Topochemical Strategies for the Formation of Alkali-metal Halide Arrays within Perovskite Hosts
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Topochemical Strategies for the Formation of Alkali-Metal Halide Arrays within Perovskite Hosts Liliana Viciu1, Xiao Zhang1, Thomas A. Kodenkandath1, Valdimir Golub2, Elisha Josepha1, and John B. Wiley1 1 Department of Chemistry and AMRI, University of New Orleans, 2000 Lakeshore Dr., New Orleans, LA, 70148 2 AMRI, University of New Orleans, 2000 Lakeshore Dr., New Orleans, LA, 70148 ABSTRACT Multistep topochemical reactions can be used to construct alkali-metal halide arrays within layered perovskite hosts. Combinations of ion exchange and reductive intercalation (A = Li) or reductive and oxidative intercalation (A = Rb) allow one to prepare the compounds such as (A2Cl)LaNb2O7. These products consist of perovskite blocks separated by double alkali-metal halide layers where the local layer structure is dependent on the size of the alkali cation. Details on the synthesis and structures of these materials are presented, and the general utility of the topochemical strategies used in their preparation is discussed. INTRODUCTION Topochemical methods are effective tools for the manipulation of solid-state compounds while maintaining desired structural features. Extensive work in layered perovskites has lead to a series of new compounds via ion exchange and intercalation routes. A combination of ion exchange and reductive deintercalation routes were used by Schaak and Mallouk [1] to prepare simple perovskites with alternating layered features, features not accessible by traditional high temperature synthetic chemistry. Denis and Clark [2], working with layered oxysulfides (e.g., Y2Ti2O5S2), have shown that by controlling the reaction conditions for reductive intercalation, they can direct lithium into perovskite slabs or intermediate rock salt layers. Li and Greaves [3] have inserted fluoride anions between the perovskite blocks of Ruddlesden-Popper layered perovskites by oxidative intercalation; Sr3Ru2O7, for example, can be oxidized to Sr3Ru2O7F2. Working at slightly higher temperatures, Gopaplakrishnan’s group [4] has used metathesis reactions (ion exchange) for the formation of new layered perovskites such as Ca2La2Ti3CuO12 from K2La2Ti3O10. Our group has worked on the formation of metal halide layers within perovskite blocks [5, 6]. We are currently exploiting multistep topotactic reaction strategies to build such layers [7, 8]. Herein we describe the use of two approaches to making alkali metal chloride layers, one based on a combination of ion exchange and reductive intercalation and the other based on reductive and oxidative intercalation steps. EXPERIMENTAL Synthesis. (LixCl)LaNb2O7. RbLaNb2O7 was prepared from La2O3 (Alfa, 99.99%) and Nb2O5 (Alfa, 99.9985%) and a 25% molar excess Rb2CO3 (Alfa, 99%); the mixture was annealed 12 h at 850°C followed by an additional thermal treatment at 1050 °C for 24 h [9]. (CuCl)LaNb2O7 was prepared by a single-step ion-exchange reaction between anhydrous CuCl2 (Alfa, 99%) and RbLaNb2O7 in a 2:1 molar ratio [5]. The reaction was carried out in a sealed,
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