Finite size effect and influence of temperature on electrical properties of nanocrystalline Ni–Cd ferrites

Electrical conductivity and dielectric measurements have been investigated for four different average grain sizes ranging from 3 to 7 nm of nanocrystalline Ni_0.2Cd_0.3Fe_2.5−xAl_xO₄ (0.0 ⩽ x ⩽ 0.5) ferrites. The impedance spectroscopy technique has been used to study the effect of grain and grain boundary on the electrical properties of the Al doped Ni–Cd ferrites. The analysis of data shows only one semi-circle corresponding to the grain boundary volume suggesting that the conduction mechanism takes place predominantly through grain boundary volume in the studied samples. The variation of impedance properties with temperature and composition has been studied in the frequency range of 120 Hz–5 MHz between the temperatures 300–473 K. The hopping of electrons between Fe³⁺ and Fe²⁺ as well as hole hopping between Ni³⁺ and Ni²⁺ ions at octahedral sites are found to be responsible for conduction mechanism. The dielectric constant and loss tangent (tan δ) are found to decrease with increasing frequency, whereas they increase with increasing temperature. The dielectric constant shows an anomalous behavior at selected frequencies, while the temperature increases, which is expected due to the generation of more electrons and holes as the temperature increases. The behavior has been explained in the light of Rezlescu model.     

Influence of V2O5 ion addition on the conductivity and grain growth of Ni–Zn–Cu ferrites

Polycrystalline nickel–zinc–copper ferrites with chemical formula Ni_0.6+xZn_0.2Cu_0.2V_xFe_2−2xO₄,(0.0 ⩽ x ⩽ 0.25) were prepared by the ceramic route. The X-ray diffraction (XRD) analysis of the samples results confirms single-phase spinel structure. Scanning electron microscopy (SEM) of the prepared ferrites reveal that vanadium addition resulted in a rapid grain growth with large pores trapped inside the grains as the vanadium concentration increases. The ac conductivity σ_ac has been studied as a function of frequency and temperature over the temperature range (300–600 K). The results obtained for these materials reveal a semiconductor – to semimetal transition as V⁵⁺ content increases. All studies composition exhibit a transition with change in the slope of conductivity. The obtained temperature T_c is found to be decrease with the increasing vanadium content. The hopping of electrons between Fe³⁺ and Fe²⁺ as well as the hole hopping between Ni³⁺ and Ni²⁺ are found to responsible for the conduction mechanism. The relation of the universal exponent s with temperature gives evidence for the presence of the correlation barrier hopping (CHB) mechanism in these compounds. The impedance technique has been used to study effect of grain and grain boundary on the electrical properties. The analysis data show only one semi-circle for all samples except for sample with x = 0.05. The results suggested that the conduction mechanism takes place predominantly through the grain in the studied samples.     

High-resistivity nickel–zinc ferrites by the citrate precursor method

Nickel–zinc ferrites of different compositions, Ni_1−xZn_xFe₂O₄ with x=0.2, 0.35, 0.5 and 0.6, have been prepared by a precursor method involving citrate precursors of the concerned metals and mixing them in solution state. Resistivity has been studied as a function of composition and sintering temperature. It is observed that NiZn ferrites prepared by this method have resistivity ⩾10⁸ Ω cm which is higher by atleast two orders of magnitude than that reported (⩽10⁶ Ω cm) for ferrites prepared by the conventional ceramic method. This is attributed to better purity as well as better compositional and microstructural control achievable by the citrate method. High resistivity makes these ferrites suitable particularly for high-frequency applications where eddy current losses are required to be low.     

Effects of excessive Zn2+ ions on intrinsic magnetic and structural properties of Ni0.2Zn0.6Cu0.2Fe2O4 powder prepared by chemical coprecipitation method

Zn(OH)₂ is a kind of amphoteric compound. Therefore, for chemical coprecipitation method, the precipitation of Zn²⁺ ions may be incomplete if using NaOH as precipitator. In this study, single-phase powder specimens with a nominal composition Ni_0.2Zn_0.6Cu_0.2Fe₂O₄ were prepared with chemical coprecipitation method, and the effects of excessive Zn²⁺ content (x, x = 3%, 5%, 7%, 9%) in working solution on intrinsic magnetic and structural properties were studied by vibrating sample magnetometer and X-ray diffractometer, respectively. It was found that the magnetization when H_m = 398 kA/m (5000 Oe) reached a maximum when x = 5%, and then decreased with the increase of x, which was attributed to the effect of different amount of Zn²⁺ in A sites on the A–B and B–B exchange interaction. Moreover, it was found that the lattice parameter was affected by the Zn²⁺ and Fe³⁺ ions due to their different ion radius to a certain extent.     

A comparative study of different concentrations of pure Zn powder effects on synthesis,structure, magnetic and microwave-absorbing properties in mechanically-alloyed Ni–Zn ferrite

In this study, a powder mixture of Zn, Fe₂O₃ and NiO was used to produce different compositions of Ni_1−xZn_xFe₂O₄ (x=0.36, 0.5 and 0.64) nanopowders. High-energy ball milling with a subsequent heat treatment method was carried out. The XRD results indicated that for the content of Zn, x=0.64 a single phase of Ni–Zn ferrite was produced after 30 h milling while for the contents of Zn, x=0.36 and 0.5, the desired ferrite was formed after sintering the 30 h-milled powders at 500 °C. The average crystallite size decreased with increase in the Zn content. A DC electrical resistivity of the Ni–Zn ferrite, however, decreased with increase in the Zn content, its value was much higher than those samples prepared by the conventional ceramic route by using ZnO instead of Zn. This is attributed to smaller grains size which were obtained by using Zn. The FT-IR results suggested two absorption bands for octahedral and tetrahedral sites in the range of 350–700 cm⁻¹. The VSM results revealed that by increasing the Zn content from 0.36 to 0.5, a saturation magnetization reached its maximum value; afterwards, a decrease was observed for Zn with x=0.64. Finally, magnetic permeability and dielectric permittivity were studied by using vector network analyzer to explore microwave-absorbing properties in X-band frequency. The minimum reflection loss value obtained for Ni_0.5Zn_0.5Fe₂O₄ samples, about −34 dB at 9.7 GHz, making them the best candidates for high frequency applications.     

The initial growth structure of Ni1-xFex (x=0.6–0.8) films dc-biased plasma-sputter-deposited on Ni/MgO(0 0 1), and on Fe/MgO(0 0 1)

Cross sectional and plane-view transmission electron microscopy (X- and PV-TEM) were used to investigate the initial growth phase of 5, 10, 20 and 40 nm thick Ni_1-xFe_x (x=0.6–0.8) films, prepared on MgO(0 0 1) covered with a buffer layer of Fe or Ni as well as on naked MgO(0 0 1). The 100 nm thick buffer layers of Fe and Ni were pre-grown on MgO(0 0 1). All of Ni_0.20Fe_0.80, Ni_0.40Fe_0.60, Fe and Ni films could be epitaxially grown at 250°C by dc-biased plasma sputtering at 2.9 kV in pure Ar gas.The films of Ni_0.20Fe_0.80 and Ni_0.40Fe_0.60 were grown in their own stable phase, bcc and fcc on MgO(0 0 1), respectively. However, Ni_0.20Fe_0.80 film could be grown in fcc phase pseudomorphic with Ni(0 0 1) up to 20 nm thick on Ni/MgO(0 0 1), while Ni_0.40Fe_0.60 film in bcc phase pseudomorphic with Fe(0 0 1) up to 10 nm thick on Fe/MgO(0 0 1). With increasing thickness, their growth phases transformed into their own stable phases. Whether or not the pseudomorphic phase may be induced and what its critical thickness may be should depend primarily on the lattice misfit between the crystal planes in contact. The growth mode of Ni_0.40Fe_0.60 film was investigated more in details to be compared with the simulations of the average strain energy versus thickness and with those of the critical thickness of the pseudomorphic films versus the lattice misfit between the contacted crystal planes.     

Ferromagnetic order and spin-glass behavior in multi-metallic compound Ni1.125Co0.375[Fe(CN)6] · 6.8H2O

Multi-metallic Prussian blue compound Ni_1.125Co_0.375[Fe(CN)₆] · 6.8H₂O has been synthesized. The Mössbauer spectroscopy at room temperature and IR spectra study revealed that the metal ions are bonded through cyanide ligand and the presence of low spin Fe^III(S = 1/2) and high spin Fe^III(S = 5/2) ions, as showed in these structure: Fe^III(S = 1/2)-CN-(Co^II/Ni^II)(96%) and Fe^III(S = 5/2)-NC-(Co^II/Ni^II) (4%). The Curie constant of C = 3.00 cm³ K mol⁻¹ and Weiss paramagnetic Curie temperature of θ = 16.43 K were observed in fitting according to Curie–Weiss law. These results indicate that there existed a ferromagnetic exchange interaction in the complexes. The observed value of coercive field (H_c) and remanent magnetization (M_r) at 4 K for the compound are 497 Oe and 1.03 Nβ. The presence of spin-glass behaviours in the compound is ascribed mainly to domain mobility or domain growth under different cooling conditions.     

Influences of Al doping concentration on structural,electrical and optical properties of Zn0.95Ni0.05O powders

Nanocrystalline Zn_0.95−xNi_0.05Al_xO (x = 0.01, 0.02, 0.05 and 0.10) diluted magnetic semiconductors have been synthesized by an auto-combustion method. X-ray diffraction measurements indicate that all Al-doped Zn_0.95Ni_0.05O samples have the pure wurtzite structure. Transmission electron microscope analyses show that the as-synthesized powders are of the size 40–45 nm. High-resolution transmission electron microscope, energy dispersive spectrometer and X-ray photoemission spectroscope analyses indicate that Ni²⁺ and Al³⁺ uniformly substitute Zn²⁺ in the wurtzite structure without forming any secondary phases. The Al doping concentration dependences of cell parameters (a and c), resistance and the ratio of green emission to UV emission have the similar trends.     

Jump rate and linear distance of vacancy on the structure and electrical conductivity of Ni0.65Zn0.35CuxFe2−xO4 ferrites

Ferrite compositions of Ni_0.65Zn_0.35Cu_xFe_2−xO₄ (0⩽x<1) were examined using X-ray analysis. The effect of the linear distance of vacancy jumping on the lattice parameter was studied. The jump rate of vacancy increased with increasing Cu concentration. The increase of jump rate of vacancy enhanced the linear distance which increased the conductivity and mobility of the charge carriers. The majority of charge carriers of our systems are holes. The estimated linear distance of each jump was 2.86×10⁻⁷ m. The decrease of thermal conductivity was attributed to the increase of the jump rate and also the linear distance. The formation of oxygen vacancies during the substitution of Cu²⁺ ions for Fe³⁺ ions helped the internal stress to decrease the lattice parameter. Because the ionic radius of O²⁻ (0.136 nm) is larger than that of Fe³⁺ (0.067 nm) ion.     

Conductivity and oxygen permeability of a novel oxide Pr2Ni0.8 − x Cu0.2FexO4 and its application to partial oxidation of CH4

Iron-doped Pr₂Ni_0.8Cu_0.2O₄ was studied as a new mixed electronic and oxide-ionic conductor for use as an oxygen-permeating membrane. An X-ray diffraction analysis suggested that a single phase K₂NiF₄-type structure was obtained in the composition range from x = 0 to 0.05 in Pr₂Ni_0.8 − xCu_0.2Fe_xO₄. It is considered that the doped Fe is partially substituted at the Ni position in Pr₂NiO₄. The prepared Pr₂NiO₄-based oxide exhibited a dominant hole conduction in the P_O2 range from 1 to 10^− 21 atm. The electrical conductivity of Pr₂Ni_0.8−xCu_0.2Fe_xO₄ is as high as 10² S cm^− 1 in the temperature range of 873–1223 K and it gradually decreased with the increasing amount of Fe substituted for Ni. The oxygen permeation rate was significantly enhanced by the Fe doping and it was found that the highest oxygen permeation rate (60 μmol min^− 1 cm^− 2) from air to He was achieved for x = 0.05 in Pr₂Ni_0.8 − xCu_0.2Fe_xO₄. Since the chemical stability of the Pr₂NiO₄-based oxide is high, Pr₂Ni_0.75Cu_0.2Fe_0.05O₄ can be used as the oxygen-separating membrane for the partial oxidation of CH₄. It was observed that the oxygen permeation rate was significantly improved by changing from He to CH₄ and the observed permeation rate reached a value of 225 μmol min^− 1 cm^− 2 at 1273 K for the CH₄ partial oxidation.     

Effect of Ti4+ ions doping on microstructure and dc resistivity of nickel ferrites

Nanostructured nickel ferrites (NiFe₂O₄) were prepared by doping with Ti⁴⁺ ions using solid-state reaction route. Lowest grain size of 55 nm was achieved in the specimens with 20 mole% TiO₂ doping. Magnetization in the specimens decreases with decreasing grain sizes. Lower volume fractions of ferrite phase due to dissociation of the magnetic phase into smaller particles by the disruption of super exchange interaction by the titanium substitution results a decrease in magnetizations. Coercivity showed an increasing trend. This was explained as arising due to multidomain/monodomain magnetic behavior of magnetic nanoparticles. Small polaron hopping conduction between Fe²⁺ and Fe³⁺ sites controls the dc electrical properties of the specimens. The presence of an interfacial amorphous phase between the sites is evident from Mott's analysis. Specimens containing 10 mole or more TiO₂ and sintered at 1350 °C contain NiTiO₃ as a secondary phase and show unusual dc conductivity.     

Impacts of neodymium on structural,spectral and dielectric properties of LiNi0.5Fe2O4 nanocrystalline ferrites fabricated via micro-emulsion technique

Soft ferrites are technologically advanced smart materials and their properties can be tailored by controlling the chemical composition and judicial choice of the metal elements. In this article we discussed the effect of rare earth neodymium (Nd³⁺) on various properties of LiNi_0.5Nd_xFe_2−xO₄ spinel ferrites. These ferrites have been synthesized by facile micro-emulsion route and characterized by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), a.c. electrical conductivity and thermal analysis. The influence of Nd³⁺ doping on structural and electrical parameters has been investigated. XRD analysis revealed the formation of single cubic spinel structure for x≤0.07. Few traces of secondary phase (NdFeO₃) were found for x≥0.105. The secondary phase induced owing to the solubility limit of Nd³⁺ cations in these ferrites. The lattice parameter (a) and crystallite size (D) both exhibit non-linear relation. The values of “a” and “D” were found in the range 8.322–8.329 Å and 25–32 nm respectively. These variations were attributed to the larger ionic radius of Nd³⁺ cations as compared to the host cations and lattice strain produced in these ferrites. The dielectric parameters were studied in the range 1 MHz to 3 GHz and these parameters were damped by Nd³⁺ incorporation and also by increasing the frequency. The reduced dielectric parameters observed in wide frequency range proposed that these nanocrystalline ferrites are potential candidates for fabricating the devices which are required to operate at GHz frequencies.     

Effect of Nd ion on the magnetic properties of Ni–Mn ferrite nanocrystal

The nanocrystalline Ni_0.7Mn_0.3Nd_0.1Fe_1.9O₄ ferrites were prepared by the emulsion method. X-ray diffractometer (XRD), transmission electron microscope (TEM), Mössbauer spectra and vibrating samples magnetometer (VSM) were used to study the structure, morphology and magnetic properties. The magnetic properties of Nd³⁺-doped Ni_0.7Mn_0.3Fe₂O₄ nanocrystal ferrites were investigated in detail. The Ni_0.7Mn_0.3Nd_0.1Fe_1.9O₄ nanocrystal ferrite with particle size of 10.7 nm shows superparamagnetism.     

Mixed conductivity,stability and thermomechanical properties of Ni-doped La(Ga,Mg)O3−δ

Perovskite-type LaGa_0.65Mg_0.15Ni_0.20O_3−δ exhibiting oxygen transport comparable to that in K₂NiF₄-type nickelates was characterized as a model material for ceramic membrane reactors, employing mechanical tests, dilatometry, oxygen permeability and faradaic efficiency measurements, thermogravimetry (TG), and determination of the total conductivity and Seebeck coefficient in the oxygen partial pressure range from 10^− 15 Pa to 40 kPa. Within the phase stability domain which is similar to La₂NiO_4+δ, the defect chemistry of LaGa_0.65Mg_0.15Ni_0.20O_3−δ can be adequately described by the ideal solution model with oxygen vacancies and electron holes to be the only mobile defects, assuming that Ni²⁺ may provide two energetically equivalent sites for hole location. This assumption is in agreement with the density of states, estimated from thermopower, and the coulometric titration and TG data suggesting Ni⁴⁺ formation in air at T < 1150 K. The hole conductivity prevailing under oxidizing conditions occurs via small-polaron mechanism as indicated by relatively low, temperature-activated mobility. The ionic transport increases with vacancy concentration on reducing p(O₂) and becomes dominant at oxygen pressures below 10^− 7–10^− 5 Pa. The average thermal expansion coefficients in air are 11.9 × 10^− 6 and 18.4 × 10^− 6 K^− 1 at 370–850 and 850–1270 K, respectively. The chemical strain of LaGa_0.65Mg_0.15Ni_0.20O_3−δ ceramics at 1073–1123 K, induced by the oxygen partial pressure variations, is substantially lower compared to perovskite ferrites. The flexural strength determined by 3-point and 4-point bending tests is 167–189 MPa at room temperature and 85–97 MPa at 773–1173 K. The mechanical properties are almost independent of temperature and oxygen pressure at p(O₂) = 1–2.1 × 10⁴ Pa and 773–1173 K.     

M/Li+(M = Mg2+, Zn2+, and Mn2+) ion-exchange on lithium ion-conducting perovskite-type oxides and their properties

Lithium ions of perovskite-type lithium ion conductor La_0.55Li_0.35TiO₃ were replaced by divalent Mg²⁺, Zn²⁺, and Mn²⁺ ions in an ion-exchange reaction using molten chlorides. The polycrystalline Mg-exchanged and Zn-exchanged samples are solid electrolytes for divalent Mg²⁺ and Zn²⁺ ions, whose dc ionic conductivities (σ = 2.0 × 10^− 6 S cm^− 1 at 558 K for the Mg-exchanged sample, La_0.56(2)Li_0.02(1)Mg_0.16(1)TiO_3.01(2) and σ = 1.7 × 10^− 6 S cm^− 1 at 708 K for the Zn-exchanged samples, La_0.55(1)Li_0.0037(2)Zn_0.15(1)TiO_2.98(2)) were compared to those of the known highest Mg²⁺ and Zn²⁺ inorganic solid electrolytes. The Mn-exchanged sample, then, showed paramagnetic behavior in the temperature range of 2 to 300 K. The Mn ions in the exchanged sample are divalent and the spin configuration is in high spin state (S = 5/2).     

Conductometric study of complex formations between some substituted pyrimidines and some metal ions in acetonitrile and the determination of thermodynamic parameters

The complexation reactions between Cr³⁺, Cd²⁺, Co²⁺, Ni²⁺, Hg²⁺, Zn²⁺, Pb²⁺,and Ag⁺ metal ions with diethyl-3-(4-hydroxy-6-methylpyrimidin-2-yl) guanidine (L1), 2-amino,4-hydroxy,6-methylpyrimidine (L2), and 2-(diethylamin),4-hydroxy,6-methylpyrimidine (L3) in acetonitrile (AN) were studied using a conductometric method. The formation constants of the resulting complexes were determined from computer fitting conductance-mole ratio data. The results revealed that the formation constants of L1 with metal ions are varying in order of Cr³⁺ > Pb²⁺ > Zn²⁺ > Co²⁺ > Hg²⁺ > Ag⁺ ~ Cd²⁺ > Ni²⁺. For L2 and L3 the sequence of complex formation constants are follow as Cr³⁺ > Pb²⁺ > Zn²⁺ > Co²⁺ > Cd²⁺ > Hg²⁺ > Ag⁺ > Ni²⁺ and Cr³⁺ > Pb²⁺ > Zn²⁺ > Co²⁺ > Ni²⁺ ~ Hg²⁺ ~ Cd²⁺ ~ Ag⁺, respectively. The values of the thermodynamic parameters (ΔH, ΔS and ΔG) for complexation reactions were obtained from the temperature dependence of the stability constants. In all cases, the complexes were found to be enthalpy stabilized but entropy destabilized. In addition, some ab-initio quantum-mechanical calculations were carried out, in order to obtain a clue about the degree tendency of ligands to metal ions.     

Growth mode of Pt1−xNix films biased dc-sputter-deposited on MgO(0 0 1)

A study is made by TEM, XRD and by measuring electrical/magnetic properties, of growth mode and properties of Pt_1−xNi_x alloy films deposited on MgO(0 0 1) at 250°C by dc-sputtering at 2.5–2.7 kV in Ar. A bias voltage V_s≤−160 V was applied to the substrate during deposition. It was confirmed that the Pt film was polycrystalline with the texture of Pt(1 1 1)/MgO(0 0 1) while the films of Pt_0.14Ni_0.86 and Pt_0.19Ni_0.81 were epitaxially grown with Pt–Ni(0 0 1)[1 0 0]/MgO(0 0 1)[1 0 0] similarly to the case of Ni/MgO(0 0 1). Thus the growth mode transformation between Pt–Ni(1 1 1)/MgO(0 0 1) and Pt–Ni(0 0 1)/MgO(0 0 1) may be induced at x less than 0.81 for Pt_1−xNi_x alloy films. The temperature coefficient of resistance TCR from 100 to 300 K of Pt_0.14Ni_0.86 films was estimated to be 0.0044–0.0053 K⁻¹ and saturation magnetization at 300 K to be 1.7–3.2 kG, respectively, while TCR of Pt films was estimated to be 0.0035–0.0048 K⁻¹.     

BaCO3 mediated modifications in structural and magnetic properties of natural nanoferrites

Preparing M-type barium hexaferrite and improving the magnetic response of natural ferrites by incorporating barium carbonate (BaCO₃) is ever-demanding. Series of barium carbonate doped ferrites with composition (100−x)Fe₃O₄·xBaCO₃ (x=0, 10, 20, 30 wt%) are prepared through solid state reaction method and sintered gradually at temperatures of 800 and 1000 °C. Nanoparticles of natural ferrite and commercial BaCO₃ are used as raw materials. Impacts of BaCO3 on structural and magnetic properties of these synthesized ferrites are inspected. The obtained ferrites are characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) at room temperature. Uniform barium hexaferrite particles in terms of both morphology and size are not achieved. The average crystallite size of BaFe₁₂O₁₉ is observed to be within 30–600 nm. The sintering process results phase transformation from Fe₃O₄ (magnetite) to α-Fe₂O₃ (hematite) and the formation of hexagonal barium ferrite crystals. The occurrence of barium crystal is found to enhance with the increase of BaCO₃ concentrations up to 20 wt% and suddenly drop at 30 wt%. Saturation and remanent magnetization of the doped ferrites are significantly augmented up to 16.37 and 8.92 emu g⁻¹, respectively compared to their pure counterpart. Furthermore, the coercivity field is slightly decreased as BaCO₃ concentrations are increased. BaCO₃ mediated improvements in the magnetic response of natural ferrites are demonstrated.     

Epitaxial alloyed films out of the bulk stability domain: Surface structure and composition of Ni3Al and NiAl films on a stepped Ni(1 1 1) surface

We have studied by Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) and Auger Electron Spectroscopy (AES) Ni–Al alloyed layers formed by annealing, around 780 K, Al deposits on a stepped Ni(1 1 1) surface. The surface structure and composition of the thin epitaxial Ni₃Al and NiAl films, obtained respectively below and above a critical Al initial coverage θ_c, differ markedly from those of corresponding bulk alloys.The Ni₃Al ordered films form in a concentration range larger than the stability domain of the L1₂ Ni₃Al phase. The NiAl films present a marked distortion with respect to the lattice unit cell of the B2 NiAl phase, which slowly decreases when the film thickness increases.It also appears that the value of θ_c depends on the morphology of the Ni(1 1 1) substrate, increasing from θ_c = 4.5 ML for a flat surface to θ_c = 10 ML for a surface with a miscut of 0.4°. This could be directly related to the presence of steps, which favour Ni–Al interdiffusion.     

The studies on structural and thermal properties of delithiated LixNi1/3Co1/3Mn1/3O2 (0 < x ≤ 1) as a cathode material in lithium ion batteries

The structural and thermal properties of the delithiated Li_xNi_1/3Co_1/3Mn_1/3O₂ (0 < x ≤ 1) material have been investigated by using diffraction and thermoanalytical techniques such as XRD and TG-DSC methods. XRD result shows that the delithiated materials maintain the O3-type structure with defined stoichiometric number at the range of 0.24 < x ≤ 1, exhibiting good crystal structural stability. The cobalt and nickel ions in the delithiated materials change their valence state (i.e. Co³⁺ to Co⁴⁺ and Ni³⁺ to Ni⁴⁺) when x < 0.49; the irreversible changes of the transformation may affect the first cycle of charge–discharge efficiency of the materials. A comparison of the results of TG-DSC with TPD-MS shows that the irreversible change of oxygen species during the delithiation process of Li_xNi_1/3Co_1/3Mn_1/3O₂ have great influence on the structural and thermal stability and reversibility of the materials.