Fd3m Phase Research Articles

Effect of d-block element Co2+ substitution on structural, Mössbauer and dielectric properties of spinel copper ferrites

The present work focuses on the influence of replacement of d-block element Cu2+ ion by Co2+ in Cu-spinel ferrites [Cu1−xCoxFe2O4 (x=0.0, 0.1, 0.2, 0.4, 0.6, and 1.0)] on the structural, vibrational and dielectric properties as synthesized by Solid-state reaction route. A structural transition from tetragonal (space group I41/amd)) to cubic (space group Fd3m) phase is observed due to introduction of cobalt. Cubic spinel- type structure at room temperature of Cu1−xCoxFe2O4 (0.4≤x≤1.0) is confirmed by Rietveld – refined X-ray powder diffraction patterns. Raman spectroscopic studies reveal 2 (5) optical active modes in CuFe2O4 (CoFe2O4) at room temperature. Transmission Mössbauer spectroscopy of Cu1−xCoxFe2O4 (x=0.0, 0.2 and 0.6) shows two sets of six-line hyperfine patterns for all the three samples, indicating the presence of Fe in both A and B sites. Identification of sites is accomplished by evidence from hyperfine distribution and isomer-shift data. Dielectric constant and dielectric loss tangent measured in the frequency range from 1KHz to 1MHz at room temperature are found to be decreasing with the increase in frequency.

Structural Behavior of ZnCr2S4 Spinel under Pressure

The series of Cr-chalcogenide spinels ACr2X4 (A = Zn, Cd, Hg; X = S, Se) exhibits a rich phase diagram upon compression, as revealed by our recent investigations. There exist, however, some open questions regarding the role of cations in the observed structural transitions. In order to address these queries, we have performed X-ray diffraction and Raman spectroscopic studies on the ZnCr2S4 spinel up to 42 GPa, chosen mainly due to the similarity of the Zn2+ and Cr3+ cationic radii. Two reversible structural transitions were identified at 22 and 33 GPa, into a I41/amd and an orthorhombic phase, respectively. Close comparison with the behavior of relevant Cr-spinels revealed that the structural transitions are mainly governed by the competition of the magnetic exchange interactions present in these systems, and not by steric effects. In addition, careful inspection of the starting Fd3m phase revealed a previously unnoticed isostructural transition. The latter is intimately related to changes in the electro...

Origin of the Ni/Mn ordering in high-voltage spinel LiNi0.5Mn1.5O4: The role of oxygen vacancies and cation doping

Spinel LiNi0.5Mn1.5O4 (LNMO) exhibits two different Ni/Mn arrangements, i.e., the Ni/Mn ordered P4332 phase and disordered Fd-3m phase. It has been found that the Ni/Mn disorder is correlated with the formation of oxygen vacancies, nevertheless the underlying mechanism remains unclear. Density functional theory (DFT) calculations show that formation of 1:3 ordered Ni2+ and Mn4+ ions is energetically favorable compared to the disordered Ni3+ and Mn3+ ions caused by Ni aggregation in the stoichiometric P4332 phase. However, in oxygen deficient LiNi0.5Mn1.5O4−δ, the oxygen vacancies tend to diminish the valence discrepancy between the Ni aggregated and the ordered P4332 phases, making the former energetically competitive and consequently resulting in the disordered Ni/Mn distribution. Understanding the origin of Ni/Mn disorder also sheds light on the cation doping effect. Calculations show that Mg2+ ion tends to replace Ni2+ ion in ordered P4332 phase, and maintaining the Ni/Mn order. By contrast, Al doping promotes the Ni/Mn disorder, as Al3+ ion prefers to substitute for Ni3+ and Mn3+ ions emerged in Ni/Mn disordered structure. Our findings rationalize the experimental observations, and further reveal that Ni/Mn arrangement could be controlled by adjusting the electronic structure of spinel LNMO system.

Synthesis of LiCo<sub>x</sub>Mn<sub>2-x</sub>O<sub>4</sub> by Low Temperature Solid-State Reaction for Cathode Material

LiCoxMn2-xO4 can serve as one of the main candidates for cathode materials for lithium batteries since they are abundant, low cost and environmentally friendly. This study aims to study the synthesis of LiCoxMn2-xO4 by low temperature solid-state reaction and its microstructure determination. Attention is paid more to the mole ratio of Co/Mn in LiCoxMn2-xO4. The compound was characterized by XRD, SEM-EDX, and BET while the analysis of LiCoxMn2-xO4 microstructure was carried out by Direct Method using winPLOTR package program and Diamond using XRD data. The series of LiCoxMn2-xO4 have well–developed cubic crystal structure with Fd3m phase, and the increase in the dopant does not change its structure. The lattice parameters and cell volumes of LiCoxMn2-xO4 tend to decrease with the increase in x values

Phase transition and thermodynamic properties of YAg alloy from first-principles calculations

Abstract Using the particle swarm optimization algorithm on crystal structure prediction, we first predicted that YAg alloy undergoes a first-order phase transition from CsCl phase to Fd-3m phase at about 25 GPa with a volume collapse of 7.47%. An abrupt change of the electrical resistance was discovered accompanying the phase transformation. Our results showed that external pressure has different effects on the ductility and anisotropy of the two phases. External pressure can improve the ductility of CsCl phase, while it has a negligible effect on that of Fd-3m phase. As pressure increases, the elastic anisotropy of CsCl phase increases rapidly, while that of Fd-3m phase remains unchanged. The structural stability, electronic structure, and thermodynamic properties of the two phases under pressure were also investigated systematically.

Structural and electrochemical characteristics of morphology-controlled Li[Ni0.5Mn1.5]O4 cathodes

Two types of structure- and morphology-controlled spinel Li[Ni0.5Mn1.5]O4 (LNMO) are prepared and systematically investigated as 5V, high-rate, and long-life cathode materials for rechargeable Li-ion batteries. The octahedral LNMO particles (1μm or 1–5μm mixed sizes) are prepared through a heat-treatment at 850°C after a hydrothermal reaction, and their performance is compared with that of one-dimensional LNMO nanorods (100–200nm and 1–3μm in diameter and length, respectively), which are synthesized via a two-step method consisting of a hydrothermal reaction followed by solid-state Li and Ni implantation. They show high single crystallinities with an ordered (P4332) and disordered (Fd-3m) phase for the nanorods and octahedral particles, respectively. Rietveld refinement of X-ray and neutron diffraction, FT-IR, SEM, and TEM are employed to study their phases and microstructures. Galvanostatic studies reveal that overall battery performances of the octahedral LNMO particles are superior to those of the LNMO nanorods. In particular, the disordered octahedral LNMO particles that are composed of mixed particle sizes ranging from of 1 to 5μm show not only the best rate capability and specific discharge capacity but also an excellent cycle stability with a capacity retention of 89% (corresponding to specific discharge capacity of 105mAhg−1) at a 10C cycling rate, even after 1000 cycles. This remarkable performance is attributed to the structural stability, while the highest electrode tap density (1.59gcm−3) in combination with efficient packing resulted in the coexistence of various particle sizes that can provide a shortened pathway for lithium ions between particles.

Phase behaviour of the ternary system: monoolein-water-branched polyethylenimine.

Addition of a branched polymer, polyethyleneimine, significantly alters the organization of a glycerol monooleate (GMO) lipid-water system. We present detailed data over a wide range of compositions (water content from 10 to 40%, relative to GMO and PEI fractions from 0 to 4%) and temperatures (25-80 °C). The PEI molecular weight effects are examined using polymers over a range from 0.8 to 25 kDa. Addition of PEI induces the formation of higher curvature reverse phases. In particular, PEI induces the formation of the Fd3m phase: a discontinuous phase comprising reverse micelles of two different sizes stacked in a cubic AB2 crystal. The formation of the Fd3m phase at room temperature, upon addition of polar, water soluble PEI is unusual, since such phases typically are formed only upon addition of apolar oils. The largest stability window for the Fd3m phase is observed for PEI with a molecular weight = 2 kDa. We discuss how PEI influences the formation and stability of high curvature phases.

LiNi0.5Mn1.5O4 with significantly improved rate capability synthesized by a facile template method using pine wood as a bio-template

A porous LiNi0.5Mn1.5O4 spinel with significantly improved rate capability has been synthesized by a facile template method using pine wood as a bio-template. The templated LiNi0.5Mn1.5O4 (T-LNMO) has disordered Fd-3m phase, and consists of sub-micrometric and highly dispersed particles. It exhibits significantly improved rate capability. The capacity of T-LNMO at 10C is 97mAh/g and retains 90% after 100 cycles. Contrastively, the non-templated LiNi0.5Mn1.5O4 (NT-LNMO) cannot deliver any capacity at 10C, and at 5C rate after 100 cycles, the capacity retention is only 22%.

Controllable synthesis of spinel lithium nickel manganese oxide cathode material with enhanced electrochemical performances through a modified oxalate co-precipitation method

A spinel lithium nickel manganese oxide (LiNi0.5Mn1.5O4) cathode material is synthesized with a modified oxalate co-precipitation method by controlling pH value of the precursor solution and introducing excessive Li source in the precursor. All the samples synthesized through this method are of Fd3m phase with a small amount of P4332 phase. It is found that pH value of the precursor solution considerably affects the morphology, stoichiometry and crystallographic structure of the target material, thereby resulting in different amounts of Mn3+ (i.e., different degree of disorder). 5% excessive Li source in the precursor may compensate for the lithium loss during the high-temperature sintering process and eliminate the LixNi1-xO impurity phase. Under the optimized synthesis conditions, the obtained high-purity LiNi0.5Mn1.5O4 spinel exhibits enhanced electrochemical performances. A reversible capacity of ca. 140 mAh g−1 can be delivered at 0.1C and the electrode retains 106 mAh g−1 at 10C rate. When cycled at 0.2C, a capacity retention of more than 98% is obtained in the initial 50 electrochemical cycles.

One-step synthesis and effect of heat-treatment on the structure and electrochemical properties of LiNi0.5Mn1.5O4 cathode material for lithium-ion batteries

Spinel LiNi0.5Mn1.5O4 (Fd-3m) powders are synthesized by a facile one-step sol-gel approach with a resorcinol formaldehyde (RF) resin as a chelating agent. The cross-linked metal-containing RF xerogel particles are sintered at different high temperatures from 750 to 950°C to produce several micron-sized LiNi0.5Mn1.5O4 powders. Electrochemical measurements suggest that the 850°C-sintered (in air) sample (Fd-3m phase) performs the best with a discharge capacity of 141 mAh g−1 at 0.1C and 110 mAh g−1 at 10C, and capacity-retention of 96.3% after 60 cycles at 0.25C and 89% after 200 cycles at 1C. For comparison, the LiNi0.5Mn1.5O4 sample sintered at 850°C in O2 (P4332 phase) presents limited rate performance (45 mAh g−1 at 10C) and higher values in both AC impedance and DC-method derived resistance. A characteristic double “w”-shape curve of DC resistance against cell potential can be possibly considered as an indicator to probe the material structure transition during the charge/discharge process of the cell.

Formation of Inverse Topology Lyotropic Phases in Dioleoylphosphatidylcholine/Oleic Acid and Dioleoylphosphatidylethanolamine/Oleic Acid Binary Mixtures

The addition of saturated fatty acids (FA) to phosphatidylcholine lipids (PC) that have saturated acyl chains has been shown to promote the formation of lyotropic liquid-crystalline phases with negative mean curvature. PC/FA mixtures may exhibit inverse bicontinuous cubic phases (Im3m, Pn3m) or inverse topology hexagonal phases (HII), depending on the length of the acyl chains/fatty acid. Here we report a detailed study of the phase behavior of binary mixtures of dioleoylphosphatidylcholine (DOPC)/oleic acid (OA) and dioleoylphosphatidylethanolamine (DOPE)/oleic acid at limiting hydration, constructed using small-angle X-ray diffraction (SAXD) data. The phase diagrams of both systems show a succession of phases with increasing negative mean curvature with increasing OA content. At high OA concentrations, we have observed the occurrence of an inverse micellar Fd3m phase in both systems. Hitherto, this phase had not been reported for phosphatidylethanolamine/fatty acid mixtures, and as such it highlights an additional route through which fatty acids may increase the propensity of bilayer lipid membranes to curve. We also propose a method that uses the temperature dependence of the lattice parameters of the HII phases to estimate the spontaneous radii of curvature (R0) of the binary mixtures and of the component lipids. Using this method, we calculated the R0 values of the complexes comprising one phospholipid molecule and two fatty acid molecules, which have been postulated to drive the formation of inverse phases in PL/FA mixtures. These are -1.8 nm (±0.4 nm) for DOPC(OA)2 and -1.1 nm (±0.1 nm) for DOPE(OA)2. R0 values estimated in this way allow the quantification of the contribution that different lipid species make to membrane curvature elastic properties and hence of their effect on the function of membrane-bound proteins.

Pressurizing the HgCr2Se4 spinel at room temperature

The cubic HgCr2Se4 spinel undergoes two structural transitions upon pressure increase. Initially, the ambient-pressure Fd-3m phase transforms into a tetragonal I41/amd structure above 15 GPa. We speculate that this Fd-3m-I41/amd transition is accompanied by an insulator-to-metal transition, resulting in the vanishing of the Raman signal after the structural transformation. Further compression of HgCr2Se4 leads to structural disorder beyond 21 GPa. Our spin-resolved band structure calculations reveal significant changes in the electronic structure of HgCr2Se4 after the Fd-3m-I41/amd transition, whereas the ferromagnetic interactions are found to dominate in both structures.

Multiple pressure-induced transitions in HgCr2S4

We have performed combined experimental and theoretical high-pressure investigations on magnetoelectric HgCr2S4 spinel. Overall, HgCr2S4 exhibits three reversible structural transitions under pressure: the starting Fd-3m phase adopts a tetragonal I41/amd structure at 20 GPa, an orthorhombic distortion occurs above 26 GPa, whereas a third structural transition takes place beyond 37 GPa. During the Fd-3m to I41/amd structural transition, HgCr2S4 experiences a volume change of 4% which is unexpected from space group symmetry considerations alone. Furthermore, the Fd-3m to I41/amd transformation appears to be concomitant with an insulator-to-metal transition whereas compression of HgCr2S4 leads to a gradual suppression of its ferromagnetism.

Theory of the formation of the ordered LiZn0.5Mn1.5O4 phase

A theory of the structural phase transition in LiZn0.5Mn1.5O4 cathode material is proposed. The symmetry of the order parameter, thermodynamics, and mechanisms of formation of the atomic structure of low-symmetry ordered cubic lithium-zinc-manganese oxide spinel have been studied. The critical order parameter inducing the phase transition has been found. It is shown that the calculated LiZn0.5Mn1.5O4 structure is formed due to displacements and orderings of lithium, zinc, manganese, and oxygen atoms. Within the Landau theory of phase transitions, it is shown that the phase states may change from high-symmetry cubic disordered Fd3m phase to the low-symmetry ordered cubic P213 phase as a result of first-order phase transitions.

How the chain configuration governs the packing of inverted micelles in the cubic Fd3m-phase

The inverted micellar cubic Fd3m phase has attracted significant interest in drug delivery and is also of special biological relevance in the early steps of fat digestion. Applying small angle X-ray diffraction (SAXD) the stability of the Fd3m phase with respect to the chain configuration has been examined. In particular, the fully hydrated monoelaidin system containing the trans elaidic or its counterpart cis oleic fatty acid was investigated. Re-analyzed data of a fully hydrated monoolein–oleic acid (MO–OA) mixture complete this study (data taken from Luzzati et al.). On the basis of determined electron density maps, minute structural details are presented. The decomposition of the cubic Fd3m nanostructure into its apolar and polar moieties allowed the estimation of chain length and water radius depending on the fatty acid content. An increase in the elaidic chain concentration increases the membrane monolayer thickness, and in particular reduces the sphericity of the polar envelope of the small micelles. Furthermore, the electron density maps revealed the geometry of the space filling 512 and 51264 cages of the Fd3m phase as well as the loci of the greatest packing stress, which are situated at vertices of the 51264 cages that are placed opposite the hexagonal faces.

Raman study of the spinel-to-layered phase transformation in sol–gel LiCoO2 cathode powders as a function of the post-annealing temperature

LiCoO2 powders synthesized by a sol/gel process followed by an annealing heat treatment in the range 400–900°C are systematically characterized using SEM, X-ray diffraction, FTIR and Raman spectroscopy. The composition of the final powder is found to be controlled by the heat treatment temperature. A thorough multipeak fitting analysis of Raman spectra gives access for the first time to the quantitative estimation of the R-3m and Fd3m relative amounts in the LiCoO2 powders as a function of the post-annealing temperature. The gradual LT–HT phase transformation can therefore be observed: a pure Fd3m phase is obtained at 400°C, and then the two LiCoO2 forms coexist from 500°C to below 700°C, the R-3m relative amount increasing from 20% at 500°C to 90% at 600°C, to be 100% at 700°C. Cyclic voltammetric measurements confirmed this evolution, showing the typical behaviour of the high performance R-3m layered phase from 600°C. These results put forward an original and appropriate use of Raman spectroscopy in the field of electrode materials for lithium batteries.

Facile polymer-assisted synthesis of LiNi0.5Mn1.5O4 with a hierarchical micro–nano structure and high rate capability

We report the facile preparation of spinel type LiNi0.5Mn1.5O4 with hierarchical micro–nano structures (LNMO-HMs) and their application as cathode materials for rechargeable lithium-ion batteries. The LNMO-HMs, which were synthesized through a poly(ethylene glycol) (PEG)-assisted co-precipitation route, have a particle size of 5–10 μm, which are composed of nano-particles with a size of about 200 nm. The effect of PEG on the phase purity and morphology of the LNMO products was studied. It was found that as the molecular weight of PEG increased, the lithium nickel oxide impurity decreased at first and then increased slightly. The average size of the nano-particles also showed a similar trend of first a decrease and then an increase, while the secondary micro-particles were enlarged with longer PEG chains. Raman mapping technology proved that the P4332 phase and the Fd3m phase LNMO coexist in the as-prepared samples, but the latter is the mainstay. When applied as cathode materials for lithium-ion batteries, PEG4000-assisted LNMO-HMs showed a remarkably high rate capability and cycling stability. The deliverable discharge capacity exceeded 120 mAh g−1 at 40 C current rate and the capacity retention approached 89% after 150 cycles at 5 C current rate, showing the potential in the application of high rate discharge.

One step synthesis of lamellar R-3m LiCoO 2 thin films by an electrochemical–hydrothermal method

In this study, we report the synthesis of lamellar R-3m LiCoO 2 thin films electrodes for lithium rechargeable batteries by a single step method based on an electrochemical–hydrothermal synthesis in a concentrated LiOH solution with a cobalt salt. This process combines the effect of temperature (between 150 °C and 200 °C), pressure and galvanostatic current. The obtained films were not annealed after the electrochemical–hydrothermal synthesis. For the first time, the theoretical study of the potential–pH diagram of cobalt was carried out at high temperature and high concentration. These calculations show that a pH value higher than 12 is necessary to avoid the direct precipitation of cobalt hydroxide Co(OH) 2 inside the solution. An improvement of the soluble species stability with an increase of the temperature and a decrease of the cobalt concentration is predicted. The influence of the deposition conditions (temperature and concentration) at a constant current density was experimentally studied. X-ray diffraction (XRD) shows the formation of well-crystallized LiCoO 2 thin films. Raman spectroscopy confirmed the achievement of the electrochemically active R-3m LiCoO 2 phase without any trace of the Fd3m phase at temperatures as low as 150 °C. Electrochemical measurements demonstrate good performances of the material synthesized between 150 °C and 200 °C with better capacity retention at higher temperature.

Hydrostatic pressure effects on a hydrated lipid inverse micellar Fd3m cubic phase

Over a range of hydration, unsaturated diacylglycerol/phosphatidylcholine mixtures adopt an inverse micellar cubic phase, of crystallographic space group Fd3m. In this study hydrated DOPC:DOG mixtures with a molar ratio close to 1 : 2 were examined as a function of hydrostatic pressure, using synchrotron X-ray diffraction. The small-angle diffraction pattern at atmospheric pressure was used to calculate 2-D sections through the electron density map. Pressure initially has very little effect on the structure of the Fd3m cubic phase, in contrast to its effect on hydrated inverse bicontinuous cubic phases. At close to 2 kbar, a sharp transition occurs from the Fd3m phase to a pair of coexisting phases, an inverse hexagonal H(II) phase plus an (ordered) lamellar phase. Upon increasing the pressure to 3 kbar, a further sharp transition occurs from the H(II) phase to a (fluid) lamellar phase, in coexistence with the ordered lamellar phase. These transitions are fully reversible, but show hysteresis. Remarkably, the lattice parameter of the Fd3m phase is practically independent of pressure. These results show that these two lipids are miscible at low pressure, adopting a single lyotropic phase (Fd3m); they then become immiscible above a critical pressure, phase separating into DOPC-rich and DOG-rich phases.

Effects of Pressure and Temperature on the Self-Assembled Fully Hydrated Nanostructures of Monoolein−Oil Systems

Synchrotron small-angle X-ray scattering (SAXS) was applied for studying the effects of hydrostatic pressure and temperature on the structural behavior of fully hydrated tetradecane (TC)-loaded monoolein (MO) systems. Our main attention focused on investigating the impact of isobaric and isothermal changes on the stability of the inverted type discontinuous Fd3m cubic phase as compared to the inverted type hexagonal (H(2)) liquid crystalline phase. The present results show that compressing the TC-loaded Fd3m phase under isothermal conditions induces a significant increase of its lattice parameter: it approximately increases by 1 A per 75 bar. Further, the Fd3m phase is more pressure-sensitive as compared to the Pn3m and the H(2) phases. At ambient temperatures, we observed the following structural transitions as pressure increases: Fd3m --> H(2) --> Pn3m. Our findings under isobaric conditions reveal more complicated structural transitions. At high pressures, we recorded the interesting temperature-induced structural transition of (Pn3m + L(alpha)) --> (Pn3m + L(alpha) + H(2)) --> (L(alpha) + H(2)) --> H(2) --> Fd3m --> traces of Fd3m coexisting with L(2). At high pressures and low temperatures, the TC molecules partially crystallize as indicated by the appearance of an additional diffraction peak at q = 3.46 nm(-1). This crystallite disappears at high temperatures and also as the system gets decompressed. The appearance of the Pn3m and the L(alpha) phases during compressing the fully hydrated MO/TC samples at high pressures and low temperatures is generally related to a growing hydrocarbon chain condensation, which leads to membrane leaflets with less negative interfacial curvatures (decreasing the spontaneous curvatures |H(0)|). Both the effects of pressure and temperature are discussed in detail for all nonlamellar phases on the basis of molecular shape and packing concepts.