Strategies for Enhancing Lithium-Ion Conductivity of Triple-Layered Ruddlesden-Popper Oxides: Case Study of Li2-xLa2-yTi3-zNbzO10

We report strategies of enhancing the ionic conductivity of triple-layered Ruddlesden-Popper oxides through design and synthesis of seven compounds belonging to the series A₂A'₂B₃O₁₀ (A = Li, A' = La, B = Ti/Nb), investigated by neutron diffraction, impedance spectroscopy, and dielectric analyses. We demonstrate, for the first time, that lithium diffusion in triple-layered Ruddlesden-Popper oxides is a result of cooperative effect of both inter- and intrastack sites, i.e., A and A'. As shown by neutron diffraction, the structure of these materials comprises triple-layered stacks of octahedra (BO₆), separated by A-site cations, while A' ions reside in intrastack spaces. We first synthesized Li₂La₂Ti₃O₁₀ and showed that its lithium-ion conductivity can be systematically enhanced by incorporation of cation deficiency in interstack sites through synthesis of Li_1.9La₂Ti_2.9Nb_0.1O₁₀, Li_1.8La₂Ti_2.8Nb_0.2O₁₀, and Li_1.75La₂Ti_2.75Nb_0.25O₁₀. The latter represents the limit of cation deficiency on the A-site and has the highest conductivity among the A-site-deficient materials. We then investigated the enhancement of lithium-ion conductivity by incorporation of cation defects in intrastack A'-sites through synthesis of Li₂La_1.9Ti_2.7Nb_0.3O₁₀ and Li₂La_1.8Ti_2.4Nb_0.6O₁₀, where the latter represents the limit of cation deficiency on the A'-site and has the best conductivity among the A'-deficient materials. Finally, we hypothesized that cooperative effect of defects in both inter- and intrastack sites should have an even higher impact on ionic conductivity. This hypothesis was confirmed by synthesis of Li_1.9La_1.9Ti_2.6Nb_0.4O₁₀, which showed the highest conductivity among all materials synthesized in this work. Detailed analysis of real and imaginary components of impedance spectroscopy, as well as dielectric and loss tangent, have been conducted. This systematic study is aimed at answering a fundamental question related to materials chemistry of Ruddlesden-Popper oxides, namely, determination of the sites that contribute to ionic conductivity.

A series of new layered lithium europium(II) oxoniobates(V) and -tantalates(V)

The new Ruddlesden-Popper-related phases A n +1 B n O3 n +1 (n = 3) with the compositions Li2Eu2Nb3O10, Li2Eu1.5Ta3O10, Li2EuKNb3O10, and Li2EuKTa3O10 were synthesized by solid-state reactions from Li2[CO3] (+ K2[CO3]) and the corresponding refractory metals along with their oxides in a high-frequency furnace at temperatures above T = 1600°C. Their structures have been determined by single-crystal X-ray diffraction studies. Characteristic features are triple layers of corner-sharing [MO6]7− octahedra (M = Nb and Ta), which are connected via [LiO4]7− tetrahedra. The Eu2+ cations are cuboctahedrally surrounded by 12 oxygen atoms and according to the Eu–O distances of around 275 pm, they have the oxidation state +2, as confirmed by XPS measurements. In the potassium-containing samples they share their positions with K+ cations. The black compounds are stable in air at room temperature. Measurements of the magnetic susceptibilities in the range of T = 5–300 K revealed Li2Eu2Nb3O10, Li2Eu1.5Ta3O10 and Li2EuKTa3O10 to be paramagnetic without any ordering.

Thermal structural characterization of the acentric layered perovskite LiHSrTa2O7: X-ray and neutron diffraction, SHG and Raman experiments

The present work concerns the thermal structural characterization of the acentric Ruddlesden-Popper LiHSrTa2O7. A previous study, performed with powder neutron diffraction data, has revealed that at room temperature, LiHSrTa2O7 crystallizes in the Ama2 space group and that the acentric character is mainly due to the unequal distribution of the Li(+) and H(+) cations on their sites. In this new paper, the thermal behaviour has been studied by several techniques: powder X-ray and neutron diffraction, SHG experiments and Raman spectroscopy. All of them have revealed that LiHSrTa2O7 undergoes a reversible structural transition from an orthorhombic to a tetragonal symmetry around 200 °C. This transition is associated with the progressive vanishing of the TaO6 octahedra tilting, becoming completely straight in the high temperature form (S.G. I4/mmm), and with a variation of the Li(+) and H(+) distribution in the interlayer spacing.

Nature of the Bottom t2g-Block Bands of Layered Perovskites. Implications for the Transport Properties of Phases Where These Bands Are Partially Filled

The electronic structure of a series of double layer (Sr(3)V(2)O(7), KLaNb(2)O(7), Li(2)SrNb(2)O(7), RbLaNb(2)O(7), Rb(2)LaNb(2)O(7)) as well as triple layer (Ca(4)Ti(3)O(10), K(2)La(2)Ti(3)O(10), Sr(4)V(3)O(9.7), CsCa(2)Nb(3)O(10)) Dion-Jacobson and Ruddlesden-Popper phases and quadruple layer A(n)()B(n)(O(3)(n)(+2) phases (Sr(2)Nb(2)O(7) as well as the low-temperature and room-temperature structures of Sr(2)Ta(2)O(7)) has been studied by means of a first-principles density functional theory approach. The results are rationalized on the basis of a simple tight-binding scheme, which provides a simple yet precise scheme allowing the correlation of the crystal structure details and the nature of the bottom t(2g)-block band levels. Both the quantitative and the qualitative approaches are used to analyze the nature of the carriers in intercalated samples of the d(0) semiconducting phases as well as those of the metallic d(x) (0 < x < or = 1) systems. The Ruddlesden-Popper and Dion-Jacobson materials with partially filled t(2g)-block bands must be genuine two-dimensional metals except when the M-O(ap) distances of the outer layer octahedra are similar and the band filling is not low. The conducting electrons in these phases are almost equally distributed among the different layers. It is shown that the A(n)()B(n)O(3)(n)(+2) phases with partially filled bands are potentially interesting materials because they are structurally two-dimensional materials exhibiting one-dimensional band structure features. Finally, the possible application of the simple scheme to related materials such as layered perovskite oxynitrides and the effect of disorder are briefly discussed.