Structural and electrical characterization of MxNb3Se4 (M = Cu, H)

The compounds Cu_xNb₃Se₄ (0≤x≤0,45) and H_xNb₃Se₄ (0≤x≤2·10⁻³) were prepared by electrochemical titration from Nb₃Se₄. The samples were characterized by X-ray analysis and q-probe conductivity measurements as a function of temperature. The Cu-compound is isostructural with Nb₃Se₄ for 0≤x≤0.2 and shows new phases for 0,2≤x≤0,45. The H-compound shows an impurity controlled conductivity in the temperature range from 20 to 200 °C and an intrinsic type conductivity in the temperature range from 330 to 450 °C. The activation engines are 0 and 0.15 eV, respectively. Rapid proton conduction in H_xNb₃Se₄ makes it difficult to control the composition as demonstrated by exposure of the samples to different atmospheres. An increasing H-concentration decreases drastically the conductivity by several orders of magnitude.     

Phase equilibria in the La-Me-Co-O (Me=Ca, Sr, Ba) systems

The phase equilibria of the La-Me-Co-O systems (Me = Ca, Sr and Ba) were studied in air at 1100 °C. Two types of solid solution of general composition La_1−xMe_xCoO_3−δ and (La_1−y Me_y)₂CoO₄ were found to exist in the systems. The limiting composition of La_1−xMe_xCoO_3−δ lies at x=0.8 for Me = Sr, Ba and between 0.3–0.5 for Me = Ca. It is shown that the rhombohedral distortion of the perovskite type La_1−xMe_xCoO_3−y decreases while x increases. La_1−xMe_xCoO_3−δ (Me = Sr, Ba) shows an ideal cubic structure at x=0.5. The stability range of (La_1−yMe_y)₂CoO₄ was found to be 0.25≤y≤0.35 for Me = Ca, 0.3≤y≤0.55 for Me = Sr and 0.3≤y≤0.375 for Me = Ba. All phases have tetragonal K₂NiF₄-type crystal structure. Based on the XRD and neutron diffraction patterns of quenched samples, the phase diagrams (Gibbs triangles) are constructed for all systems. The phase equilibrium at low oxygen pressure is shown for the example of the La-Sr-Co-O system. The decomposition mechanism of La_1−xSr_xCoO_3−δ at 1100 °C for the samples with 0.5<x<0.8 within the oxygen pressure range −0.678>log(Po₂)>−2.25 can be written as follows: La_1−x′ Sr_x′CoO_3−δ′=n La_1−x″Sr_x″CoO_3−δ″+m SrCoO_2.5+q/2 O₂ where x′>x″. The decomposition mechanism of La_1−xSr_xCoO_3−δ for the samples with x < 0.5 within the oxygen pressure range −2.25>log(Po₂)>−3.55 changes and can be written as follows: La_1−xSr_xCoO_3−δ′=r La_1−x′Sr_x′CoO_3−δ″+w (La_1−y′Sr_y′)₂CoO₄+v CoO+f/2 O₂. The results are shown in “logPo₂-composition” diagrams. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Ionic conductivity and structural properties of MnO-doped Bi4V2O11 system

Bi₄V₂O₁₁ exists in three phases viz. α, β, and γ. High temperature γ-phase can be stabilized to room temperature owing to its higher conductivity by the partial substitution of metallic cations for vanadium in Bi₄V₂O₁₁. Phase transitions from α → β and β → γ are composition and temperature-dependent. Mn²⁺-doped compounds Bi₄V_2−x Mn _x O_{11− δ} (0 ≤ x ≤ 0.4) have been synthesized by solid state reaction technique and investigated by X-ray diffraction and ionic conductivity measurement. High ionic conducting γ-phase is stabilized for x ≥ 0.2. The ionic conductivity of the series of Bi₄V_2−x Mn _x O_{11− δ} samples has been measured by using ac impedance spectroscopy technique. The conductivity data do show departure from its simple Arrhenius behavior for all of the compositions. The highest conductivity observed for x = 0.2 sample can be attributed to lower activation energy.     

Ionic and electronic transport in perovskite-type La(Ga,M)O3−δ (M=Mg, Cr, Fe, Co, Ni, Nb)

Oxygen ion conduction in La_0.9Sr_0.1Ga_1−xM_xO_3−δ (M=Cr, Fe; x=0 – 0.20), LaGa_1−xM_xO_3−δ (M=Co, Ni; x=0.20 – 0.60), LaGa_1−x−yCo_xMg_yO_3−δ (x=0.35 – 0.60; y=0.10 – 0.25) and LaGa_0.85−xMg_0.15(Nb_0.33Mg_0.66)_xO_3−δ (x=0 – 0.20) is reported. At temperatures below 1200 K the ionic conductivity of La(Ga,M)O_3−δ (M=Co, Ni) increases with increasing oxygen nonstoichiometry, but is lower than for La(Ga,Mg)O_3−δ and (La,Sr)GaO_3−δ. Co-doping with Nb and Mg was found to result in decreasing ionic transport in La(Ga,Nb,Mg)O_3−δ due to blocking of oxygen sites by Nb⁵⁺. Small additions of Fe to the B-site of La_0.9Sr_0.1GaO_3−δ increase the ionic conductivity, whereas substitution of Cr for Ga has the opposite effect. Incorporation of transition metal cations into the Ga site leads to a higher p-type electronic conductivity in all studied perovskites. Paper presented at the 6th Euroconference on Solid Sate Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Vibrational studies of spinel LixMn2O4 (0.3≤x≤1.0) cathodes

We report on the vibrational properties of spinel LiMn₂O₄ and its electrochemically delithiated forms Li_xMn₂O₄. Raman scattering and infrared absorption spectra have been studied as a function of the delithiation content in the wavenumber range 50–700 cm⁻¹. Results show that lithium ions can be extracted at room temperature to obtain Li_x[Mn₂]O₄ (0.3≤x≤1.0) without disrupting the [Mn₂]O₄ array. The normal modes of the spinel LiMn₂O₄ have been discussed in the O _h ⁷ symmetry and vibrations due to lithium ions with their oxygen neighbors have been identified at ca. 400 cm⁻¹. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Cd x Hg 1−x−y Mn y Se solid solutions are promising materials for filters operating in the near- and middle-IR regions of the spectrum

Transmission spectra of new fourfold semimagnetic semiconductors Cd_xHg_1−x−yMn_ySe (x≤0.6; y≤0.01–0.1) at 77 and 300 K and the effect of annealing of them in vapors of the components on their optical properties were investigated. It is shown that Cd_xHg_1−x−yMn_ySe crystals possess high light transmission (≥60%) and can be used for devices operating in the near- and middle-IR regions of the spectrum. Chernovtsy State University, 2, Kotsyubinskii Str., Chernovtsy, 274012, Ukraina. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 282–286, March–April, 1999.     

Electrochemical behavior of Li intercalation processes into a Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 cathode

Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ (X=0.17, 0.25, 0.33, 0.5) compounds are prepared by a simple combustion method. The Rietvelt analysis shows that these compounds could be classified as having the α-NaFeO₂ structure. The initial charge-discharge and irreversible capacity increases with the decrease of x in Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂. Indeed, Li[Ni_0.50Mn_0.50]O₂ compound shows relatively low initial discharge capacity of 200 mAh/g and large capacity loss during cycling, with Li[Ni_0.17Li_0.22Mn_0.61]O₂ and Li[Ni_0.25Li_0.17Mn₀.₅₈]O₂ compounds exhibit high initial discharge capacity over 245 mAh/g and stable cycle performance in the voltage range of 4.8 -2.0 V. On the other hand, XANES analysis shows that the oxidation state of Ni ion reversibly changes between Ni²⁺ and about Ni³⁺, while the oxidation state of Mn ion sustains Mn⁴⁺ during charge-discharge process. This result does not agree with the previously reported ‘electrochemistry model’ of Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂, in which Ni ion changes between Ni²⁺ and NI⁴⁺. Based on these results, we modified oxidation-state change of Mn and Ni ion during charge-discharge process.     

Synthesis and conductivities of the apatite-type systems, La9.33+xSi6−yMyO26+z (M=Co, Fe, Mn) and La8Mn2Si6O26

Apatite-type oxides of formula (La/Sr)_10−xSi₆O_26+y have been attracting significant interest recently, because of their high oxide ion conductivity. In this paper we report the synthesis and conductivities of phases based on doping La_9.33Si₆O₂₆ with Co, Fe, Mn on the Si site, according to the formula La_9.33+x/3Si_6−xM_xO₂₆ (M=Co, Fe, Mn). Substitution limits observed were x≤1.5 (Co), x≤1.25 (Fe), x≤0.5 (Mn). Higher Mn levels could be achieved by substituting onto the La site, with it being possible to prepare the phase La₈Mn₂Si₆O₂₆. The highest conductivities were observed for the Co doped samples, although investigations into the dependence of conductivity on p(O₂) (0.2–10⁻⁵ atm.) indicated that the conductivity was dominated by the electronic component in these cases. In contrast, the conductivities for the Fe and Mn doped samples were mainly ionic in the same p(O₂) range. Experiments into varying the oxygen content of these doped phases indicated that increasing the oxygen content above the nominally stoichiometric O₂₆ appears to increase the oxide ion conductivity. Preliminary studies of the reactivity of the electrolyte La_9.33Si₆O₂₆ with potential SOFC cathode materials (La_1−xSr_xMO₃; M=Co, Fe, Mn) suggests that reaction can occur at high temperatures leading to the incorporation of the transition metal into the apatite electrolyte. However, the fact that these doped phases exhibit high conductivities suggests that this may limit any problems caused by such a reaction at the electrolyte-electrode interface. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Co-precipitation synthesis and performance of multi-doped LiCrxNixMn2-2xO4-zFz cathode materials for lithium ion batteries

Multiple ion-doped lithium manganese oxides LiCr_xNi_xMn_2-2xO_4-zF_z (0 < x ≤ 0.25, z =  0.05, 0.1) with a spinel structure and space group Fd m were prepared by using the co-precipitation procedure carried out in water–alcohol solvent using adipic acid as the chelating agent. The electrochemical measurements indicated that the charge/discharge capacities of the samples prepared at 600 °C are higher than that of the treatment at 800 °C or microwave heating. The capacitance-voltage (CV) curves of LiCr_xNi_xMn_2-2xO_4-zF_z (0 < x ≤ 0.25, z = 0.05, 0.1) showed that when x ≤ 0.1, the samples had two reduction–oxidation peaks at 4.0 to 4.2-V region, whereas when x > 0.1, the samples had only one reduction–oxidation peak at 4.0- to 4.2-V region in CV measurements and could offer more stable voltage plateau in a 4-V region and also had stable electrical conductivity after 20 cycles. Another reduction–oxidation peak appeared in 4.6-4.8-V region (Ni²⁺–Ni⁴⁺ reduction–oxidation peaks); this suggests that the LiCr_xNi_xMn_2-2xO_4-zF_z (0.1 < x≤ 0.25, z = 0.05, 0.1) cathode material could offer 4.6 to 4.8-V charge/discharge plateaus, and its specific capacity increases with increasing Ni²⁺. The impedance measurements of the cell proved that the F⁻ anion doped can not only prevent Mn³⁺ from disproportion but also can prevent the passivation film from forming and can help keep stable the cell’s electrical properties. The LiCr_0.05Ni_0.05Mn_1.9O_3.9F_0.1 sintered at 600 °C shows the best cycle performance and the largest capacity in all prepared samples; its first discharge capacity is 120 mAh/g, and the discharge capacity loses only 1.78% after 20 cycles. After 100 cycles, it still remains in the spinel structure.     

The NASICON solid solution Li1−x La x /3Zr2(PO4)3: optimization of the sintering process and ionic conductivity measurements

The Li_1−x La _{x /3}Zr₂(PO₄)₃ NASICON-type compounds (0 ≤ x ≤ 1) have been synthesized in powder form by a sol-gel method and sintered for ionic conductivity measurements. In order to improve the compactness of the ceramic without decomposition of the compound, several sintering processes have been tested for one member of the solid solution (x = 0.6): the use of sintering aids (ZnO, B₂O₃, TiO₂ and LiNO₃), a ball-milling of the synthesized powder, a flash heating, high isostatic pressure, and spark plasma sintering. Finally, a satisfactory compactness of 85% is obtained compared to the referenced value (63%) obtained by uniaxial and isostatic pressing. The ionic conductivity study was performed by impedance spectroscopy. It shows that, despite the formation of vacancies, the substitution Li⁺→ 1/3 La³⁺ + 2/3 □ has unfortunately no influence on the conduction for 0 ≤ x ≤ 0.7 since the ionic conductivity remains identical to the LiZr₂(PO₄)₃ one. For higher x values, the ionic conductivity strongly decreases.     

Anomalous electrical behavior in ferromagnetic La1−xBaxCoO3−δ materials (0<x≤0.50)

We prepared in this work La_1−xBa_xCoO_3−δ (0<x≤0.50) compounds using the nitrate decomposition method, and focus in the study of the transport properties of the ferromagnetic compounds (x>0.15, T_c≈200 K) in the temperature range 77 K≤T≤300 K. We find that while for x<0.20 these materials show semiconducting behavior, their electrical conduction being dominated by small-polaron holes, for x>0.20 they show metallic behavior for T>100 K, with a transition to semiconducting behavior being observed for T ≤ 100 K. For x=0.20, small changes in the oxygen stoichiometry of the samples - due to slight variations in the thermal treatments - greatly affect the transport properties of the materials that can show either two metal-insulator transitions as the temperature increases or a semiconducting behavior. Very interestingly all these ferromagnetic samples are very sensitive to the polarity of the applied electrical current, and display peculiar “diodic” behaviors. All these observations are explained on the basis of an inhomogeneous electronic structure in the Ba-doped cobalt perovskites. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Band structure and dominating electron-scattering mechanisms in Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis>−<Emphasis Type="Italic">y</Emphasis></Subscript>Mn<Subscript>x</Subscript>Fe<Subscript>y</Subscript>Se

Magnetic and kinetic properties as well as transmission and absorption spectra of Hg _1−x−y Mn _x Fe _ySe (0.09 ≤ x ≤ 0.099 and 0.001 ≤ y ≤ 0.01) crystals are investigated at H ≈ 0.5–6 kOe in the temperature range T = 77–300 K. The band parameters are determined on the basis of experimental data. It is found that in the crystals under study at T ≈ 300 K, electron scattering by polar optical phonons dominates, direct optical band-to-band transitions occur, and replacement of a part of Mn atoms by Fe for x + y = 0.1 results in an increase in E_g ^op with Fe content. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 35–39, March, 2007.     

Kinetic properties of Ni<Subscript>3</Subscript>Al<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys in the range of transition from band to spin-localized ferromagnetism

The kinetic properties of ferromagnetic Ni₃Al_{1 − x} Mn _x alloys with _x ≤ 0.6 are studied at T ≤ 800 K and H ≤ 7 MA/m. The behavior of the electrical resistivity, the thermopower, the magnetoresistance, the Hall effect, and the spontaneous Hall effect are analyzed in the range of transition from band (Ni₃Al) to spinlocalized (Ni₃Mn) ferromagnetism at x ∼ 0.15.     

Synthesis and electrical characterisation of the perovskite niobate-titanates,Sr1−x/2Ti1−xNbxO3−δ

The synthesis and electrical characterisation over a range of oxygen partial pressures (10⁻²⁰ to 1 atm) are reported for the cubic perovskite niobate-titanates Sr_1−x/2Ti_1−xNb_xO_3−δ, which are proposed as potential anode materials for solid oxide fuel cells. Single phase samples were observed for 0≤x≤0.4, and phase purity was retained on annealing at both high and low oxygen partial pressures. Good electrical conductivity was observed on reduction in low oxygen partial pressures, with a maximum for the sample with 25% Nb (x=0.25), σ=5.6 Scm⁻¹ at 930°C (P (O₂)=10⁻¹⁸ atm). For dense samples the higher the Nb content the more resistant the reduced sample was to reoxidation as the oxygen partial pressure was increased. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Photoluminescent analysis of Zn1-xCdxO alloys

The photoluminescent (PL) spectra of Zn_1-xCd_xO (0≤x≤0.53) alloy films were obtained successfully. A new explanation from the viewpoint of band structure is brought forward to comprehend the PL nature of the alloy films. According to this explanation, the near-band-energy emissions of the Zn_1-xCd_xO (x>0) films are caused by the radiative transitions between the Zn4s–Cd5s hybrid level and the O2p level, and the broadenings of the two levels are responsible for the gradually increased line width of the PL peak of the film; Zn3d and Cd4d orbital levels have great effects on the band-gap variations of the alloys. In addition, a quadratic equation is put forward to depict the relationship between the band gaps E_g of the alloys and their Cd contents x, i.e. E_g(x)=3.30-1.22x+1.26x² (0≤x≤0.53). PACS 78.55.-m; 78.55.Et; 81.15.Cd     

Structural and conductivity investigations of doped Li-titanates

Polycrystalline lithium lanthanum titanates, Li_xLa_yTiO₃ (0<x<0.5, 0.5<y<0.7) show high ionic conductivity (10⁻⁴ to 10⁻³ S/cm, depending on x and y) at room temperature. Doping the lithium lanthanum titanates by Co and Ni results in perovskite-like structures, which may be readily synthesized by solid state reaction. Structural and conductivity characterizations are reported. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 999.     

Studies of diluted antiferromagnets MnxMg1-xTiO3 with x=0.55 and 0.70 by muon spin relaxation method

Longitudinal fields μSR measurements have been performed in order to probe the spin dynamics in the diluted antiferromagnets Mn_xMg_1-xTiO₃ with x=0.70 and 0.55. In the x=0.70 sample which forms the antiferromagnetic long‐range order, the static and fluctuating fields coexist at the muon stopping site below T_N. On the other hand, in the x=0.55 sample which shows the spin‐glass behavior, the local fields fluctuate rather fast even below T_SG. We infer that this drastic change occurs when Mn_xMg_1-xTiO₃ transforms from an antiferromagnetic system to a spin‐glass system by dilution. This revised version was published online in July 2006 with corrections to the Cover Date.     

High-temperature electrical transport in Al-doped calcium and strontium titanates

The electrical conductivity of perovskite-related oxides CaTi_1−xAl_xO_3−δ and SrTi_1−xAl_xO_3−δ (x=0−0.4) were investigated within the temperature range 900 to 1000 °C and the oxygen partial pressure range between 10⁻²⁰ and 0.21 atm using a dc four-point technique. The materials investigated show predominantly p-type electronic conductivity at high, n-type electronic conductivity at low, and ionic conductivity at intermediate oxygen partial pressures. The values of ionic conductivity in CaTi_1−xAl_xO_3−δ were found to be lower than those in CaTi_1−xFe_xO_3−δ. The effect of aluminium concentration on the high-temperature transport properties was examined. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Investigation of impurity effects in La2?xSrxCu1?yMyO4 (M=Ga,Zn) by electronic specific heat

The electronic specific heat C_el was studied at T≤10K on Ga- and Zn-doped La_2−xSr_xCuO₄ (0.16≤x≤0.22). In pure La_2−xSr_xCuO₄ (0.16≤x≤0.22), the C_el at T<T_c contains only a T² component but no T-linear one, which is peculiar to a clean d-wave superconductor. Partial substitution of Ga or Zn for Cu changes the T²-term of C_el into a quite different one described by the sum of a T-linear and a nearly T³ term. The coefficient of the T-linear term, γ, markedly increases with Zn-or Ga-content. The λ/λ_N vs. T_c/T_co relation for Zn-doped samples with x≥0.2 is in good agreement with the theoretical curve for resonant impurity scattering in a d-wave superconductor, while those for Ga-doped samples and for Zn-doped samples with x<0.2 deviate slightly from the theoretical curve. Such a deviation will be discussed in relation to the change in the magnetic properties of the present system caused by impurity-doping.     

Magnetothermal properties of Gd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Y<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript>2</Subscript> compounds

The heat capacity of pseudobinary intermetallic compounds Gd_1−x Y _x Ni₂ (0≤x≤1) has been studied. The magnetic contribution to the total heat capacity is isolated with the use of the data obtained for the paramagnetic analogs Lu_1−y Y _y Ni₂ possessing the same molar masses as the gadolinium compounds. It has been found that the difference between the entropies of the Gd_1−x Y _x Ni₂ (x<0.8) compounds and the corresponding paramagnetic Lu_1−y Y _y Ni₂ analogs reaches larger values than those expected from the calculations performed under the assumption that only Gd ions contribute to the magnetic part of the total entropy. The existence of an additional contribution of magnetic nature to the entropy of the Gd_1−x Y _x Ni₂ compounds, as well as the large values of the γ coefficient in the linear-in-temperature term of the heat capacity, is assigned to the spin fluctuations induced by the f—d exchange in the subsystem of Ni 3d electrons.