H. Duroy

Modeling Li-ion conductivity in fast ionic conductor La2/3−xLi3xTiO3

Abstract Monophasic samples of fast ionic conductors La2/3−xLi3xTiO3 (LLTO), with x varying from 0.06 to 0.13, are prepared by solid-state reaction. The total dc-conductivity is measured by complex impedance spectroscopy in the 10 MHz –1 Hz frequency range. Considering only the resulting location of oxygen atoms and employing bond valence equations, the conduction geometry and dc ionic conductivity are modeled. An averaged pathway for the Li+ conduction is proposed in this paper, assuming that the time-averaged position for Li+ is the geometrical centre of the A-cage. The saddle point of this pathway (Vumax) can be related to the activation energy for the ionic jump. Moreover, in order to model dc-ionic conductivity, not only activation energy, but also number of carriers and site occupancies have been considered. We propose three possibilities for the Li+ location in the structure in order to predict bulk conductivity in LLTO phase. Experimental evidence allows the exclusion of one of the three possibilities, while the other two are both in agreement with experimental values.

In search of the cubic phase of the Li+ ion-conducting perovskite La2/3−xLi3xTiO3: structure and properties of quenched and in situ heated samples

Abstract In search of the cubic phase of the Li + ion-conducting perovskite La 2/3− x Li 3 x TiO 3 (LLTO), several samples, quenched and heated at different temperatures, are examined by powder X-ray diffraction (XRD). The best structural model (for 0.06 x P 4/ mmm ; a ≈ a p , c ≈2 a p ) in which La 3+ ions and vacancies are always unequally distributed in the two different A sites, centers of the perovskite dodecahedral cages. The quenching experiments bring the evidence of a very fast disordering/ordering of La 3+ ions between their two unequivalent positions, and the in situ thermodiffractometry experiments show that the La 3+ ion thermal diffusion becomes noticeable above 800 °C and depends on the solid solution composition x . Although the thermal diffusion of the La 3+ ions occurs above 800 °C, the disordering of these ions remains limited for low x values (with c /2 a values slightly greater than 1) even at 1200 °C, while a nearly complete disordering is reached at 1200 °C for x =0.11 (with c /2 a very close to 1). As reported by earlier authors, the kinetics of the fast La 3+ ions disordering/ordering in the temperature range 900–1200 °C has to be fully considered during the preparation of the samples, and the thermal history can noticeably change the La 3+ ion partition and hence the electrical conductivity of the prepared phase. The equilibrated samples can be obtained by using proper heating time and temperature.

Topotactic H+/Li+ ion exchange on La2/3−xLi3xTiO3: new metastable perovskite phases La2/3−xTiO3−3x(OH)3x and La2/3−xTiO3−3x/2 obtained by further dehydration

We have synthesized novel La2/3−xTiO3−3x(OH)3x (0.07 < x < 0.13) compounds by ion exchange of the lithium ions from La2/3−xLi3xTiO3 in dilute HNO3 at 60°C. The proton derivatives crystallize in tetragonal perovskite cells (a ≈ ap, c ≈ 2ap) similar to those of the parent oxides. Interestingly, the protonated oxides (characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), and 1H nuclear magnetic resonance (NMR) yield new metastable A-site- and oxygen-deficient perovskites La2/3−xTiO3−3x/2 at about 800°C.

Synthesis and structure of novel layered perovskite oxides: Li2La1.78Nb0.66Ti2.34O10, and a new family, Li2[A0.5nBnO3n + 1]

Several new members (n = 2, 3, 4 members) of the family of layered perovskites Li2[AxBnO3n + 1] (A = Ca, Sr, La; B = Nb, Ta, Ti, Fe) have been synthesized for the first time. The structure analysis of Li2SrNb2O7 and Li2SrTa2O7 determined by powder X-ray diffraction (XRD), and of Li2La1.78Nb0.66Ti2.34O10 and Li2Sr2Nb3.97Fe0.03O12.97 determined by single crystal XRD data, shows that these novel phases are related to the Ruddlesden–Popper series of compounds, A′2[An – 1MnO3n + 1]. While the structures of n = 2 members are identical to the RP series, in the higher members the A sites are partially occupied. Interestingly, the compounds for A = Ca, Sr and B = Nb, Ta, Fe indicate the formation of a new family of oxides of general formula Li2A0.5nBnO3n + 1 where different members of the family can be synthesized with the same set of A and B atoms.

Synthesis, characterization and dehydration study of H2A0.5nBnO3n + 1·xH2O (n = 2 and 3, A = Ca, Sr and B = Nb, Ta) compounds obtained by ion-exchange from the layered Li2A0.5nBnO3n + 1 perovskite materials

We have synthesized several new layered protonated materials related to Ruddlesden–Popper phases, H2A0.5nBnO3n + 1·xH2O (A = Ca, Sr; B = Nb, Ta), by ion-exchange of the lithium ion from Li2A0.5nBnO3n + 1, a family of layered perovskites recently synthesized by us, in dilute HNO3 at 60 °C. The protonated derivatives show interesting dehydration properties with formation of layered anhydrous phases followed by a transformation to novel metastable A0.5BO3 compounds. The latter oxides, irrespective of the n-value of the mother phase, transform on further heating to the thermodynamically stable AB2O6 materials. The structural transformations occurring during the dehydration process are characterized by X-ray powder diffraction and electron microscopy techniques.

New A-deficient perovskites in the series LixLa2/3Ti1-xFexO3 (0.12 ≤ x ≤ 0.33) and La(2+x)/3Ti1-xFexO3 (0.5 ≤ x ≤ 1.0)

Two new series of A-deficient perovskites have been synthesized and structurally characterized from Rietveld treatment of their X-ray diffraction powder patterns. The first one, LixLa2/3Ti1-xFexO3, for 0.12 ≤ x ≤ 0.33, results from the substitution mechanism Ti4+ → Fe3+ + Li+. The structure is closely related to that of La2/3-xLi3xTiO3 (a ≈ ap, b ≈ ap and c ≈ 2 ap) with a symmetry evolution leading to the existence of two domains: for 0.12 ≤ x ≤ 0.19 the symmetry is orthorhombic (Pmmm), while for 0.20 ≤ x ≤ 0.33 a tetragonal symmetry is obtained (P4/mmm). In both cases, the population of La3+ ions, unequally distributed on two adjacent sites for smaller x values, is directly affected by the increase of the Li content and tends to be equalized with larger x values. The second series is found to result from another substitution mechanism Ti4+ → Fe3+ + 1/3La3+, leading to the formula La(2+x)/3Ti1-xFexO3 in the domain 0.5 ≤ x ≤ 1.0. The structure is then closely related to that of LaFeO3, where La3+ ions occupy now only one crystallographic site in the Pnma space group. Transmission Electron Microscopy and Mossbauer spectrometry confirm the above models.

Synthesis and crystal structure of new layered perovskite compounds: Li2La0.833(Nb1.5Ti0.5)O7 and Li2La2.25(Nb1.25Ti2.75)O13

Single crystals of Li2La0.833(Nb1.5Ti0.5)O7 and Li2La2.25(Nb1.25Ti2.75)O13 were grown in the system Li2O–La2O3–Nb2O5–TiO2. Both these compounds crystallize in the space group P42/mnm (No. 136), Z = 4: a = 5.5334(2), c = 18.3907(2) A and a = 5.4915(6), and c = 33.812(4) A, respectively. Their crystal structures, determined from single crystal X-ray diffraction data, are related to the Ruddlesden–Popper phases An+1BnO3n+1, which could be better written here as Li2[AxBnO3n+1]: Li2La0.833(Nb1.5Ti0.5)O7 is an n = 2 member and Li2La2.25(Nb1.25Ti2.75)O13 corresponds to an n = 4 member. In both structures, Nb5+ and Ti4+ ions statistically occupy distorted B octahedral sites of the perovskite, La3+ ions partially fill the cages of the perovskite, and Li+ ions are found in tetrahedral coordination between the perovskite layers.