Oxygen Ion and Proton Transport in Alkali-Earth Doped Layered Perovskites Based on BaLa2In2O7

Inorganic materials with layered perovskite structures have a wide range of physical and chemical properties. Layered perovskites based on BaLanInnO3n+1 (n = 1, 2) were recently investigated as protonic conductors. This work focused on the oxygen ion and proton transport (ionic conductivity and mobility) in alkali-earth (Sr2+, Ba2+)-doped layered perovskites based on BaLa2In2O7. It is shown that in the dry air conditions, the nature of conductivity is mixed oxygen–hole, despite the dopant nature. Doping leads to the increase in the conductivity values by up to ~1.5 orders of magnitude. The most proton-conductive BaLa1.7Ba0.3In2O6.85 and BaLa1.7Sr0.15In2O6.925 samples are characterized by the conductivity values 1.2·10−4 S/cm and 0.7·10−4 S/cm at 500 °C under wet air, respectively. The layered perovskites with Ruddlesden-Popper structure, containing two layers of perovskite blocks, are the prospective proton-conducting materials and further material science searches among this class of materials is relevant.

Materials AIILnInO4 with Ruddlesden-Popper Structure for Electrochemical Applications: Relationship between Ion (Oxygen-Ion, Proton) Conductivity, Water Uptake, and Structural Changes

In this paper, the review of the new class of ionic conductors was made. For the last several years, the layered perovskites with Ruddlesden-Popper structure A^IILnInO₄ attracted attention from the point of view of possibility of the realization of ionic transport. The materials based on Ba(Sr)La(Nd)InO₄ and the various doped compositions were investigated as oxygen-ion and proton conductors. It was found that doped and undoped layered perovskites BaNdInO₄, SrLaInO₄, and BaLaInO₄ demonstrate mixed hole-ionic nature of conductivity in dry air. Acceptor and donor doping leads to a significant increase (up to ~1.5-2 orders of magnitude) of conductivity. One of the most conductive compositions BaNd_0.9Ca_0.1InO_3.95 demonstrates the conductivity value of 5 × 10^-4 S/cm at 500 °C under dry air. The proton conductivity is realized under humid air at low (<500 °C) temperatures. The highest values of proton conductivity are attributed to the compositions BaNd_0.9Ca_0.1InO_3.95 and Ba_1.1La0_.9InO_3.95 (7.6 × 10^-6 and 3.2 × 10^-6 S/cm correspondingly at the 350 °C under wet air). The proton concentration is not correlated with the concentration of oxygen defects in the structure and it increases with an increase in the unit cell volume. The highest proton conductivity (with 95-98% of proton transport below 400 °C) for the materials based on BaLaInO₄ was demonstrated by the compositions with dopant content no more that 0.1 mol. The layered perovskites A^IILnInO₄ are novel and prospective class of functional materials which can be used in the different electrochemical devices in the near future.

Defect engineering of oxide perovskites for catalysis and energy storage: synthesis of chemistry and materials science

Oxide perovskites have emerged as an important class of materials with important applications in many technological areas, particularly thermocatalysis, electrocatalysis, photocatalysis, and energy storage. However, their implementation faces numerous challenges that are familiar to the chemist and materials scientist. The present work surveys the state-of-the-art by integrating these two viewpoints, focusing on the critical role that defect engineering plays in the design, fabrication, modification, and application of these materials. An extensive review of experimental and simulation studies of the synthesis and performance of oxide perovskites and devices containing these materials is coupled with exposition of the fundamental and applied aspects of defect equilibria. The aim of this approach is to elucidate how these issues can be integrated in order to shed light on the interpretation of the data and what trajectories are suggested by them. This critical examination has revealed a number of areas in which the review can provide a greater understanding. These include considerations of (1) the nature and formation of solid solutions, (2) site filling and stoichiometry, (3) the rationale for the design of defective oxide perovskites, and (4) the complex mechanisms of charge compensation and charge transfer. The review concludes with some proposed strategies to address the challenges in the future development of oxide perovskites and their applications.

Inversion symmetry breaking by oxygen octahedral rotations in the Ruddlesden-Popper NaRTiO4 family

Rotations of oxygen octahedra are ubiquitous, but they cannot break inversion symmetry in simple perovskites. However, in a layered oxide structure, this is possible, as we demonstrate here in A-site ordered Ruddlesden-Popper NaRTiO4 (R denotes rare-earth metal), previously believed to be centric. By revisiting this series via synchrotron x-ray diffraction, optical second-harmonic generation, piezoresponse force microscopy, and first-principles phonon calculations, we find that the low-temperature phase belongs to the acentric space group P42(1)m, which is piezoelectric and nonpolar. The mechanism underlying this large new family of acentric layered oxides is prevalent, and could lead to many more families of acentric oxides.

Crystal-chemistry guidelines for noncentrosymmetric A2BO4 Ruddlesden-Popper oxides

Noncentrosymmetric (NCS) phases are seldom seen in layered A2BO4 Ruddlesden-Popper (214 RP) oxides. In this work, we uncover the underlying crystallographic symmetry restrictions that enforce the spatial parity operation of inversion and then subsequently show how to lift them to achieve NCS structures. Simple octahedral distortions alone, while impacting the electronic and magnetic properties, are insufficient. We show using group theory that the condensation of two distortion modes, which describe suitable symmetry unique octahedral distortions or a combination of a single octahedral distortion with a "compositional" A or B cation ordering mode, is able to transform the centrosymmetric aristotype into a NCS structure. With these symmetry guidelines, we formulate a data-driven model founded on Bayesian inference that allows us to rationally search for combinations of A- and B-site elements satisfying the inversion symmetry lifting criterion. We describe the general methodology and apply it to 214 iridates with A(2+) cations, identifying RP-structured Ca2IrO4 as a potential NCS oxide, which we evaluate with density functional theory. We find a strong energetic competition between two closely related polar and nonpolar low-energy crystal structures in Ca2IrO4 and suggest pathways to stabilize the NCS structure.

Rietveld refinement of KLaTiO(4) from X-ray powder data

Potassium lanthanum titanate(IV), KLaTiO(4), has been synthesized by conventional solid-state reaction. It crystallizes isotypically with the NaLnTiO(4) (Ln = La, Pr, Nd, Sm, Eu, Gd, Y and Lu) family. Five of the six atoms in the asymmetric unit (one K, one La, one Ti and two O atoms) are situated on sites with 4mm symmetry, whereas one O atom has 2mm. site symmetry. The crystal structure can be described as being composed of single layers of distorted corner-sharing TiO(6) octa-hedra extending parallel to (001). The layers are alternately separated by K(+) and La(3+) cations along [001]. The coordination number of both K(+) and La(3+) cations is nine, resulting in distorted KO(9) and LaO(9) polyhedra.

Reversible Cation/Anion Extraction from K2La2Ti3O10: Formation of New Layered Titanates, KLa2Ti3O9.5 and La2Ti3O9

A new soft-chemical transformation of layered perovskite oxides is described wherein K2O is sequentially extracted from the Ruddlesden-Popper (R-P) phase, K2La2Ti3O10 (I), yielding novel anion-deficient KLa2Ti3O(9.5) (II) and La2Ti3O9 (III). The transformation occurs in topochemical reactions of the R-P phase I with PPh4Br and PBu4Br (Ph = phenyl; Bu = n-butyl). The mechanism involves the elimination of KBr accompanied by decomposition of PR4+ (R = phenyl or n-butyl) that extracts oxygen from the titanate. Analysis of the organic products of decomposition reveals formation of Ph3PO, Ph3P, and Ph-Ph for R = phenyl, and Bu3PO, Bu3P along with butane, butene, and octane for R = butyl. The inorganic oxides II and III crystallize in tetragonal structures (II: P4/mmm, a = 3.8335(1) A, c = 14.334(1) A; III: I4/mmm, a = 3.8565(2) A, c = 24.645(2) A) that are related to the parent R-P phase. II is isotypic with the Dion-Jacobson phase, RbSr2Nb3O10, while III is a unique layered oxide consisting of charge-neutral La2Ti3O9 anion-deficient perovskite sheets stacked one over the other without interlayer cations. Interestingly, both II and III convert back to the parent R-P phase in a reaction with KNO3. While transformations of the R-P phases to other related layered/three-dimensional perovskite oxides in ion-exchange/metathesis/dehydration/reduction reactions are known, the simultaneous and reversible extraction of both cations and anions in the conversions K2La2Ti3O10 right harpoon over left harpoon KLa2Ti3O9.5 right harpoon over left harpoon La2Ti3O9 is reported here for the first time.