The study of lithium fast ion conductors of Li3xLa0.67−xScyTi1−2yNbyO3 system

Perovskite-type lithium fast ion conductors of Li_3xLa_0.67−xSc_yTi_1−2yNb_yO₃ system were prepared by solid state reaction. X-Ray powder diffraction shows that perovskite solid solution form in the ranges of x=0.10, y≤0.10. AC impedance measurements indicate that the bulk conductivities and the total conductivities are of the order of 10⁻⁴ S·cm⁻¹ and 10⁻⁵ S·cm⁻¹ at 25 °C respectively. The compositions have low bulk activation energies of about 17 kJ/mol in the temperature ranges of 298 – 523 K and total activation energies of about 37 kJ/mol in the temperature ranges of 298 – 523 K.     

Solid electrolytes based on lithium-containing lanthanum metaniobates and metatantalates with defect-perovskite structure

The structure and transport properties of lithium-containing lanthanum metaniobates and metatantalates with defect-perovskite structure, La_2/3−xLi_3x□_4/3−2xNb[Ta]₂O₆, have been studied. It has been shown that the materials under investigation possess a high lithium ion-conductivity Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Effect of V5+ in the complex Li3xLa2/3−xTiO3-LaPO4 system

Lithium fast ion conductors of the composition Li_0.3La_2/3Ti_0.7P_0.3−xV_xO_3.3 (LTV) based on mixtures of Li_3xLa_2/3−xTiO₃ and LaPO₄ were prepared by solid state reaction at high temperature (≈ 1300 °C). AC impedance measurements indicate total conductivities of about 1 × 10⁻⁴ Scm⁻¹ for compositions of x=0∼0.3 at room temperature with an activation energy of ≈18 kJ·mol⁻¹ in the temperature range from 30 to 400 °C. X-ray powder diffraction patterns showed that the LTV system is composed of Li_3xLa_2/3−xTiO₃ perovskite solid solution and LaP_1−xV_xO₄ solid solution.     

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.     

Lithium ion conductivity of Li5+xBaxLa3?xTa2O12 (x?=?0–2) with garnet-related structure in dependence of the barium content

The stoichiometry range and lithium ion conductivity of Li_5+x Ba _x La_3−x Ta₂O₁₂ (x = 0, 0.25, 0.50, 1.00, 1.25, 1.50, 1.75, 2.00) with garnet-like structure were studied. The powder X-ray diffraction data of Li_5+x Ba _x La_3−x Ta₂O₁₂ indicated that single phase oxides with garnet-like structure exist over the compositional range 0 ≤ x ≤ 1.25; while for x = 1.5, 1.75 and 2.00, the presence of second phase in addition to the major garnet like phase was observed. The cubic lattice parameter increases with increasing x and reaches a maximum at x = 1.25 then decreases slightly with further increase in x in Li_5+x Ba _x La_3−x Ta₂O₁₂. The impedance plots of Li_5+x Ba _x La_3−x Ta₂O₁₂ samples obtained at 33 °C indicated a minimum grain-boundary resistance (R _gb) contribution to the total resistance (R _b + R _gb) at x = 1.0. The total (bulk + grain boundary) ionic conductivity increases with increasing lithium and barium content and reaches a maximum at x = 1.25 and then decreases with further increase in x in Li_5+x Ba _x La_3−x Ta₂O₁₂. Scanning electron microscope investigations revealed that Li_6.25Ba_1.25La_1.75Ta₂O₁₂ is much more dense, and the grains are more regular in shape. Among the investigated compounds, Li_6.25Ba_1.25La_1.75Ta₂O₁₂ exhibits the highest total (bulk + grain boundary) and bulk ionic conductivity of 5.0 × 10⁻⁵ and 7.4 × 10⁻⁵ S/cm at 33 °C, respectively.     

The effect of LaPO4 in the LixLa2/3Ti1−xPxO3+x system

Lithium ion conductors of the overall composition Li_xLa_2/3Ti_1−xP_xO_3+x (hereafter referred to as LTP) based on La_2/3TiO₃ were prepared by solid state reaction at high temperature (1300 °C). AC impedance measurements indicate that the total conductivities are of the order of 10⁻⁴ S·cm⁻¹ when x=0.28 − 0.35 at room temperature and have an activation energy of 18 kJ·mol⁻¹ in the temperature range from room temperature to 400 °C. X-ray powder diffraction patterns showed that the LTP system has a complex composition, which contains the solid solution perovskite Li_3xLa_2/3−xTiO₃ and LaPO₄.     

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.     

Effect of B-site substitution of (Li,La)TiO3 perovskites by di-, tri-, tetra- and hexavalent metal ions on the lithium ion conductivity

We report the synthesis and lithium ion conductivity of di-, tri-, tetra- and hexavalent metal ion B-site substituted (Li,La)TiO₃(LLT) perovskites. All 5–10 mol% Mg, Al, Mn, Ge, Ru and W ion substituted LLTs crystallize in a simple cubic or tetragonal perovskite structure. Among the oxides investigated, the Al-substituted perovskite La_0.55Li_0.36□_0.09Ti_0.995Al_0.005O₃ (□=vacancy) exhibits the highest lithium ion conductivity of 1.1 × 10⁻³ S/cm at room temperature which is slightly higher than that of the undoped (Li,La)TiO₃ perovskite (8.9 × 10⁻⁴ S/cm) at the same temperature. The lithium ion conductivity of substituted LLTs does not seem to depend on the concentration of the A-site ion vacancies and unit cell volume. The high ionic conductivity of Al-substituted LLT is attributed to the increase of the B(Al)-O bond and weakening of the A(Li,La)-O bond. The conductivity behavior of the doped LLT is being described on the basis of Gibbs free energy considerations.     

Calculation of chemical bonds and diffusion path of Li ion in (LaLi)Ti2O6 by DV-Xα cluster method

The chemical bonds and lithium diffusion of La_4/3−y Li_3y Ti₂O₆ (y = 0.21) were investigated by using the DV-Xα cluster method. The cluster model used is the formula La₈Li₂Ti₂O₁₁. A Li ion was moved on the ab plane at z = 1/2. The Na ion was moved along the x axis in the cluster model La₈Na₂Ti₂O₁₁ for comparison. The total bond overlap population (BOP) between the moving Li ion and the other ions was calculated on the ab plane at z = 1/2. The total BOP of the Li ion along the x axis increased near the oxygen ion site, whereas the BOP of the Na ion decreased. The decrease in total BOP indicates the decrease in covalent interaction between the Na and the other ions. The change of the net charge of the Li ion was almost the same as that of the Na ion. This suggests that the smaller change of covalent interaction in the mobile Li ion determines the diffusion path of Li ion.     

Fast ionic conduction in cubic hafnium garnet Li<Subscript>7</Subscript>La<Subscript>3</Subscript>Hf<Subscript>2</Subscript>O<Subscript>12</Subscript>

A new member of the family of garnets with fast lithium ion conduction has been found with the composition Li₇La₃Hf₂O₁₂. The anion arrangement corresponds to the oxygen framework in garnets, e.g., in Ca₃Fe₂Si₃O₁₂. Hafnium is coordinated octahedrally while the lanthanum environment can be described as a distorted cube. Lithium occupies a large number of positions with tetrahedral, trigonal planar, and metaprismatic coordination. Li₇La₃Hf₂O₁₂ shows a lithium bulk ion conductivity of 2.4 × 10⁻⁴ Ω⁻¹ cm⁻¹ at room temperature with an activation energy of 0.29 eV.     

Comparative analysis of the effect of La and Co on the superconductivity and energyband spectrum of YBa2Cu3Oy for different oxygen contents

Temperature dependences of the resistivity and Seebeck coefficient of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y samples (x=0–0.25) have been measured under maximum sample saturation with oxygen, as well as following their anneal in an oxygen-deficient atmosphere. The T _c (x) dependences for as-prepared samples were found to pass through a maximum at x=0.05, which persists after annealing for Y(Ba_1−xLa_x)₂Cu₃O_y and disappears for YBa₂Cu_3−x Co_xO_y. A phenomenological model of the band spectrum in normal phase has been used to determine the parameters of the conduction band and of the carrier system, and to analyze their variation with the dopant type and content, as well as with annealing. Despite the differences observed in the T _c (x) dependence, the critical temperatures for all the sample series studied were found to correlate with the conduction-band effective width. The mechanism of the effect of impurities on the band-structure parameters and the reasons for the different influence of annealing on the properties of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 389–394 (March 1997)     

A comparison of the transport properties of La2−xSrxNi1−yFeyO4+δ where 0<x<0.2 and 0<y<0.2

Recently there has been substantial interest in optimising perovskite type ceramics as mixed ionic-electronic conductors (MIECs) for use in ceramic oxygen generators and solid oxide fuel cells. However these materials suffer from thermomechanical deficiencies and therefore there is a need to develop alternative materials. Using the IEDP/SIMS technique the La_2−xSr_xNi_1−yFe_yO_4+δ series of compounds has been investigated and the oxygen tracer diffusion and surface exchange coefficients determined. It has been found that the oxide ion diffusivity of the x and y=0 material is very close to that of the fast oxide ion conducting perovskites such as La_1−xSr_xCoO₃ (LSC) but on acceptor doping with Sr a considerable decrease in the oxygen tracer diffusion coefficient was observed. Further studies on the effects of B site doping, where y>0, indicate that this has very little effect on the transport properties. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

The ionic conductivity of Li6BaLa2M2O12 with coexisting Nb and Ta on the M sites

In a view to balancing cost and lithium ion conductivity, Li₆BaLa₂Nb _x Ta_2???x O₁₂ (x?=?0–2) was prepared by solid-state reaction, and its corresponding AC impedances were tested at temperatures ranging from 20 to 250 °C in air. Li₆BaLa₂Ta₂O₁₂ exhibits the highest conductivity, 8.77?×?10^?6?S/cm, and the second highest is Li₆BaLa₂Nb₂O₁₂ with 6.69?×?10^?6?S/cm. Partial replacement of Ta with Nb cannot bestow the advantages of cost saving or the enhancement of lithium ion conductivity. X-ray diffraction patterns revealed a gradual change as an increasing amount of Nb replaces Ta in Li₆BaLa₂Nb _x Ta_2???x O₁₂ (x?=?0–2), and it is thought that the trending of Nb and Ta to rest on the crystallographic planes is different.     

Lithium ion conductivity of the B-site substitution in Li3xLa0.67−xRE y III Ti1−2yPyO3 system (REIII=Sc3+, Y3+, Nd3+, Sm3+, Eu3+, Yb3+)

The B-site substituted perovskite solid solution systems Li_3xLa_0.67−xRE_yTi_1−2yP_yO₃ (RE=Sc, Y, Nd, Sm, Eu, Yb) have been investigated. Perovskite solid solutions formed in the range of x=0.10, y<0.10 for RE=Sc³⁺, Y³⁺, Yb³⁺, x=0.10, y≤0.05 for RE=Nd³⁺, Sm³⁺, Eu³⁺. Li_0.3La_0.57Nd_0.05Ti_0.9P_0.05O₃ has the highest bulk conductivity of 4.31×10⁻⁴ S·cm⁻¹ and the highest total conductivity of 2.52×10⁻⁴ S·cm⁻¹ at room temperature in all prepared compositions. The compositions have low activation energies of about 24–30 kJ/mol in the temperature ranges of 298–523 K. SEM studies showed that the sample made by solid-state reaction has a sphere-like morphology and a rough particle with particle size of about 50 μm. The research results also indicated that the reaction temperature decreases and the electrochemical stabilities of the titanate-based perovskite-type solid solutions are improved by using RE³⁺ and P⁵⁺ replaced Ti⁴⁺ on B-site in the Li_3xLa_0.67−xTiO₃ parent.     

An ab initio molecular dynamics study of ionic conductivity in hexagonal lithium lanthanum titanate oxide La0.5Li0.5TiO3

To date, the fastest lithium ion-conducting solid electrolytes known are the perovskite-type ABO₃ oxide, with A = Li, La and B = Ti, lithium lanthanum titanate (LLTO) Li_3x La_{( 2 \mathord}

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.     

The effect of Ca2+ in Li0.24+xLa0.59−xCaxTiO3-LaPO4 system

Li_0.2375+xLa_0.5875−xCa_xTiO₃-LaPO₄ fast ionic conductor was synthesized by high temperature solid-state reaction. A.C. impedance measurements show that the compositions of system have better conductivities in low doping content of Ca²⁺, but the conductivity goes down from 1.039·10⁻⁴ Scm⁻¹ to 1.173·10⁻⁵ Scm⁻¹ with increasing the content of Ca²⁺. The activation energy is about 20 kJ·mol⁻¹ in the temperature range of 25–400 °C. X-ray powder diffraction shows that the doping Ca²⁺ would not affect the structure of compositions in the system. The main phase is β-Li_0.24+xLa_0.59−xCa_xTiO₃ perovskite solid solution, LaPO₄ as a second phase also can be found. With the increasing content of Ca²⁺ (x>0.05), another phase CaTiO₃ appears obviously. IR measurement also indicates that the structure of compositions in the system would not be affected by Ca²⁺ doping. The decomposition voltage of Li_0.24+xLa_0.59−xCa_xTiO₃-LaPO₄ keeps on 1.6 V.     

Calculation of 7Li MAS NMR chemical shift in La4/3−yLi3yTi2O6

The chemical shifts of ⁷Li MAS nuclear magnetic resonance spectra in La_4/3−yLi_3yTi₂O₆ (LLTO) showed negative values and decreased with increasing lithium concentration. The chemical shifts were interpreted by Pople’s theory in which the ⁷Li chemical shifts were due to the local paramagnetic currents of the closest oxygen ions. Lattice parameters and coordination of oxygen were obtained by Rietveld analysis of X-ray diffraction data. The gross population and electron excitation energy were calculated by DV-Xα method.     

Transport properties of a and c axis oriented (Y1?xCax)Ba2Cu3Oy thin films

Anisotropy and Hall effect measurements have been performed in calcium-doped, i.e., overdoped YBa₂Cu₃O_y ((Y_1−xCa_x)Ba₂Cu₃O_y) thin films witha andc axis orientations. In highly overdoped films (x=0.4), the anisotropy of the normal resistivity decreases and a drastic change in Hall conductivity in the mixed state is observed. The change in Hall conductivity in the overdoped region is consistent with recent experimental results for La_2−xSr_xCuO₄ films and seems to be common in highT _c superconductors.     

Dielectric properties of the Li0.07Na0.93Ta0.1Nb0.9O3 and Li0.07Na0.93Ta0.111Nb0.889O3 solid solutions

Dielectric properties of perovskite ferroelectric solid solutions Li_0.07Na_0.93Ta_0.1Nb_0.9O₃ and Li_0.07Na_0.93Ta_0.111Nb_0.889O₃ are studied over the 290?C700 K range of temperature within the frequency range of 25?C10⁶ Hz. A decrease of the Curie temperature (??75 K) compared with Li_0.07Na_0.93Ta_0.1Nb_0.9O₃ synthesized from mechanical mixture of pentoxides Ta₂O₅ and Nb₂O₅ is observed in Li_0.07Na_0.93Ta_0.111Nb_0.889O₃ synthesized from co-precipitated pentoxide Ta_2y Nb_{2(1 ? y)}O₅.