Electrical and structural studies of lithium doped cobalt ferrite

Lithium doped cobalt ferrites (Li_0.2CoFe_1.8O₄) were prepared by a sol–gel combustion method by using Lithium nitrate, cobalt nitrate, and ferric nitrate as source materials. Here, Citric acid and polyvinyl alcohol used as a burning agent and agglomeration reducing agent respectively. The pH value of precursor was maintained between 7 and 8 while preparation of the sample. The average particle size of the ferrite obtained at 300°C about ～15–20 nm. Further, single phase spinel structure has been confirmed by X-ray diffraction (XRD) and Cubic morphology was confirmed by High resolution scanning electron microscopy (HR-SEM). The composition of elements has been confirmed by Infrared spectroscopy (FT-IR). Moreover, the electrical studies of the prepared sample have been analyzed by four point probe method. The observed results inferred that conductivity of the ferrite sample has been reduced by addition of lithium.     

Lead-free BFN ceramics doped by chromium,lithium or manganese

The material of the study was lead-free BaFe_0.5Nb_0.5O₃ ceramics subject to modification. The base composition BaFe_0.5Nb_0.5O₃ as well as the chromium, lithium and manganese modified ones were obtained using conventional mixed oxides and carbonates method. Synthesis was performed by the powder calcination method at high temperature 1250 °C for 4 h, while the densification was carried out by free sintering method under conditions 1350 °C/4 h. The paper presents a complex study of admixtures influence on the crystal structure, microstructure and dielectric properties of the BFN type samples. The mentioned dopants chromium, lithium or manganese in the BFN-type ceramics among other caused the reduction of the electric permittivity maximum as well as significant decrease in value of dielectric loss.     

X-ray study of some spinels containing gallium and manganese

The ternary oxides CrMnGaO₄, NiMnGaO₄, CuMnGaO₄ and ZnMnGaO₄, crystallize in the cubic spinel structure with lattice parametera=8.41±0.02 Å, 8.34±0.02 Å, 8.36±0.02 Å and 8.32±0.02 Å, respectively. The oxidation state of manganese in these spinels was determined x-ray spectroscopically. The site distribution was determined from the structural properties and calculated site preference energies of cations in the lattice. The ionic structures were found to be Ga³⁺ [Mn²⁺ Cr³⁺] O ₄ ^2? . Ga³⁺ [Cu²⁺ Mn³⁺] O ₄ ^2? , Mn²⁺ [Ga³⁺ Ni³⁺] O ₄ ^2? and Zn²⁺ [Mn³⁺ Ga³⁺] O ₄ ^2? .     

Electrochemical and structural characteristics of tungstic acids as cathode materials for lithium batteries

The electrochemical characteristics and structural changes associated with discharge and charge of several tungstic acids such as H₂WO₄ and H₂WO₄ · H₂O have been investigated. The suitability of these substances as new cathode materials for nonaqueous lithium batteries has been assessed. H₂WO₄, having only coordinated water molecules, showed a discharge capacity of about 410 Ah kg^–1 of acid weight and a discharge potential around 2 V vs. Li/Li⁺. This capacity was much higher than the 40 180 Ah kg^–1 of anhydrous WO₃. H₂WO₄ showed a good charge-discharge cycling behavior at a capacity below 1e ^–/W. However, the formation of a stable phase such as Li₂WO₄ during the cyclings limited the cycling number. In addition, the crystal structure of H₂WO₄ changed from orthorhombic to tetragonal during discharge, but the original layered lattice was kept on discharge to 1.5e ^–/W. On the other hand, a significant decrease in the layer spacing of H₂WO₄ · H₂O took place with discharge, due to the direct interaction between the interlayer water molecule and the lithium inserted between the layers. In this paper, in particular, the effect of the coordinated and hydrated water molecules in the acid structure on the electrochemical behavior is discussed.     

Comparison of negative magnetoresistance mechanisms in manganese perovskites and chromium spinels

A transition of the field dependence of the electrical resistivity from a square law (∼H ²) above T _c to a linear function (∼H) below T _c is observed in the degenerate ferromagnetic semiconductor HgCr₂Se₄(n). Together with the large negative magnetoresistance, these magnetoelectric effects correspond to effects observed in the perovskite-type oxides La_1−x Ca _x MnO _δ . Inasmuch as the undoped semiconductor HgCr₂Se₄ is a ferromagnet with approximately the same critical temperature as the doped semiconductor and in view of the total lack of data on the Jahn-Teller effect in this compound, we infer that our results cast doubt on existing hypotheses (polaron and binary exchange) regarding the origin of the giant magnetoresistance in La_1−x Ca _x MnO _δ . Impurity sd scattering is discussed as a possible magnetoresistance mechanism for both compounds. Fiz. Tverd. Tela (St. Petersburg) 41, 1800–1803 (October 1999)     

Significance of Mg doped LiMn2O4 spinels as attractive 4 V cathode materials for use in lithium batteries

LiMn₂O₄ spinel is one of the most promising cathode materials for lithium-ion batteries because of its cheapness and eco-friendliness. Due to Jahn-Teller distortion, the capacity fades, however, upon repeated cycling. Attempts are being made to improve the cycle life of the spinel by substitution of manganese with other cations. In this paper we report the effect of partial substitution of manganese by Mg²⁺ ions in the LiMn₂O₄ phase. LiMg_yMn_2−yO₄ (y=0 – 0.3) has been synthesized by a thermal method and characterized using XRD, TG/DTA and FTIR. The electrochemical performance is correlated with the dopant concentration.     

Ionic conductivity of lithium nitride doped with hydrogen

Lithium ionic conductivity of Li₃N single crystals is reported for temperatures from 120 K to 350 K. The intrinsic ionic conductivity is rather small (< 10^?6 Ω^?1 cm^?1 at 300 K) and shows no strong anisotropy. The activation energy is near 0,6 eV. It is shown that hydrogen is the critical impurity in the crystals grown and studied at this laboratory. The relative impurity concentration is determined from infrared transmission measurements near 3130 cm^?1. An estimate for absolute values is obtained from dielectric studies. Increases in ionic conductivity with hydrogen doping by a factor 5000 are reported for E⊥c but no significant effects are found for E6c. The proposed defect is an impurity-vacancy complex consisting of an NH^?? and a lithium vacancy.     

Chloride spinels: A new group of solid lithium electrolytes

The chloride spinels Li₂MCl₄ with M = Mg, Mn, Fe and Cd show very high lithium ionic conductivity in the solid state. The ionic conductivity in the compounds under investigation was established with the help of emf measurements. The specific conductivities measured by both frequency response analysis and the four probe ac method are 1.3 Ω^?1 · cm^?1 for Li₂CdCl₄, and about 0.9Ω^?1 · cm^?1 for Li₂MnCl₄, Li₂MgCl₄, and Li₂FeCl₄ at 773 K. There are several indications that the ternary chlorides become highly disordered at elevated temperatures. Thus the Arrhenius plots, i.e. In σ · T vs 1/T-curves, exhibit significant bends, the slopes below the transition temperature being considerably higher than those above.     

Structural, thermal and morphological studies of magnesium substituted-lithium manganese oxide spinels

Coexistence of two phases having space groups of Fd3m and P4₃32 in the Mg-doped LiMn₂O₄ spinel is being reported for the first time in this article. Mg-doped LiMn₂O₄ powders have been synthesized by sol-gel method using citric acid as a chelating agent. X-ray powder diffraction (XRD) studies show that the crystal structure of LiMg_xMn_2−xO₄ for x<0.25 is a single-phase cubic spinel, which has space group of Fd3m. The cubic spinel structures having space group of Fd3m and P4₃32 are found to coexist in the compound for x=0.25. The structure becomes single-phase cubic spinel with space group P4₃32 for x>0.25. Field emission scanning electron microscopy (FESEM) shows that particle size of various synthesized powders ranges from 100 to 350 nm. Particle size decreases with increase in Mg content. Differential thermal analysis (DTA) and thermogravimetry (TG) studies show an exponential decay relationship between Mg-doping content and the decomposition temperature to form nonstoichiometry (LiMg_xMn_2−xO_4−δ) in air atmosphere. Fourier transform infrared spectroscopy (FTIR) analysis shows increase in the number of vibrational bands with increase in Mg content, which indicates ordering of the ions in the case of ordered spinel structure, and consequent reduction of the space group symmetry from O_h⁷ to O⁷.     

Fermi surface of doped lithium

The Korringa-Kohn-Rostoker method with Green’s function averaged over the atomic configurations in a complex Ising lattice and a muffin-tin potential was used to calculate the electronic-band structure in lithium containing vacancies and s, p, and d impurities. It is shown that substantial changes in the profile of the Fermi surface do not lead to necking, as was postulated previously, but cause splitting of the electronic states at the face of the Brillouin zone. This is attributed to the reduced symmetry of the crystal lattice with impurity excitation of the electronic-subsystem. Fiz. Tverd. Tela (St. Petersburg) 40, 1185–1186 (July 1998)     

Optical characterization of gallium antimonide highly doped with manganese

Gallium antimonide crystals highly doped with Mn were prepared by a liquid-phase-electroepitaxy growth method. The crystals exhibited high hole concentrations up to 6×10¹⁸ cm⁻³. Photoluminescence (PL) and transmission techniques were used for their investigation. Spectral line-shapes typical for highly doped semiconductors were observed. The lines revealed the features corresponding to band gap narrowing and valence-band filling phenomena. Values of the band-gap narrowing ΔE_g and the degree of the valence-band filling ΔE_F were estimated from the PL spectra. The ionization energy of the Mn acceptor E_i was estimated to be approximately 15.1-15.6 meV. At low temperatures, the PL maxima shifted relatively strongly towards higher energy with temperature. The shifts most probably resulted from a dramatic change in the electron density of states near the bottom of the conduction band. The extent of low-energy tails of the PL bands correlates with the doping levels. The transmission spectra exhibited an absorption band centred at around 774-780 meV. The band most probably originated in electron transitions from the level of spin-orbit splitting to the top of the valence band.     

Magnetic,electrical and structural properties of some ternary and quaternary spinels with chromium

Spinel compounds with the general formulas ACr₂X₄ and ACr_2−xB_xX₄ (where A=Cd, Zn, Hg, Ga, Cu; X=S, Se, Te, O; B=Ga, In, Sb), including also substitution of two different cations in the tetrahedral subarray, exhibit different correlations of the magnetic and electrical properties. These properties are correlated with the ionic radii of cations and anions. The solid solutions of the boundary compounds reveal, e.g. correlations of metallic conductivity and a semiconductor–metal phase transition. In some spinel solid solutions the latter phase transition is accompanied by the magnetic phase transitions, i.e. either AF-F or Fi-F. Magnetic and electrical properties and their correlations in different spinel series with chromium are discussed in the frames of the phase diagram of the spinel-type compounds with chromium. Different mechanisms leading to the spin-glass state are discussed and illustrated.     

Photorefractive properties of potassium lithium niobate doped with copper

Potassium lithium niobate doped with copper (Cu:KLN) were grown by the Czochralski method for the first time. The structure of Cu:KLN was measured by the x-ray powder diffraction method, and its lattice constants were obtained. The position of copper ions in KLN crystal was determined. The exponential gain coefficient, response time and erasure time were measured. It was found that the exponential gain coefficient of Cu:KLN is 10.5 cm⁻¹, as two times high as that of KLN, and its response time of 1.53 s is one order of magnitude shorter than that of Cu:LiNbO₃. The type of light exciting carriers in Cu:KLN has been investigated. The result showed that the electron acts the main role in Cu:KLN.     

Electrochemical reactions of lithium with transition metal stannides

High-quality crystalline MSn₂ (M = Cr and Co) thin films have been successfully fabricated by reactive pulsed laser deposition. The physical and electrochemical properties of the as-deposited thin films have been investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), galvanostatic cycling and cyclic voltammetry (CV). XRD measurement indicates that the as-deposited thin films prepared at 400 °C consisted mainly of MSn₂ (M = Cr and Co) with a small quantity of metal tin. The specific reversible capacities of CrSn₂ and CoSn₂ thin film electrodes are found to be 467 mA h/g and 465 mA h/g, respectively. A mechanism involving an irreversible decomposition of MSn₂ (M = Cr and Co) and a classical alloying process of Sn is proposed. MSn₂ (M = Cr and Co) as the starting anode materials for conversion to the Li–Sn alloy can improve its electrochemical performance with high reversible capacity and good stable cycle.     

Electromagnetic properties of lithium zinc ferrites doped with aluminum

We present the results of the effect of Al substitution on the magnetic and electrical properties of Li_0.2Zn_0.6Fe_2.2−xAl_xO₄ ferrites (0.0≤x≤0.5) prepared by the standard ceramic technique. The characterization has been performed using XRD, SEM, magnetic and dielectric response in frequency. XRD analysis confirms that the system exhibits polycrystalline single phase cubic spinel structure only for low dopant content. Doping decreases the dielectric loss tangent and the ferrite conductivity in more than two orders of magnitude in the whole analyzed frequency range. Attenuation has a maximum intensity (86 dB) near 90 MHz for x=0.4. The wider bandwidth at 20 dB (94.6 MHz) is for x=0.3.     

Luminescence of yttrium aluminum borate single crystals doped with manganese

The optical absorption and luminescence spectra of single crystals of yttrium aluminum borate YAl₃(BO₃)₄ doped with manganese ions are measured. It is established that the optical absorption spectra of yttrium aluminum borate single crystals doped with manganese ions are determined primarily by the contribution from the Mn⁴⁺ ions. The luminescence spectra of the YAl₃(BO₃)₄ single crystals doped with manganese exhibit narrow lines attributed to the Mn⁴⁺ ions and an extended multiband structure which is associated primarily with the contribution from the Mn²⁺ ions.     

铁磁性锰氧化物掺杂的ZnO压敏电阻性能研究

利用通常的电子陶瓷制备工艺制备了铁磁性锰氧化物La_07Sr_03MnO₃掺杂的ZnO陶瓷. 晶界处存在La_07Sr_03MnO₃(LSMO)和LaMnO₃(LMO)两种杂相. 样品中绝缘相LMO的含量显著影响着样品的电学性能. 掺杂后的样品仍具有一定的铁磁性. 在样品上施加磁场后,样品电阻值增加,表现为正磁电阻性质. 正磁电阻的出现,是由于磁场的存在 关键词： ZnO 压敏电阻 锰氧化物 正磁电阻     

Laser damage in ferrites of MnZn spinels and other possible interactions

Pulsed laser damage of ferrites of MnZn spinels was studied. Some of the samples were treated by nuclear gamma radiation and α particles and then exposed to laser beams in different working conditions. The sample and damage quality was evaluated on the basis of X-ray, optical and SEM microscopy and micro-hardness experiments. The characteristic brittleness after nuclear gamma irradiation of ferrites and other materials was confirmed in the cases of interactions with laser beams. Characteristic cases of material cracks with CO₂ laser were found.     

Electrochemical performance of the (Ni, Cr) doped spinel material as cathode in lithium cells

The (Ni:Cr) doped lithiated manganese oxide electrode is prepared by wet chemical route using citric acid as the precursor material. The various physical and chemical properties of the prepared electrode material were studied by DSC, XRD, FTIR, Raman and SEM measurements. The electrical conducting property of the electrode was studied by the d.c. conductivity measurements. The electrochemical property was also evaluated by using test cells with the (Ni:Cr) doped spinel electrode. The cycle life was found to be increased upon increasing dopant concentration. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.