sulfides, whereby nickel is enriched in the Cr-rich compounds. The decomposition of the spinel-type sulfides is greatly retarded, even at temperatures about 900 K, due to both thermodynamic and kinetic reasons. The cation distribution established corresponds to a site preference as Cr3+>Ni2+>In3+at the octahedral 16d site and □ (vacancies)>In3+>Ni2+at the tetrahedral 8a site of the spinel structure. The metal-sulfur distances of the solid solutions refined are compared with those calculated from characteristic metal–sulfur distances in close-packed structures, taking into account the site occupation established. For the respective indium-sulfur distances, the following improved values are recommended: Intet–S=247.1 pm; Inoct–S=260.6 pm.

Doped Li−Mn Spinels: Physical/Chemical Characteristics and Electrochemical Performance in Li Batteries

Several doped spinel-type Li−Mn oxides of formula Li1+xMyMn2-(x+y)O4+z, where Mn is partly replaced by Li, Cu, Zn, Ni, Co, Fe, Cr, Ga, Al, B, or Ti, were prepared by a solid-state reaction at 730 °C. These spinels were investigated by X-ray powder diffraction, thermal analysis, slow step voltammetry, and galvanostatic cycling at medium-high rates. Even substitutional levels as low as 0.05 atom/formula unit produce structural effects that are reflected in the electrochemical characteristics. The substituents may occupy either tetrahedral 8a sites or octahedral 16d sites. Examples of the first type of substitution are provided by Ga3+ and Zn2+, while octahedral sites are preferred by, e.g., Ni2+ and Cr3+. Cycling at practical rates produces capacities of ∼100 mA h/g for the best materials with limited capacity losses (3−6·10-2 mA h/g·cycle). The lower Mn3+ content with respect to undoped spinel explains the initially lower capacities.

Electrochemical and magnetic properties of lithium manganese oxide spinels prepared by oxidation at low temperature of hydrothermally obtained LiMnO 2

Cubic and tetragonally distorted lithium manganese oxide spinels (LiMnO 2 + x ) were obtained by progressive oxidation of LiMnO 2 between 300 and 400 °C. Orthorhombic LiMnO 2 was prepared by hydrothermal treatment of either γ-MnOOH or Mn(COOCH 3) 3 · 2H 2O with LiOH · H 2O in large excess. The reversible specific capacity for LiMnO 2 + x was found to be sensitive to average manganese valency changeable by the oxidizing condition. The LiMnO 2 + x exhibits the ferromagnetic behavior with T c = 20–40 K and the occurrence of spontaneous magnetic moment is associated with the decrease in the development of ferromagnetic interaction accompanying the decrease in the average shortest MnMn distance between Mn ions on octahedral 16d sites.

Elastic/anelastic behaviour during the phase transition in spinel LiMn2O4

Both the Young`s modulus sound velocity (VE) and the internal friction (Q-1) have been measured for a polycrystalline sample of LiMn2O4 in the temperature range between 35 and 350 K, using a vibrating-reed technique. The resonance frequency (fr) employed for the measurement ranged from 0.81 kHz to 2.69 kHz at 300 K. At fr(300 K)=0.81 kHz, as temperature is lowered from 350 K, VE decreases down to 225 K (TA) Then, VE increases rapidly but monotonically with lowering temperature, though the magnitude of the slope (-dVE/dT) reduces. The overall increase in the velocity between TA and 100 K is about 68%. In the Q-1-versus-T curve, a large peak is clearly observed at around 210 K, as is the forming of a large shoulder on the low-temperature side; additionally, there exists a small Q-1 peak at around 120 K, though VE indicates no significant change at around 120 K. Furthermore, both the VE-versus-T curve and the Q-1-versus-T curve shift toward the higher-temperature side, by ~45 K and ~25 K, as fr(300 K) is increased from 1.32 kHz to 2.69 kHz. The observed elastic anomalies suggest that a phase transition occurs at temperatures below 290 K. This phase transition is probably due to an ordering of Mn3+ and Mn4+ ions on the octahedral 16d site in the spinel lattice. On the other hand, the origin of the Q-1 peak is considered to lie in a stress-induced relaxation of the distributions of Mn ions or Li ions.

Preparation of LiTi 2O 4 single crystals with the spinel structure

Single crystals of the spinel-type LiTi{sub 2}O{sub 4} (Cubic, Fd3m, a = 8.4095(12) {angstrom}) have been prepared by the reaction of lithium metal and TiO{sub 2} at 1373 K in a sealed iron vessel. The structure refinement by a single crystal X-ray diffraction study confirmed a normal spinel cation distribution; the lithium cations are located at the tetrahedral 8a sites, the titanium cations occupy the octahedral 16d sites, and the oxygen parameter u = 0.2622(4) with the final R = 3.8% (R{sub w} = 2.7%) for 60 independent observed reflections. The octahedral Ti-O distance is 2.005(3) {angstrom}, and the tetrahedral Li-O distance is 1.999(3) {angstrom}. The superconductivity for the crystals was observed by the magnetization measurements.

Etude cristallographique du systeme FeSSc 2S 3 preparations et structures de FeSc 2S 4 et Fe 0.85Sc 2.10S 4

The crystal structures of FeSc 2S 4 and Fe 0.85Sc 2.10S 4 have been determined by three dimensional X-ray diffraction. The cubic cells, space group Fd3m, have lattice constants a = 10,501 A ̊ for FeSc 2S 4 and a = 10,444 A ̊ for Fe 0.85Sc 2.10S 4. Iron is divalent and scandium trivalent. FeSc 2S 4 is a direct spinel structure, Fe 0.85Sc 2.10S 4 is near a spinel structure. Only 0,5 iron atom is on every 8a position; 0,175 Fe and 1,05 Sc lie on octahedral 16d and 16c site.